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Spiral galaxies are taking over

Sunday, February 28, 2010

Everybody knows what a spiral galaxy looks like. Here's a typical nearby example (M74):



Among the largest and brightest galaxies close to our own, about 72% are of this spiral type, like M74. There is a classification system for galaxy shapes, and the remaining 28% of large, bright, nearby galaxies fall into classes called "elliptical", "lenticular", or simply "peculiar". (For simplicity and for other reasons that will become apparent, we're ignoring smaller, dimmer galaxies – such as the Magellanic Clouds – which may be very small and difficult to detect. Such galaxies tend to have more irregular shapes.)

Actually, two types of spiral galaxies are normally recognized: "ordinary" spirals, such as M74, above. In these the spiral arms reach all the way in to the center of the galaxy. However, in the variant known as a "barred" spiral the arms originate at either end of a "bar" through the center of the galaxy. The figure to the right shows how the Milky Way is thought to appear if viewed from outside.

Our own galaxy, the Milky Way, has been placed in this category. Although we can't see it as it looks from a distance, the classification is based on detailed surveys of star frequencies in varying directions and distances. The Milky Way's bar is fairly small. Here's another, more dramatic, barred spiral, NGC 1300:



The classification system for galaxy shapes was developed by Edwin Hubble, and so it's called the Hubble sequence. In addition to the two types of spirals, it includes elliptical galaxies and lenticular galaxies. A final type that Hubble recognized is the irregular galaxy, which is anything that doesn't fit in another classification.

Elliptical galaxies are classified as such based on their 2-dimensional appearance being elliptical. The actual 3-dimensional shape is generally ellipsoidal, in which the three principal axes may be of different lengths. A purely spherical galaxy would be a special case. Elliptical galaxies lack any distinct internal features besides smoothly increasing brightness towards the center. A lenticular galaxy is a transition type between elliptical and spiral. It has a central bulge that is ellipsoidal in shape, but also a flattened disk outside the bulge, like the disk of a spiral galaxy, except that there are no distinct spiral arms.

The Hubble classification scheme recognizes distinctions within each type, based on the degree of elongation (for an ellipse) or how tightly wound the spiral arms are. For our purposes here, it's enough to consider just the main types: ellipticals, lenticulars, spirals, and everything else (irregulars).

Hubble constructed his classification based on nearby galaxies, whose structure was easily observable at the time (1936). There is no compelling reason to think that a very similar sequence could be used to classify galaxies in existence at some very different age of the universe, whose present age is ~13.7 billion years. So, now that our instruments are capable of studying galaxies at much greater distances, and hence at earlier times, a very interesting question is how much different would a useful classification system be for a very different time period. Would there be entirely different types? Or would the types be much the same, but perhaps in different proportions relative to each other?

Astronomers have been wondering about this for some time, and reaching out to greater distances as their equipment allows. Now there is some recent research that covers a substantial number of selected galaxies (143) at redshifts around z~.65, which are about 6 billion light-years distant, and hence observed as they were 6 billion years ago, about 7.7 billion years after the big bang. We have no way to know how old any particular galaxy is, but most must have been little more than 7 billion years old.

For comparison, an appropriate sample of 116 local galaxies was selected, to determine the relative proportion of each type at the present time. All galaxies sampled, both local and distant, were required to have at least a certain minimum intrinsic brightness.

In a nutshell, what the research found is that the types observed are just the same as today. There are no obviously distinct new types such as (say) circular rings with little in the center. However, the percentages of spiral and irregular galaxies are dramatically different, though the percentages of ellipticals and lenticulars are almost the same as in the present-day universe.

The research deliberately included only intrinsically bright, massive galaxies in both samples – because, in the distant group, dimmer, less massive galaxies would mostly not be sufficiently bright to reveal useful details of their shape, if they were even detectable at all. But on top of that, even today the less massive galaxies tend to have more irregular shapes that don't fit in the Hubble classification.

In the resulting sample, fully 72% of local galaxies were spirals, but only 31% of distant ones were. On the other hand, irregular galaxies made up only 10% of the local sample but 52% of the distant one. Yet there was hardly any difference in the percentages for ellipticals and lenticulars, being 3% and 15% (respectively) in the local case and 4% and 12% in the distant case.

Stated differently, on a percentage basis spirals are 2.3 times as abundant now as they were 6 billion years ago. Yet irregulars are now 5.2 times less abundant.

Here's the research abstract:

How was the Hubble sequence 6 Gyr ago?
The way galaxies assemble their mass to form the well-defined Hubble sequence is amongst the most debated topic in modern cosmology. One difficulty is to link distant galaxies, which emitted their light several Gyr ago, to those at present epoch. Such a link is affected by the evolution or the transformation of galaxies, as well as by numerous selection and observational biases. We aim to describe the galaxies of the Hubble sequence, 6 Gyr ago. We intend to derive a past Hubble sequence that can be causally linked to the present-day one.... We found that our single criterion is particularly appropriate to relating distant and nearby galaxies, either if gas is transformed to stars in relatively isolated galaxies or, alternatively, if they accrete significant amounts of gas from the intergalactic medium. Subsequent mergers during the elapsed 6 Gyr, as well as evolution of the stellar populations, are found to marginally affect the link between the past and the present Hubble sequence.... We do find an absence of number evolution for elliptical and lenticular galaxies, which strikingly contrasts with the strong evolution of spiral and peculiar galaxies. Spiral galaxies were 2.3 times less abundant in the past, which is compensated exactly by the strong decrease by a factor 5 of peculiar galaxies. It strongly suggests that more than half of the present-day spirals had peculiar morphologies, 6 Gyr ago, and this has to be taken into account by any scenario of galactic disk evolution and formation. The past Hubble sequence can be used to test these scenarios and to test evolution of fundamental planes for spirals and bulges.

One obvious concern with this kind of analysis is to eliminate as much as possible the chance that the results might be biased by selection effects. This is why the primary criterion from including galaxies in either the local or distant sample is that they all have more than a minimum intrinsic brightness. It goes a long way to ensure that enough detail is present in images of the remote galaxies that accurate classification is possible. There is additional analysis to show that effects such as very recent mergers of galaxies and overall aging of stellar populations should not make substantial differences.

Further, the criteria used to classify galaxies are the same for both samples, and simple enough that they can be applied just as accurately to the distant galaxies as the nearby ones. Consequently, it should not be the case that so many irregular (or "peculiar", in the terminology of the paper) types are found in the distant sample simply because the level of detail is inadequate.

Here are some of the characteristics necessary for a galaxy to be classified as irregular: (1) If more than half the object's light comes from a central region less than 1 kiloparsec (3.26 thousand light-years) in radius, the galaxy is classified as "compact". (2) The object appears to have more than one component – a possible merger of two or more galaxies. (3) The object is asymmetrical, with a central bright area and "tail" on only one side. (4) The light in the central area is bluer than the surrounding part (in which young, blue stars of spiral galaxies are usually found) – in the spiral galaxy pictures above you can see that the arms are bluer than the center.

Among objects that are not classified as irregular, the further distinction between elliptical, lenticular, and spiral is based on the percentage of light coming from the central region, with the highest percentage leading to an elliptical classification and the lowest (<50%) to spiral.

The results of this study are actually rather surprising. It has long been assumed that galaxies at present are largely the product of multiple mergers over time between smaller galaxies, and further, that this should lead to more rather than fewer irregularly shaped galaxies.

Instead, just the opposite seems to the case. The percentage of galaxies in both samples that are considered elliptical or lenticular is small and very similar, and therefore probably not representative of the typical evolutionary progression. This is so even if some galaxies in either class transitioned from (or to) the classes of spirals and irregulars.

The most natural conclusion would seem to be that a large percentage of galaxies 6 billion years ago either never had a regular shape or else were midway in the process of merging with others, whereas this is uncommon now. And further, the result at present of all those mergers consists mostly of symmetrical spiral galaxies, rather than ellipticals, lenticulars, or asymmetrical "peculiar" galaxies.

So it seems that the normal course of galaxy evolution is to produce galaxies with a spiral shape much like that of the Milky Way. This is surprising and unexpected.

The obvious question, then, is how so much symmetry and regularity evolved. The same research group responsible for the paper just described has made some specific hypotheses about how this process worked. But that is reported in another paper (here or here) – which we'll look at another time.



ResearchBlogging.org
Delgado-Serrano, R., Hammer, F., Yang, Y., Puech, M., Flores, H., & Rodrigues, M. (2010). How was the Hubble sequence 6 Gyr ago? Astronomy and Astrophysics, 509 DOI: 10.1051/0004-6361/200912704



Further reading:

Forming the present-day spiral galaxies (2/4/10)

Today's Spiral Galaxies Were Once the Ugly Ducklings (2/8/10)

Spiral Galaxies Exist — But Why? (2/14/10)

Older galaxies more peculiar, census shows (2/8/10)

How was the Hubble sequence 6 Gyrs ago? – open access arXiv version of the paper

The Hubble sequence: just a vestige of merger events? – companion paper (arXiv version)

Selected readings 2/26/10

Friday, February 26, 2010

Interesting reading and news items.

These items are also bookmarked at my Diigo account.


Oceans losing ability to absorb greenhouse gas
Like a dirty filter, the Earth's oceans are growing less efficient at absorbing vast amounts of carbon dioxide, the major greenhouse gas produced by fossil-fuel burning, reports a study co-authored by Francois Primeau, UC Irvine Earth system science associate professor. [Physorg.com, 1/11/10]

Sedentary TV time may cut life short
Couch potatoes beware: every hour of television watched per day may increase the risk of dying earlier from cardiovascular disease, according to research reported in Circulation: Journal of the American Heart Association. [Physorg.com, 1/11/10]

How come intelligence, religion, and fertility are linked?
Here's a new study looking at the connection between religion, fertility, and IQ at a national level. We know from previous studies that countries where people are, on average, more religious also tend to have higher average fertility and lower average IQ. The problem is that we also know that countries that have lower average IQ also have higher fertility. So teasing out the two factors is not obvious. [Epiphenom, 2/20/10]

Alien Planet Safari
The premiere observatory of the next decade, the James Webb Space Telescope, will launch in 2014 in search of "big game"--namely, the first stars and galaxies ever formed in our Universe. But the "little game" could turn out to be just as interesting. There's a dawning awareness among astronomers that the world's largest infrared telescope is going to be a canny hunter of planets circling faraway stars. [Physorg.com, 1/14/10

Abstract Thoughts? The Body Takes Them Literally
The body embodies abstractions the best way it knows how: physically. What is moral turpitude, an ethical lapse, but a soiling of one’s character? Time for the Lady Macbeth Handi Wipes. One study showed that participants who were asked to dwell on a personal moral transgression like adultery or cheating on a test were more likely to request an antiseptic cloth afterward than were those who had been instructed to recall a good deed they had done. [New York Times, 2/1/10]

What Is Life? A New Theory
Biology is often called the study of life, yet in the history of the field, experts have never agreed on just what, exactly, life is. Many attempts to classify life focus on a list of requirements, such as the ability to reproduce, to carry out metabolic reactions, to grow, to defend against injury, and others. ... Biologist Gerard Jagers op Akkerhuis of Wageningen University in the Netherlands has come up with a novel solution that does not ask life to meet a long list of abilities. [Space.com, 2/11/10]

Martian Hunting: The Search for Extraterrestrial Genomes
The iguanas of the Galapagos Islands have evolved many unique characteristics due to their isolation from mainland iguanas. Because they can't swim long distances, biologists believe that the first Galapagos iguanas arrived on natural rafts made from vegetation. The same thing may have happened across the ocean of space. Some researchers speculate that life on Mars – if there is any – may be composed of "island species" that were carried away from Earth on interplanetary meteorites. [Space.com, 2/16/10]

Space Is Getting Bigger, and It's Getting Bigger Faster
Few scientists can say their work forever changed how we see the universe. Saul Perlmutter is one of them, for his central role in the 1998 discovery of dark energy. That invisible energy, which accounts for a whopping 73 percent of everything in the cosmos, is stretching the fabric of space and could cause a runaway expansion of the universe. [Discover, 2/22/10]

The Man Who Builds Brains
In its trial runs Markram’s Blue Gene has emulated just a single neocortical column in a two-week-old rat. But in principle, the simulated brain will continue to get more and more powerful as it attempts to rival the one in its creator’s head. “We’ve reached the end of phase one, which for us is the proof of concept,” Markram says. “We can, I think, categorically say that it is possible to build a model of the brain.” In fact, he insists that a fully functioning model of a human brain can be built within a decade. [Discover, 2/5/10]

Problem-solving crows may not be as smart as we thought
Among all these overachievers, crows seem to be the shining exemplar of intelligence. You see, a crow, when first faced with a bit of meat dangling from a bit of string, figures out a solution pretty much instantly. This has led researchers to posit that crows build mental models that generate solutions, instead of relying on trial and error. Now, a bunch of Kiwis have published research in PLoS One that suggests crows don't actually build models. [Nobel Intent, 2/22/10]

As more planets emerge, astronomers are confident they'll find one like Earth
It seems increasingly likely that, as they stare at the heavens, astronomers are going to find an Earth out there, or at least something that they can plausibly claim is a rocky planet where water could splash at the surface and -- who knows? -- harbor some kind of life. ... [However,] the roughly 400 planets that astronomers have found outside our solar system have not been Earthlike by any stretch of the imagination. Most are hot Jupiters, which is to say they're gas giants in scorching orbits. [Washington Post, 1/12/10]

Inflaming dangers of a fat-laden meal
In the heavyweight division, immune cells embedded in fat pack some extra disease-causing punches, a new study shows. Those punches involve potentially dangerous proteins linked to inflammation, heart disease and diabetes. [Science News, 2/24/10]

Ancient dawn's early light refines age of universe
Six papers posted online present new satellite snapshots of the earliest light in the universe. By analyzing these images, cosmologists have made the most accurate determination of the age of the cosmos, have directly detected primordial helium gas for the first time and have discovered a key signature of inflation, the leading model of how the cosmos came to be. The analysis, based on the first seven years of data taken by NASA’s Wilkinson Microwave Anisotropy Probe, also provides new evidence that the mysterious entity revving up the expansion of the universe resembles Einstein’s cosmological constant .... In addition, the data reveal that theorists don’t have the right model to explain the hot gas that surrounds massive clusters of galaxies. [Science News, 2/2/10]

Quantum on Quantum
Almost three decades ago, Richard Feynman — known popularly as much for his bongo drumming and pranks as for his brilliant insights into physics — told an electrified audience at MIT how to build a computer so powerful that its simulations “will do exactly the same as nature.” Not approximately, as digital computers tend to do when faced with complex physical problems that must be addressed via mathematical shortcuts... Feynman meant exactly, as in down to the last jot. [Science News, 2/12/10]

Starting anew
Bely’s finding and other recent results have encouraged researchers who are trying to figure out why some animals can reconstruct their body parts while others can’t. Most species have the ability to regenerate some body parts, yet this talent is highly variable. Humans, for instance, can renew skin and bone, but salamanders can re-create entire limbs or tails, or just about any other structure that can be lopped off without killing them. And the real superstars are animals such as sea stars, flatworms and sponges: They can regenerate every part of their body, even from a tiny fragment. [Science News, 1/29/10]

Has the speed of light changed?
So would scientists notice a changing speed of light, given that the units for distance and time are defined in terms of that speed? The answer, as you might guess, is yes. There's two classes of constantly ongoing observations that come to mind. We'll call them the practical and the theoretical. [Built on Facts, 2/24/10]

The Maverick Bacterium
Whether it’s powering through the cytoplasm leaving a trail of polymerized actin, activating an arsenal of virulence factors through changes in RNA structure, or storing the code for RNA transcripts on the wrong side of DNA, Listeria makes up its own rules for survival. [The Scientist, 1/1/10]

Game theory shows evolution follows most successful member
Game theory has become a useful way to evaluate strategies for survival in evolution scenarios. In a new study, scientists set up a model where human players engage with each other and compete for resources, and can change their strategies for doing so in various ways. They found that as more rounds of the game were played, the human players developed a tendency to imitate the best player, causing the players as a group to tend to play the game the same way. [Nobel Intent, 1/19/10]

Is your brain making you fat?
Our brains developed ways to maintain our fat stores by detecting the levels of a hormone called leptin, which is secreted into the blood by fat cells. The brain mostly tries to keep this hormone level constant, by making us hungry and burn less energy when our leptin levels drop. It's an effective system for people whose lives depend on having reserves of energy to survive famine, but for those of us battling obesity in a time of plenty it's got some serious downfalls. [ABC Science, 1/21/10]

Better live in Sweden than in the US: Why More Equal Societies Almost Always Do Better
It is common knowledge that in rich societies the poor have shorter lives and suffer more from almost every social problem. In a quite fascinating book, The Spirit Level: Why More Equal Societies Almost Always do Better, epidemiologists Richard Wilkinson and Kate Pickett demonstrate that more unequal societies are bad for almost everyone - the well-off as well as the poor. [International Cognition and Culture Institute, 2/25/10]

Science education standards: A broken system?
What horrifies Cooper most is that aspiring teachers go almost directly from learning science in dysfunctional courses to teaching it in grade school classrooms, creating a cycle of poor teaching and learning. To break this cycle, she said, we need to scrap trivial, information-based assessments and find ways to judge deep conceptual understanding and scientific thinking. [National Association of Science Writers, 2/24/10]

Most modern European males descend from farmers who migrated from the Near East
More than 80% of European Y chromosomes descend from incoming farmers. In contrast, most maternal genetic lineages seem to descend from hunter-gatherers. To us, this suggests a reproductive advantage for farming males over indigenous hunter-gatherer males during the switch from hunting and gathering, to farming - maybe, back then, it was just sexier to be a farmer. [Physorg.com, 1/19/10]


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Snowflake-Shaped Galaxy From Hubble

Snowflake-Shaped Galaxy From Hubble (1/15/10)
A bluish-white spiral galaxy hangs delicately in the cold vacuum of space. Like snowflakes, no two galaxies are exactly alike. Known as NGC 1376, this snowflake-shaped beauty has features that make it a one of a kind. Bright blue knots of glowing gas highlight regions of active star formation. Concentrated along the spiral arms, these areas of star formation show an excess of light at ultraviolet wavelengths for they contain brilliant clusters of hot, newborn stars that are emitting UV light. The less intense, red areas near the core and between the arms consist mainly of older stars. The reddish dust lanes are colder, denser regions where interstellar clouds may collapse to form new stars. Intermingled between the spiral arms are a sprinkling of reddish background galaxies.




NGC 1376 – click for 800×542 image

Where have all the protons gone?

Wednesday, February 24, 2010

Astronomers have long known that there is a rather close relationship between the intrinsic luminosity of a spiral galaxy and the rotational velocity of stars (around the galactic center) in the outer portions of the galaxy. This relationship even has a name: the Tully-Fisher relation.

It has also been known that small, nearby dwarf galaxies, which are irregular in shape, are not nearly as bright as they "should" be, according to the Tully-Fisher relation, given the measured average velocities of their stars.

Recent research shows that, nevertheless, the Tully-Fisher relation can actually be extended, with slight modification, to very large structures: entire clusters of galaxies. In that case, the intrinsic brightness of a cluster is mostly in the X-ray part of the spectrum (because it's due to very hot intergalactic gas), yet the correlation of cluster brightness to the average velocities of galaxies in the cluster is still quite good.

There's actually a very good explanation for the correlation, in that intrinsic brightness and average velocity of constituents are both closely tied to the total mass of the object.

And this is where things get very interesting. One has to consider the mass of ordinary "baryonic" matter separately from the mass of non-luminous dark matter. Many different kinds of independent observations point to the existence of almost 5 times as much mass of the universe in the form of dark matter as there is in the form of ordinary matter. Stated differently, ordinary matter makes up only 17% (a bit more than 1 part in 6) of the total mass of matter in the universe.

As long as the intrinsic luminosity of an object is proportional to its total mass, then mass can be taken as a proxy for luminosity, and a relationship between total mass and average constituent velocity is to be expected. This relationship is in fact predicted even by Newtonian mechanics – total mass should be proportional to the 4th power of velocity (M ∝ V4).

If one could further assume that the ratio of mass in the form of ordinary matter to mass in the form of dark matter in a galaxy or cluster is the same as the ratio in the universe as a whole (1 : 5), then the Tully-Fisher relation makes perfect sense. And this is so even though luminosity is entirely produced by ordinary matter, not the invisible dark matter. Indeed, this holds up very well – for spiral galaxies.

Surprisingly, there is also a fairly good relationship between luminosity and average velocity even in galaxy clusters – but there's a slight difference in the exponent: M ∝ V3. Again, this holds regardless of whether one considers total mass (including dark matter), or just visible ordinary matter. (The mass of a large cluster can be determined independently by techniques such as gravitational lensing.)

It's customary to plot mass vs. velocity (on vertical and horizontal axes, respectively) with logarithmic scales on both axes. When this is done, one gets straight lines that have slopes of approximately 4 (for spiral galaxies) and 3 (for galaxy clusters).

However, when plotting visible mass vs. velocity, the relationship breaks down almost completely for nearby dwarf galaxies. The smallest and dimmest dwarf galaxies are far below the curve. Their visible mass and luminosity – not counting dark matter – is far too small. On a log-log plot, such galaxies fall, with quite a large scatter, around a straight line having a slope of 5 or more.

There's a simple way to restate this observation: dwarf galaxies have far less visible ordinary matter than predicted by a traditional Tully-Fisher relation, and even much less than that if the ratio of ordinary matter to dark matter in the dwarf galaxies were close to what it is in the universe as a whole. In most dwarf galaxies, the ratio is less than 1% of what it "should" be.

In other words, there's an awful lot of ordinary matter missing and unaccounted for in dwarf galaxies. Hence the question (since ordinary matter is mostly hydrogen (protons)): where have all the protons gone?

Observations of nearby dwarf galaxies are pretty reliable – these are our closest neighbors. Assuming Newtonian gravity, we know the masses of these objects very reliably from the velocities of the stars (which we can see individually) within them. There's no hot hydrogen in these galaxies, as there is in distant galaxy clusters, since we see no signal of it in any part of the spectrum down to the infrared. Astronomers are also pretty certain that there's not a lot of cold hydrogen, which should emit strongly at radio frequencies – the famous HI 21-centimeter line.

So where are all the protons? Quite possibly they've been blown outside of the dwarf galaxy entirely, by supernova winds. Escape velocity from a dwarf galaxy is a lot less than what it is for a typical spiral, yet supernovae have just as much bang as they do anywhere else. Very recent detailed simulations have supported this idea, as I discussed here.

An alternative, and rather more radical, possibility is that Newtonian gravity is wrong – the protons still aren't there (why?) but neither is any "dark matter". Instead, the total mass of visible stars – as surprisingly small as it seems to be – is still enough to account for observed stellar velocities, using some form of "modified Newtonian dynamics" (MOND).

Unfortunately, for believers in MOND, the theory was concocted as an alternative to dark matter for explaining rotational velocities in spiral galaxies. MOND theories are typically adjusted carefully to fit the spiral galaxy data. They would need to work differently in dwarf galaxies. And they are already known not to work right for large galaxy clusters either.

Lots of intriguing questions here...

Original abstract:

The Baryon Content of Cosmic Structures
We make an inventory of the baryonic and gravitating mass in structures ranging from the smallest galaxies to rich clusters of galaxies. We find that the fraction of baryons converted to stars reaches a maximum between M500 = 1012 and 1013 M, suggesting that star formation is most efficient in bright galaxies in groups. The fraction of baryons detected in all forms deviates monotonically from the cosmic baryon fraction as a function of mass. On the largest scales of clusters, most of the expected baryons are detected, while in the smallest dwarf galaxies, fewer than 1% are detected. Where these missing baryons reside is unclear.




ResearchBlogging.org
McGaugh, S., Schombert, J., de Blok, W., & Zagursky, M. (2010). THE BARYON CONTENT OF COSMIC STRUCTURES The Astrophysical Journal, 708 (1) DOI: 10.1088/2041-8205/708/1/L14


Further reading:

The Baryon Content of Cosmic Structures – preprint at arXiv

Team Shines Cosmic Light on Missing Ordinary Matter (1/7/10)

Dark Matter and Dark Energy Update (1/9/10)

Inventory Asks: Where Is All the Non-Dark Matter Hiding? (1/15/10)

Dwarf galaxies start making sense

Sunday, February 21, 2010

Cosmology has, for a decade, had its "standard model", which largely explains most of the cosmological phenomena that astronomers are able to observe. Except for a relatively small number of things that don't seem to make sense in the model. Prominent among the latter are dwarf galaxies – by one definition, galaxies having less than 10% of the total mass of the Milky Way.

The standard model of cosmology is known officially as the Λ-cold-dark-matter model – ΛCDM. (This theory has no particular relation to the Standard Model of particle physics.) Cold dark matter (CDM) refers to the hypothesis that a large part of the detectable mass content of the universe consists of particles that are not accounted for by the Standard Model of particle physics. The dark matter is said to be "cold", because it appears to consist mostly of "non-relativistic" particles, meaning particles moving at speeds not close to the speed of light. That excludes, for example, neutrinos.

As weird as the idea of dark matter might seem, there is abundant evidence for it, which can't easily be better explained in other possible ways. (Although, many other possibilities have been proposed.) I haven't written a lot about this recently, since the evidence for CDM just keeps piling up, but here's one important study. Dark matter is "observed" indirectly through its gravitational effects on ordinary visible matter. For instance, the motions of stars in the Milky Way have recently been analyzed closely enough to show that the dark matter in which the Milky Way is embedded has the shape of a squashed beach ball. (See here, here, here.)

Λ is the conventional symbol used for the "cosmological constant", which is a concept from Einstein's general theory of relativity. It is supposed to account for the observed phenomenon of "dark energy". This too is controversial, but there is much evidence for it, from a variety of different studies that are not all based on the same kinds of observations. I last wrote at length on the evidence here.

I need to write a lot more about recent evidence for dark energy, but I'll be very brief about it here. There is very recent evidence involving the motion of galaxies quite near our own (see here). Other than that, the evidence for dark energy is based on observations of distant Type Ia supernovae (about which there's a lot of recent news), "weak lensing" (see here), and "baryon acoustic oscillations" (a large topic).

In spite of all this evidence, ΛCDM isn't without its problems. As already suggested, one set of problems involves dwarf galaxies. There are at least two (somewhat related) parts to this problem. The larger part of the problem is simply that not enough very small dwarf galaxies (masses less than a percent of the Milky Way's) have been detected. This is often known as the "missing satellite problem".

Dwarf galaxies, being very small, are also intrinsically dim, and thus difficult to observe at all unless they're very nearby. However, only about 11 dwarf galaxies are known to be satellites of the Milky Way – and such satellites should be the easiest of dwarf galaxies to detect. This is a serious problem, since simluations of expected galaxies sizes based on the way that dark matter should be expected to clump together predict as many as 500 dwarf satellites of the Milky Way.

The other problem is known as the cuspy halo problem. "Halo" refers to the cloud of cold dark matter in which all visible galaxies are expected to be embedded. Simulations indicate that the dark matter should be concentrated in the center of the halo instead of being evenly distributed throughout. This is intuitively reasonable – after all, most of the ordinary matter in our solar system is concentrated right in the middle, in the Sun itself.

This problem exists somewhat even for large galaxies like the Milky Way, but it is much more severe for dwarf galaxies. In fact, it seems as though the smaller the galaxy is, the greater the tendency for the dark matter (as indicated by orbital motion of stars within the galaxy) to be distributed fairly smoothly, with little or no density cusp in the center.

Related to this is a recent finding (see here) that smaller galaxies seem to have a smaller proportion of ordinary baryonic matter to dark matter than does the universe as a whole. And, in fact, the smaller the galaxy, the smaller the proportion of ordinary matter. In the universe as a whole, there is much evidence, based on detected abundances of light elements and observations of the cosmic microwave background, that there should be about 5 times as much mass in the form of dark matter as there is of ordinary matter. One might expect this proportion to be about the same in galaxies. Yet instead, in the smallest galaxies, astronomers can detect less than 1% as much ordinary matter (in the form of visible stars) as one would expect to find.

This would suggest that an important reason we can't detect very many small galaxies is that they simply have too few stars and are too dim to see. But it still doesn't explain why this should be the case.

In fact, I wrote 2½ years ago about a study that reported finding many small galaxies consisting of 99% or more of dark matter (here). The authors of the study even speculated that the reason such galaxies were mostly composed of dark matter was that "the fierce ultraviolet radiation given off by the first stars, which formed just a few hundred million years after the Big Bang, may have blown all of the hydrogen gas out of the dwarf galaxies forming at that time." And they added, "The loss of gas prevented the galaxies from creating new stars, leaving them very faint, or in many cases completely dark. When this effect is included in theoretical models, the numbers of expected and observed dwarf galaxies agree."

Kind of makes sense, doesn't it? In fact, even for galaxies that began to form later, a large number of supernovae early in the life of a galaxy might be enough to blow away most of the hydrogen from which additional stars could form. And indeed, a recent much more detailed simulation of galaxy formation supports precisely this idea.

Why is it that previous simulations had not caught this? The reason is very simple: detailed simulations of galaxy formation and evolution are exceedingly demanding of computer resources. In order to make such simulations even possible – up until now – astrophysicists considered only the effect of gravitational collapse of a mixture of ordinary and dark matter. The effects resulting from star formation and subsequent supernovae were omitted entirely.

Duh.

Actually, this simplification is pretty understandable. The simulation that is the subject of the research under discussion here, that did take into account stellar formation processes, consumed an almost incredible amount of computing time. According to one report, "The simulation was carried out using about 250 processors running for about two months." That's more than 40 processor-years.

And that's just for one simulation, involving a single set of initial conditions.

Here's the abstract:

Bulgeless dwarf galaxies and dark matter cores from supernova-driven outflows
For almost two decades the properties of ‘dwarf’ galaxies have challenged the cold dark matter (CDM) model of galaxy formation. Most observed dwarf galaxies consist of a rotating stellar disk embedded in a massive dark-matter halo with a near-constant-density core. Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles, because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers. Processes that decrease the central density of CDM halos have been identified, but have not yet reconciled theory with observations of present-day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different dark-matter particle candidate. Here we report hydrodynamical simulations (in a framework assuming the presence of CDM and a cosmological constant) in which the inhomogeneous interstellar medium is resolved. Strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of bulges and decreases the dark-matter density to less than half of what it would otherwise be within the central kiloparsec. The analogues of dwarf galaxies—bulgeless and with shallow central dark-matter profiles—arise naturally in these simulations.

Basically what the simulation has to do is to incorporate a level of granularity that reflects the size of a typical star-forming region: "Baryonic processes are included, as gas cooling, heating from the cosmic ultraviolet field, star formation and supernova-driven gas heating. The resolution is such that dense gas clumps as small as 105 M are resolved, similar to real star-forming regions."

It certainly wasn't possible to do a simulation where the granularity was on the order of the size of a single star – that could take 105 times as long. Yet the results are very reasonable. The simulation produced a galaxy that closely resembles dwarf galaxies actually observed. In particular, the simulated galaxy has no "cusp" of dark matter density at the center, and no central bulge of visible stars in the center either.

And so the simulation adequately accounts for properties of real dwarf galaxies, which no previous simulation has done. The intense outflowing "winds" from supernovae that result from the heaviest initially-formed stars sweep all ordinary baryonic matter out of the central region. These winds are simply high-energy photons, which interact only with ordinary matter, not dark matter. However, the ordinary matter does interact gravitationally with the dark matter, which also then gets pulled away from the center.

The simulation does not directly settle the question of why so few very small dwarf galaxies are observed. Presumably, many small dwarfs actually do form. They just have so little ordinary matter that is able to coalesce into stars that the galaxies are too dim to detect at any great distance. This is in accord with other studies that show that the smallest galaxies have only a very small proportion of visible ordinary matter.



ResearchBlogging.org
Governato, F., Brook, C., Mayer, L., Brooks, A., Rhee, G., Wadsley, J., Jonsson, P., Willman, B., Stinson, G., Quinn, T., & Madau, P. (2010). Bulgeless dwarf galaxies and dark matter cores from supernova-driven outflows Nature, 463 (7278), 203-206 DOI: 10.1038/nature08640


Further reading:

Supernova winds blow galaxies into shape (1/13/10)

Supernovae put dark matter in the right place (1/13/10)

New research resolves conflict in theory of how galaxies form (1/13/10)

Astrophysicists unwind 'Cold Dark Matter Catastrophe' conundrum (1/14/10)

Puzzling Dwarf Galaxies Finally Make Sense (1/13/10)

Galaxy formation: Gone with the wind? (1/13/10)

Selected readings 2/19/10

Friday, February 19, 2010

Interesting reading and news items.

These items are also bookmarked at my Diigo account.


Searching ALL the Tevatron data for exotic physics
The majority of published particle physics papers involve looking for a very specific elementary particle process. They conclude by either showing that it exists or constraining limits on how often that process could possibly occur. But is there a way to take advantage of a collider’s whole data set and seeing if it is consistent with an expansive theoretical model, such as the Standard Model of particle physics? [Symmetry breaking, 2/13/10]

The Cosmological Constant and the Dark Sector
The dark sector refers to dark energy and dark matter, which are two distinct phenomena which seem to have no direct connection other than in name. In this post I am going to talk about the cosmological constant, dark energy, and look at some landmark literature on the subject. [The Astronomist, 2/7/10]

Physicists Solve Difficult Classical Problem with One Quantum Bit
Most research on quantum information systems has concentrated on models that use multiple quantum bits. In a new study, physicists have demonstrated how to solve a difficult classical problem that completely encapsulates a quantum model that requires only one quantum bit. The scientists, Gina Passante, et al., from the University of Waterloo in Ontario, Canada, have presented their experimental results for the quantum solution of the approximation of the Jones polynomial, which is a knot invariant. [Physorg.com, 1/8/10]

Quantum computer calculates exact energy of molecular hydrogen
In an important first for a promising new technology, scientists have used a quantum computer to calculate the precise energy of molecular hydrogen. This groundbreaking approach to molecular simulations could have profound implications not just for quantum chemistry, but also for a range of fields from cryptography to materials science. [Physorg.com, 1/10/10]

Warp-Speed Algebra: New Algorithm Does Algebra in a Snap
Quantum computers can do wondrous things: too bad they do not exist yet. That has not stopped physicists from devising new algorithms for the devices, which can calculate a lot faster than ordinary computers—in fact, exponentially faster, in quite a literal sense. Once quantum computers do become available, the algorithms could become a key part of applications that require number crunching, from engineering to video games. [Scientific American, 1/1/10]

Fermi telescope closes in on mystery of cosmic ray acceleration
The Large Area Telescope collaboration, led by KIPAC researchers Takaaki Tanaka, Uchiyama, and Hiroyasu Tajima, released the first image of a supernova remnant in the giga-electronvolt energy range (about 200 million times the energy of visible light). By revealing the spatial distribution of cosmic rays in the remnant, this result is a significant step toward definitively determining how cosmic rays are accelerated in supernova remnants. [Symmetry breaking, 1/7/10]

Do particle theorists have a blind spot?
Theorist Matthew Strassler from Rutgers University challenged particle theorists to not be too simple in their analyses. Most people would probably not claim that theoretical particle physics is too simple, but Strassler argued that nature is likely to be even more complicated than hpysicists expect. And if theorists only properly examine the simplest classes of models, where simple is a relative term, they might be led astray in interpreting future Large Hadron Collider data. [Symmetry breaking, 2/14/10]

How to Change A Skin Cell Into A Nerve Cell or Cellular Anarchy & The Great Leap Sideways
A team at Stanford's Institute for Stem Cell Biology & Regenerative Medicine, led by Marcus Wernig and graduate student Thomas Vierbuchen, recently announced that a combination of only three transcription factors that would change a skin cell into a nerve cell -- with no intermediate (undifferentiated) steps. [h+ Magazine, 2/7/10]

Imaging the Brain Better, Faster, Thinner
fMRI's a tool, an amazing one in a lot of ways, but like any tool it needs to be used well. Along with others, I've criticized various aspects of recent fMRI practice, but only because it's frustrating to see such a powerful tool not being used to its full potential. So I was very pleased by a recent paper by Sabatinelli et al, The Timing of Emotional Discrimination in Human Amygdala and Ventral Visual Cortex. The authors set out to test a hypothesis - that seeing an emotionally charged picture would activate the amygdala and the inferotemporal cortex (IT) before activating the extrastriate occipital cortex. [Neuroskeptic, 2/3/10]

Uncovering the Genetic Controls of Cellular Aging
A fascinating thing about DNA replication is that the actual process lacks the ability to replicate the very ends of chromosomes. That means chromosomes should get shorter with every round of cell division (DNA replication), but they remain more or less the same length, getting gradually shorter with aging. The natural shortening of chromosomes is refered to as cellular aging. So how do chromosomes maintain their ends if not by replication? [DNA Dude, 2/9/10]

God's will and beliefs are your own, not god's
According to these results believers project their own values and beliefs on their god (or gods) to a great extent, which could certainly help explain not only the great diversity and variability of religious belief and expression, but also the ambiguous nature of religious interpretation. [Ego sum Daniel, 1/27/10]

Hunting Fossil Viruses in Human DNA
The borna virus is at once obscure and grotesque. It can infect mammals and birds, but scientists know little about its effects on its victims. ... The virus now turns out to have an intimate bond with every person on Earth. In the latest issue of Nature, a team of Japanese and American scientists report that the human genome contains borna virus genes. The virus infected our monkey-like ancestors 40 million years ago, and its genes have been passed down ever since. [New York Times, 1/11/10]

The Origin of the Future: Death by Mutation?
The evolutionary biologist Michael Lynch has published a provocative paper (to mark his inauguration into the National Academy of Sciences) in which he makes another kind of forecast. Our future evolution, he warns, is going to lead to a devastating decline in our health. [The Loom, 1/7/10]

Spiral Galaxies Exist — But Why?
It's a minor miracle (and a topic of considerable debate) how all the spirals we see today managed to endure all that mayhem unscathed. "The formation of spirals is a problem," admits Christopher Conselice, a galaxy specialist at the University of Nottingham. "We don't know how they formed, or how they survive all those mergers." [Sky & Telescope, 2/16/10]

WMAP Refines "Precision Cosmology"
Without much public notice, the team running the Wilkinson Microwave Anisotropy Probe (WMAP) recently released results from the satellite's "seven-year data set," updating the five-year data released in 2008. ... The two more years of data have further beaten down the statistical uncertainties in the cosmic background map, allowing analysts to refine what it tells us about the cosmos as a whole. If the new, revised results didn't make much news, it's because they show modern cosmology to be steady on course. The better data only firm up confidence in what we already thought we knew. [Sky & Telescope, 2/14/10]

Cell phone radiation may fight Alzheimer's... in mice
The study examined the performance of mice that received a daily dose of radiation similar to that produced by cell phones, and found that, over a period of several months, their memory improved. When the same procedure was performed with mice engineered to be predisposed to Alzheimer's pathology, it was actually able to reverse some of the cognitive decline. [Nobel Intent, 1/6/10]

Zinc Fingers Could Be Key to Reviving Gene Therapy
The technique, which depends on natural agents called zinc fingers, may revive the lagging fortunes of gene therapy because it overcomes the inability to insert new genes at a chosen site. Other researchers plan to use the zinc finger technique to provide genetic treatments for diseases like bubble-boy disease, hemophilia and sickle-cell anemia. [New York Times, 12/28/09]

Climate change: No hiding place?
The fact that no record high happened in the 2000s does not mean that there was no warming over the decade—trends at scales coarser than the annual continued to point upwards, and other authorities suggest there have been record years during the period. Nor was the length of time without an annual record exceptional. Models simulating centuries of warming normally have the occasional decade in which no rise in surface temperatures is observed. [The Economist, 1/7/10]

Induced Pluripotent Stem Cells Fall Short of Potential Found in Embryonic Version
The act of reprogramming cells to make them as capable as ones from embryos apparently can result in aberrant cells that age and die abnormally, suggesting there is a long way to go to prove such cells are really like embryonic stem cells and can find use in therapies. [Scientific American, 2/11/10]

Pop Goes the Pulsar
One of the challenges of astrophysics is interpreting what we observe. Here on Earth we can set up experiments to test phenomena, but when it comes to the cosmos all we can do is sit back and watch. We've sent probes to the furthest regions of our solar system, but even that is just a tiny corner of the heavens. So how can we possibly know that there are galaxies light years away, or that the universe is billions of years old? The answer is that we take what we know about physics here and apply it to what we observe there. [Upon Reflection, 2/17/10]

Should Evolutionary Theory Evolve?
Some evolutionary biologists say that the body of knowledge concerning evolutionary processes has simply outgrown the confines of the Modern Synthesis, which was crafted before science had a strong grasp of genomics, molecular biology, developmental biology, and other, more recently derived disciplines, such as systems biology. [The Scientist, 1/1/10]

Violating Parity with Quarks and Gluons
Quarks and gluons interact in interesting ways, and in the many fluctuations that happen in these high-temperature collisions we can get “bubbles” that pick out a direction in space. In the presence of these bubbles, quarks treat left and right differently, even though they treat both directions exactly the same when they’re in empty space. [Cosmic Variance, 2/16/10]


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Far out!

Monday, February 15, 2010

If you're interested in something out of the ordinary, astronomically speaking, the best place to look for the exotic may be as far away (in both space and time) as possible.

Perhaps that's why I like to consider really far out stuff, like the most distant gamma-ray burst seen yet. Or maybe I just like to get away from the depressing chaos and confusion of "modern" life.

In any case, there's always something new, just beyond the farthest thing we've seen yet. That far-out gamma-ray burst (GRB 090423) discussed in the post just linked – which resides at z~8.2, about 13 billion light-years away – has already been superseded in remoteness by 3 galaxies around z~10, 13.2 billion light-years away. z~10 objects are seen as they were only about 480 million years after the big bang. (See here for a refresher on how redshift works.)

How are high-z objects actually detected? It's surprisingly easy, in principle, even though astronomers are working at the outer limits of their instruments. The term of art for the technique used is "Lyman-break" detection, which we've discussed in detail here.

Here's the short summary of that technique. We assume that the objects of interest are galaxies composed of stars, instead of something really strange. (GRBs can be ruled out, since they appearance is very brief, and gamma-rays are far outside the range of emissions produced by stars, even given substantial redshifts.) The thing is that even the hottest stars produce relatively little output (such as x-rays) on the high-energy side of the part of the ultraviolet spectrum known as the "Lyman limit", which has a wavelength of 91.1 nm. And almost all higher energy photons will be absorbed by the interstellar medium anyhow.

At z=10, the shift factor (z+1) is 11, yielding a wavelength of 1002 nm – i. e. about 1 micron, which is in the infrared. So an object that's really at z~10 will not have any observable light to the blue side (shorter wavelength) of 1002 nm.

The main instrument used in the recently refurbished Hubble telescope to detect the high-redshift galaxies is known as the Wide Field Camera 3 (WFC3). It was installed in May 2009, and in August it did the infrared imaging on which the research described here is based. As noted, the research team identified 3 objects at z~10, using WFC3. Two of those objects were also captured in an image of the same area, using Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

WFC3 has three filters covering near-infrared wavelengths. The light from a galaxy at z~10 will be visible through the filter that passes only longer infrared photons, but not through the other two filters that pass only shorter (bluer) photons. So z~10 galaxies will stand out on account of their absence from images made using the shorter-wave filters.

The neat thing about this technique is that it does not require collecting a somewhat complete spectrum, which is a much more difficult feat. In principle, all sufficiently luminous z~10 galaxies should be caught. The problem is false positives – things that aren't really z~10 galaxies, such as dim reddish galaxies that are actually much closer (z<3), or even dim nearby stars.

The research that reports on this has used careful statistical tests to rule out false positives. The analysis in fact suggests that a slightly earlier study that claimed to find 20 z~10 galaxies could be all false positives.

This is the study we're discussing now:

Constraints on the First Galaxies: z~10 Galaxy Candidates from HST WFC3/IR
The first galaxies likely formed a few hundred million years after the Big Bang. Until recently, it has not been possible to detect galaxies earlier than ~750 million years after the Big Bang. The new HST WFC3/IR camera changed this when the deepest-ever, near-IR image of the universe was obtained with the HUDF09 program. Here we use this image to identify three redshift z~10 galaxy candidates in the heart of the reionization epoch when the universe was just 500 million years old. These would be the highest redshift galaxies yet detected, higher than the recent detection of a GRB at z~8.2. The HUDF09 data previously revealed galaxies at z~7 and z~8. Galaxy stellar population models predict substantial star formation at z>9-10. Verification by direct observation of the existence of galaxies at z~10 is the next step. ... Our z~10 sample suggests that the luminosity function and star formation rate density evolution found at lower redshifts continues to z~10, and pushes back the timescale for early galaxy buildup to z>10, increasing the likely role of galaxies in providing the UV flux needed to reionize the universe.

There was a more serious purpose behind this effort than simply bagging a few more objects at record-breaking distances. The ultimate goal is to understand the sequence of events when the earliest stars and galaxies formed in the early universe, and what characteristics those first stars and galaxies had.

As we discussed here and here, at the time of "recombination", about 380,000 years after the big bang, most of the hydrogen and helium gas in the universe was in the form of neutral (un-ionized) atoms. Much later (relatively speaking), in what is known as the "reionization" period, this gas became ionized again by the intense light from young, very hot stars and galaxies. This probably happened over a period of several hundred million years and was essentially complete by 900 million years after the big bang.

What is not so clear is when the reionization (and hence the first stars) began, or how rapidly it proceeded. The best way to understand this process is to determine the numbers of galaxies per unit volume of space throughout this period, and the intrinsic brightness of these galaxies.

Astronomers have gradually been able to make reliable counts of the number of bright galaxies for z<7, corresponding to ~780 million years after the big bang and later. Of course, many galaxies for even much smaller z are too dim to be visible, but it is possible to count the number of galaxies, having at least a minimum intrinsic brightness, per unit volume of space. There is a relatively simple function that fits the data and describes the number of galaxies we can see in a given patch of sky at different redshift values.

We can see more galaxies at smaller values of z not only because the necessary intrinsic brightness decreases with z, but also because there are actually increasing numbers of galaxies having any given intrinsic brightness as the universe grows older – up to a point. This is simply because galaxies themselves became larger and brighter over time as new stars formed.

Another way to view the accumulated data is in terms of the rate of star formation. The more rapidly stars are forming, the more rapidly galaxies grow to reach any particular intrinsic brightness. When read in this way, the data show that the number of stars being formed per year increased monotonically from as far back as we can tell up to a peak with z between 2 and 3, and that the rate of star formation drops after z~2, 3.3 billion years after the big bang.

However, the actual data, so far, become very sparse for z>6. If stars were forming at z~10 at the same rate as they do at z~6, astronomers should find 20±5 galaxies at z~10. Or, using z~7 as a baseline, there should be 9±3 galaxies at z~10. Instead, there were only 3. (Keep in mind this is all for a patch of sky of the same size.)

What this means is that for z>7, stars were forming at even slower rates. And in fact, if one extrapolates the function for the number of observable galaxies back to z~10, 2 or 3 fits very nicely. Further, if that is the case, then the energy emitted by galaxies at z~10 is only about 13% of what would be needed to completely ionize the interstellar gas. Which means, finally, that the largest part of reionization occurred more than 500 million years after the big bang.

Astronomers would, however, like to know much more than that. Ideally one would like more precisely the rates of star formation during the whole time period beginning with the first stars, perhaps 200 or 300 million years after the big bang. As well as other things, such as the distribution of shapes and sizes of galaxies in that period.

Such information isn't important just for its own sake, either. It will also help astronomers answer other questions, such as: What were the characteristics of the very first stars and galaxies, and how did they form? How was dark matter distributed at that time, and how did that affect the formation of stars and galaxies? What role did black holes play in the formation and evolution of galaxies?

As impressive as the recent upgrades to Hubble's instruments have been, they are still at the limits of their capability. So astronomers are eagerly awaiting the James Webb Space Telescope – a much bigger piece of equipment (with a 6.5 m diameter mirror) than Hubble – now scheduled for launch in 2014.



ResearchBlogging.org
R. J. Bouwens, G. D. Illingworth, I. Labbe, P. A. Oesch, M. Carollo, M. Trenti, P. G. van Dokkum, M. Franx, M. Stiavelli, V. Gonzalez, & D. Magee (2009). Constraints on the First Galaxies: z~10 Galaxy Candidates from HST
WFC3/IR Nature arXiv: 0912.4263v2



Update: The published paper may be found here: A candidate redshift z ≈ 10 galaxy and rapid changes in that population at an age of 500 MyrarXiv.org

Further reading:

New-found Galaxies May Be Farthest Back In Time And Space Yet (1/3/09)

Hubble Spots Oldest Galaxies Yet (1/5/10)

Earliest Known Galaxies Spied in Deep Hubble Picture (1/5/10)

Hubble Reaches the 'Undiscovered Country' of Primeval Galaxies (1/5/10)

Hubble Ultra Deep Field 2009 detects earliest galaxies (1/6/10)

Oldest Galaxies Show Stars Came Together in a Hurry (1/15/10)

Selected readings 2/13/10

Saturday, February 13, 2010

Interesting reading and news items.

These items are also bookmarked at my Diigo account.


Easy = True
One of the hottest topics in psychology today is something called “cognitive fluency.” Cognitive fluency is simply a measure of how easy it is to think about something, and it turns out that people prefer things that are easy to think about to those that are hard. On the face of it, it’s a rather intuitive idea. But psychologists are only beginning to uncover the surprising extent to which fluency guides our thinking, and in situations where we have no idea it is at work. [Boston.com, 1/31/10]

Mammogram Math
The panel of scientists advised that routine screening for asymptomatic women in their 40s was not warranted and that mammograms for women 50 or over should be given biennially rather than annually. The response was furious. Fortunately, both the panel’s concerns and the public’s reaction to its recommendations may be better understood by delving into the murky area between mathematics and psychology. [New York Times, 12/10/09]

A Deluge of Data Shapes a New Era in Computing
In a speech given just a few weeks before he was lost at sea off the California coast in January 2007, Jim Gray, a database software pioneer and a Microsoft researcher, sketched out an argument that computing was fundamentally transforming the practice of science. [New York Times, 12/14/09]

Earth-Like Planets May Be Made of Carbon
Other Earth-mass planets may be enormous water droplets, balls of nitrogen or lumps of iron. Name your favorite element or compound, and someone has imagined a planet made of it. The spectrum of possibilities depends largely on the ratio of carbon to oxygen. After hydrogen and helium, these are the most common elements in the universe, and in an embryonic planetary system they pair off to create carbon monoxide. The element that is in slight excess ends up dominating the planet’s chemistry. [Scientific American, 1/1/10]

People Share News Online That Inspires Awe, Researchers Find
Perhaps most of all, readers wanted to share articles that inspired awe, an emotion that the researchers investigated after noticing how many science articles made the list. In general, they found, 20 percent of articles that appeared on the Times home page made the list, but the rate rose to 30 percent for science articles. [New York Times, 2/8/10]

The Depressing News About Antidepressants
Yes, the drugs are effective, in that they lift depression in most patients. But that benefit is hardly more than what patients get when they, unknowingly and as part of a study, take a dummy pill—a placebo. As more and more scientists who study depression and the drugs that treat it are concluding, that suggests that antidepressants are basically expensive Tic Tacs. [Newsweek, 1/29/10]

Mystery Swirls Around 'Dark Stars'
When the very first stars lit up, they may have been fueled by the dark matter that has long eluded scientists. These "dark stars," first born nearly 13 billion years ago, might still exist today. Although they would not shed any visible light, astronomers might detect these invisible giants - some 400 to 200,000 times wider than our sun and 500 to 1,000 times more massive. [Space.com, 12/21/09]

Researchers store working memory in brain slices
Even though we have a decent idea of how individual neurons work, scientists are still struggling to understand how these cells manage to store and convey information. A study published over the weekend by Nature Neuroscience describes how researchers were able to track the maintenance of short-term, working memories in the neurons of a rat brain and, in the process, managed to read and write individual bits into a brain slice. [Nobel Intent, 12/28/09]

Hormones in Concert
Multiple hormones act in concert to regulate blood sugar and food intake. The idea has already led to a new diabetes therapy; will it also yield new strategies for obesity? [The Scientist, 12/1/09]

New-found galaxies may be farthest back in time and space yet
By pushing the refurbished Hubble Space Telescope to its very limits as a cosmic time machine, astronomers have identified three galaxies that may hail from an era only a few hundred million years after the Big Bang. The faint galaxies may be the most distant starlit bodies known, each lying some 13.2 billion light-years from Earth. [Science News, 1/3/10]

Benford's Mathemagical Law
If you collected data on city sizes for the country you live in, and looked at all the first digits, you would find that many more cities have a population count starting with one, than cities with populations that begin with any other number as their first digit. You could also look at consumer price index development for the European Union, or a (long enough) time series for the Dow Jones Index, and you would find the same thing: More numbers start with 1, than start with 2, than start with 3, and so on. [Ingenious Monkey, 12/2/09]

Memories Glow Under the Microscope
How does memory work? What changes in the brain when we learn something? We don't know for sure. But two outstanding Nature papers have just provided an important piece of the puzzle, using a truly amazing technique which allowed them to examine the brain of a living, breathing mouse under the microscope. [Neuroskeptic, 12/8/09]

Black Holes, Brownian Motion
Black holes also undergo Brownian motion, and astronomers can use that fact to their advantage. Within most (if not all) galaxies is a supermassive black hole. These typically have a mass a hundred thousand to a billion times larger than our sun. They reside in the center of the galaxy, surrounded by a dense cluster of stars. Just as a dust-mote is knocked about by the tiny atoms surrounding it, the black hole is knocked about by the (relatively) tiny stars surrounding it. Obviously we can't observe this motion in real time, but its effect is clearly measurable. Black holes undergoing Brownian motion on a cosmic scale. [Upon Reflection, 12/7/09]

Stress Now, Mental Illness Later
The effects of stress can be felt acutely (i.e., in the short-term) or many years later (e.g., the average time span between onset of sexual abuse and the development of clinical depression is 11.5 years. This poses an interesting question: can the age at which one experience "stress" predict both the onset and type of mental illness? That's what Lupien et al. wanted to answer in an interesting paper that was published in Nature Reviews Neuroscience earlier this year. [The MacGuffin, 12/15/09]

Researchers Identify Genetic Variant Linked to Faster Biological Aging
Chronological age is very different from biological age—the condition of chromosomes after each cellular division—according to Nilesh Samani of the University of Leicester, co-author of a February 7 report published in Nature Genetics. Biological age, Samani says, is related to the length of telomeres—stretches of DNA at the ends of chromosomes, which protect these precious packages of genes from daily wear and tear. We're born with telomeres of a certain length, and these get shorter as our cells divide, resulting in aging, scientists think. [Scientific American, 2/8/10]

Cancer: 'Primitive' gene discovered
To find the causes for cancer, biochemists and developmental biologists at the University of Innsbruck, Austria, retraced the function of an important human cancer gene 600 million years back in time. For the first time, they have identified the oncogene myc in a fresh water polyp and they have shown that this oncogene has similar biochemical functions in ancestral metazoan and in humans. [Eurekalert.org, 2/11/10]

Tumors as ecosystems
What’s a tumor? In some ways, that’s a bad question (never mind the answer) because it implies that a tumor is a single thing. But we know that’s not true. A tumor, by the time we can detect it, is a collection of many cells, at least billions of them, and those cells are not all the same. [Mystery Rays from Outer Space, 2/3/10]


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Galaxy Exposes its Dusty Inner Workings

Friday, February 12, 2010

Galaxy Exposes its Dusty Inner Workings
NASA's Spitzer Space Telescope has captured an action-packed picture of the nearby Small Magellanic Cloud, a small galaxy that looks like a wispy cloud when seen from Earth.

From Spitzer's perch up in space, the galaxy's clouds of dust and stars come into clear view. The telescope's infrared vision reveals choppy piles of recycled stardust -- dust that is being soaked up by new star systems and blown out by old ones.

To some people, the new view might resemble a sea creature, or even a Rorschach inkblot test. But to astronomers, it offers a unique opportunity to study the whole life cycle of stars close-up. ...

Recent research has shown that the galaxies may not, as previously suspected, orbit around the Milky Way. Instead, they are thought to be merely sailing by, destined to go their own way. Astronomers say the two galaxies, which are both less evolved than a galaxy like ours, were triggered to create bursts of new stars by gravitational interactions with the Milky Way and with each other. In fact, the Large Magellanic Cloud may eventually consume its smaller companion.




Small Magellanic Cloud – click for 1500×1350 image



More: here, here

Selected readings 2/6/10

Saturday, February 6, 2010

Interesting reading and news items.

These items are also bookmarked at my Diigo account.


Five things you should know about climate change
It can be really difficult for anyone not well-versed in the debate to get any sense of the science at all, something that's clear from the huge gap between the scientific community's acceptance of climate change and the public's wariness about the topic. So it's probably useful to step back from the latest findings, and look at science's basic understanding of how greenhouse gasses can force climate change, which often gets lost in the arguments. [Nobel Intent, 11/29/09]

Genes vs. environment and the role of genomic "dark matter"
The argument over the relative weight of nature and nurture-genes vs. the environment-has a history that predates anything that even resembles formal biology. With the advent of molecular biology and the completion of the human genome, we've now got a much better idea of what, precisely, genes contribute to human differences. [Nobel Intent, 12/8/09]

Skin Cells Turned into Brain Cells
Skin cells called fibroblasts can be transformed into neurons quickly and efficiently with just a few genetic tweaks, according to new research. The surprisingly simple conversion, which doesn't require the cells to be returned to an embryonic state, suggests that differentiated adult cells are much more flexible than previously thought. [Technology Review, 1/28/10]

Why Your DNA Isn't Your Destiny
At its most basic, epigenetics is the study of changes in gene activity that do not involve alterations to the genetic code but still get passed down to at least one successive generation. These patterns of gene expression are governed by the cellular material — the epigenome — that sits on top of the genome, just outside it. [Time, 1/6/10]

Fossils on the Edge of Forever
Astrobiologists have not yet found alien life on other planets. But the fossil record has evidence of aliens of another sort: the Ediacarans that lived on Earth millions of years ago. [Physorg.com, 12/14/09]

Acid oceans: the 'evil twin' of climate change
The sea lions, harbor seals and sea otters reposing along the shoreline and kelp forests of this protected marine area stand to gain from any global deal to cut greenhouse gases. These foragers of the sanctuary's frigid waters, flipping in and out of sight of California's coastal kayakers, may not seem like obvious beneficiaries of a climate treaty crafted in the Danish capital. But reducing carbon emissions worldwide also would help mend a lesser-known environmental problem: ocean acidification. [Physorg.com, 12/18/09]

Copenhagen Failed, Mexico is Already Doomed - What's Next?
Short of praying for volcanic activity to mitigate the harm of climate change, the best options are these. First, tell the truth, even when it sucks. Second, at least start paving the way for an ethic of sacrifice, so that people who are eventually forced by either events or the sudden arrival of new political realities - or most likely, both - actually have had a little time to prepare and are not wholly betrayed by the realization that this will cost us. [Casaubon's Book, 2/2/10]

Attractiveness, anger, and warrior princess blondes
So why does attractiveness (or perceived attractiveness) have a strong effect on entitlement and anger? The authors hypothesize that it has to do with social networks. Men who are strong and women who are...hot...attract people to them, men because they can protect people and because you don't want to get in their way, and women because you want a piece of that. [Neurotopia, 1/20/10]

Dark Materials
This evidence was further strengthened recently when astronomers observed the motion of stars in the Sagittarius dwarf galaxy. This galaxy collided with our Milky Way galaxy long ago, and its stars are now spread around ours in a diffuse stream. Astronomers measured the speed of these stars and again detected the effect of dark matter. Since these stars are spread all around our galaxy, astronomers could measure the distribution of the dark matter in our galaxy. The found that our galactic dark matter forms an asymmetrical squashed sphere. This clear lack of symmetry means it cannot be accounted for by modifying our gravitational theory. Dark matter is real, and it makes up the majority of mass in our galaxy. [Upon Reflection, 1/19/10]

A new, bigger kind of boom
A very massive star is supported against collapse by high energy photons (particles of light) which provide a pressure to push again gravity. However if the star is hot enough this mechanism can go horribly wrong. In a strange quirk of particle physics when photons are produced with a high enough energy, they can turn into an electron and its antiparticle, a positron. This process leads (with a few complex steps) to the star’s core becoming unstable and collapsing. This happens long before the star would have time to form an iron core. Such an event is called a Pair Instability Supernova (PISN) and only occurs with a star more massive than about 140 solar masses. [We are all in the gutter, 1/12/10]

Is the Earth even more sensitive to CO2 levels than we thought?
One of the more common arguments from skeptics of anthropogenic climate change is that the Earth has experienced periods during which atmospheric carbon dioxide levels were much much higher than they are today -- as much as 10 times higher. Why worry about a mere 30% increase over pre-industrial levels? [The Island of Doubt, 1/7/10]

Insulin resistance as a protective mechanism, a paradigm shift?
Oxidative stress has been implicated implicated in insulin resistance, and a new study by Hoehn et al. (1) adds some convincing evidence that one specific radical, superoxide generated in the mitochondria, may be a unifying cause. But the findings suggest that we may need to reconsider how we treat it. [Nutritional Blogma, 12/23/09]


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