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Selected readings 3/23/10

Tuesday, March 23, 2010

Interesting reading and news items.

These items are also bookmarked at my Diigo account.

Google’s Computing Power Refines Translation Tool
Google’s efforts to expand beyond searching the Web have met with mixed success. Its digital books project has been hung up in court, and the introduction of its social network, Buzz, raised privacy fears. The pattern suggests that it can sometimes misstep when it tries to challenge business traditions and cultural conventions. But Google’s quick rise to the top echelons of the translation business is a reminder of what can happen when Google unleashes its brute-force computing power on complex problems. [New York Times, 3/8/10]

The Psychology of the Taboo Trade-Off
The critical quality that leads people to treat rookie cards like rosaries is that of the sacred, whereby an object becomes worthy of boundless reverence, commitment, and protection. As diverse as people are in ascribing sacred status to possessions, they are equally varied in which values they consider sacred, a diversity that can breed substantial conflict. [Scientific American, 3/9/10]

Porn: Good for us?
Scientific examination of the subject has found that as the use of porn increases, the rate of sex crimes goes down. [The Scientist, 3/1/10]

High-energy physics has a case of the Higgs
One of the things that was abundantly clear in the high energy physics sessions at Physics@FOM is that everyone is very excited. The LHC is ready to roll later this winter, the Tevatron is putting out data like... well, a machine, and there is just so much stuff waiting around the corner. [Nobel Intent, 2/1/10]

MicroRNA: A glimpse into the past
The last ancestor we shared with worms, which roamed the seas around 600 million years ago, may already have had a sophisticated brain that released hormones into the blood and was connected to various sensory organs. The evidence comes not from a newly found fossil but from the study of microRNAs - small RNA molecules that regulate gene expression - in animals alive today. [Physorg.com, 2/1/10]

Free Energy and the Meaning of Life
At this very moment, tens of thousands of home computers around the world are quietly working together to solve the largest and most basic mysteries of our galaxy. Enthusiastic and inquisitive volunteers from Africa to Australia are donating the computing power of everything from decade-old desktops to sleek new netbooks to help computer scientists and astronomers at Rensselaer Polytechnic Institute map the shape of our Milky Way galaxy. [Cosmic Variance, 3/10/10]

Could life exist on Jupiter moon?
Icy moons may be the most common habitats for life in the Universe, so studying Europa will help tell us not just whether life exists elsewhere in our Solar System, but how common life may be throughout the Universe. [BBC News, 2/4/10]

Searching for Network Laws in Slime
Of all science’s model organisms, none is as weird as Dictyostelium discoideum, a single-celled amoeba better known as slime mold. When they run out of food, millions coalesce into a single, slug-like creature that wanders in search of nutrients, then forms a mushroom-like stalk, scatters as spores and starts the cycle again. In the rules governing the behavior of these creatures, researchers hope to find analogues for baffling biological mysteries, from the specialization of cells to how animals become altruistic. [Wired, 2/12/10]

Quantum gravity and space's informational entropy
Gravity is still a force, but it is generated by something more fundamental: entropy. Entropy can be described in the language of quantum mechanics and conformal field theory is one model for this sort of description. In these models, gravity kind of falls out of the equations for free—where "free" is an enormous amount of work done by someone else. [Nobel Intent, 2/14/10]

Rethinking networking
About 10 years ago, electrical engineers suggested that bundles of data could be transmitted over a network more efficiently if, instead of passing unaltered from one end to the other, they were scrambled together along the way and unscrambled at the end. In 2003, MIT electrical engineering professor Muriel Médard and her colleagues proved the counterintuitive result that, in many cases, the best way to scramble data together was to do it randomly. [Physorg.com, 2/12/10]

Dark Energy: The Biggest Mystery in the Universe
Astronomers have compiled evidence that what we’ve always thought of as the actual universe—me, you, this magazine, planets, stars, galaxies, all the matter in space—represents a mere 4 percent of what’s actually out there. The rest they call, for want of a better word, dark: 23 percent is something they call dark matter, and 73 percent is something even more mysterious, which they call dark energy. [Smithsonian Magazine, 3/22/10]

The Big Bang: Solid Theory, But Mysteries Remain
A popular picture of the early universe imagines a single Big Bang, after which space blew up quickly like a giant bubble. But another theory posits that we live in a universe of 11 dimensions, where all particles are actually made of tiny vibrating strings. This could create a universe stuck in a cycle of Big Bangs and Big Crunches, due to repeat on loop. Which scenario is closer to the truth remains to be seen, but scientists say new experiments underway could provide more answers soon. [Space.com, 3/19/10]

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Orion in a New Light

Tuesday, March 16, 2010

Orion in a New Light (2/10/10)
The Orion Nebula is a vast stellar nursery lying about 1350 light-years from Earth. Although the nebula is spectacular when seen through an ordinary telescope, what can be seen using visible light is only a small part of a cloud of gas in which stars are forming. Most of the action is deeply embedded in dust clouds and to see what is really happening astronomers need to use telescopes with detectors sensitive to the longer wavelength radiation that can penetrate the dust. VISTA has imaged the Orion Nebula at wavelengths about twice as long as can be detected by the human eye.

As in the many visible light pictures of this object, the new wide field VISTA image shows the familiar bat-like form of the nebula in the centre of the picture as well as the fascinating surrounding area. At the very heart of this region lie the four bright stars forming the Trapezium, a group of very hot young stars pumping out fierce ultraviolet radiation that is clearing the surrounding region and making the gas glow. However, observing in the infrared allows VISTA to reveal many other young stars in this central region that cannot be seen in visible light.

Orion Nebula – click for 1280×1574 image

More: here, here, here

The Stars behind the Curtain

Friday, March 12, 2010

The Stars behind the Curtain (2/3/10)
ESO is releasing a magnificent VLT image of the giant stellar nursery surrounding NGC 3603, in which stars are continuously being born. Embedded in this scenic nebula is one of the most luminous and most compact clusters of young, massive stars in our Milky Way, which therefore serves as an excellent “local” analogue of very active star-forming regions in other galaxies. The cluster also hosts the most massive star to be “weighed” so far.

NGC 3603 is a starburst region: a cosmic factory where stars form frantically from the nebula’s extended clouds of gas and dust. Located 22 000 light-years away from the Sun, it is the closest region of this kind known in our galaxy, providing astronomers with a local test bed for studying intense star formation processes, very common in other galaxies, but hard to observe in detail because of their great distance from us.

NGC 3603 – click for 1280×1291 image

More: here

Galaxies are slowly running out of gas

Thursday, March 11, 2010

Galaxies are made of stars, and stars are made of... gas. So a large part of understanding how galaxies evolve and grow is understanding how much "gas" (literally, not "gasoline") is present in galaxies – but has not yet been incorporated in stars – at different periods in the history of the universe.

What periods of the universe are most interesting in this regard? The answer is: periods somewhat less than the first half of the universe's existence since the time of the big bang, roughly the first 5.5 billion years, 40% of the total. That's because astronomers have good reason to believe that is the time when star formation, and hence galaxy growth, occurred most vigorously.

Assuming the best current estimate, that is has been about 13.7 billion years since the big bang, this means we're interested in observing the universe as it was more than 8.2 billion years ago. That's quite a long time ago, and until fairly recently observation of objects that far back in time has been infeasible. Technology is only now becoming available to study the details of such a remote time.

Astronomers find it convenient to represent distance (in either space or time) in terms of redshift. Because it takes light a finite amount of time to travel, any observable object is seen not as it looks "today", 13.7 billion years after the big bang, but instead as it looked a some time T<13.7, and so we see the object as it looked 13.7-T billion years ago. The light from such an object has taken 13.7-T billion years to reach us.

Due to the expansion of the universe, the wavelength of any photon of light has been increased by a factor of (z+1), where z is the observed redshift – z=0 corresponding to nearby objects for which the shift is negligible. z increases as a complicated function of the distance of the object, but it increases in a regular way as the distance increases. For objects at an age T=5.5 billion years, corresponding to a distance of 8.2 billion light years, the redshift would be about 1.1.

Astronomers have now done surveys of galaxies around z≈1.1. It's not easy, but there is plenty of data, even though only very large, bright galaxies can be observed in detail at that distance. It's even more difficult, though still feasible, to survey galaxies that are even more remote, say at z≈2.3, which corresponds to T≈2.9 billion years.

For the research under discussion here, the investigators relied on existing surveys to sample from, because of the difficulty of doing new surveys from scratch. There was a trade-off to be made. In order to be able to study a selected sample of galaxies in sufficient detail, it's desirable to pick the largest, brightest galaxies. On the other hand, it's also important to study galaxies that are representative of "typical" mature galaxies today, such as our Milky Way. Unfortunately, the most luminous objects at large z tend to be atypical things like quasars and merging galaxies. Those are "freaks", quite unlike typical nearby galaxies, and whatever we might learn about them might not tell us much about the typical case.

So the investigators had to select galaxies for study that were as large and bright as possible, but still "normal". In this case, they included only galaxies of estimated stellar mass (excluding dark matter) of ≥ 3×1010 M. (1 M is our Sun's mass.) Since we are inside the Milky Way and can't see all of it (because of thick dusty regions), it's hard to be sure of our galaxy's total stellar mass, but it's estimated to be about 5×1010 M. (Ref: here.)

An important objective of the research was to get a better understanding of galaxies in which new stars are actively being formed – unlike the Milky Way and other nearby large spirals, which are currently forming stars at the rate of about 5 M per year. Star formation rate is something else that's easier to determine from outside the galaxy, by measuring light flux in various parts of the spectrum (especially infrared and ultraviolet). For the present research, only galaxies with a star formation rate ≥ 40 M per year were selected.

For reasons we're coming to, the required observations are difficult and time-consuming, so for this kind of preliminary study it was necessary to work with small numbers. The net result is that the study was done with 11 galaxies selected from one survey, with z≈1.2, and 12 galaxies selected from another survey with z≈2.3.

Remember that the ultimate objective of the research is to determine how much gas is available for star formation in typical galaxies at the given values of z. That's what is so difficult that it had not been done before (for such distant galaxies).

It's relatively straightforward to determine how much of a galaxy's mass is in the form of stars. This "stellar mass" is proportional to the intrinsic luminosity of the galaxy (which is known since the galaxy distance is known), because most galaxies consist of stars with a predictable distribution of stars of given mass and luminosity ("initial mass function").

However, the total mass of a galaxy also includes non-baryonic dark matter, whose mass in the universe as a whole is known to be about 5 times as large as the mass of "ordinary" baryonic matter. The total mass of a galaxy can sometimes be inferred from measuring galaxy rotation curves. The baryonic matter of a galaxy consists of stars, gas, and (perhaps) massive nonluminous objects such as black holes. Even if one knew reliably the total mass of a galaxy, including dark matter, and one could neglect the contribution of nonluminous objects, one still could not estimate the mass of gas as the difference between the mass of a galaxy's stars and the roughly 17% of total mass that baryonic matter represents in the universe as a whole.

That's because there's no a priori reason to expect that a 1:5 ratio of baryonic matter to non-baryonic matter is present in any particular galaxy – it might well be either more or less. So there needs to be some way to measure fairly directly the amount of matter a galaxy contains in the form of gas that could form stars.

It is known that stars form only out of gas that's rather cold – with temperature less than 100 K. This is simply because hotter gas has a higher internal pressure that prevents the gravitational collapse that's necessary to form a star. Gas that cold is very hard to detect. Black body radiation at 100 K peaks at 29 μm, in the far infrared, and cooler gas emits at even longer wavelengths. Redshift stretches the wavelengths even more (by factors of 2 or 3 for z=1 or 2). Most of the radiation at such wavelengths is blocked by our atmosphere, so is observable only from space – and the necessary instruments don't exist yet.

Fortunately, black body radiation is not the only type of electromagnetic emission from cold gas. Vibrations and rotations of gas molecules also radiate at certain frequencies. Now, most of a galaxy's cold gas is in the form of atomic helium and molecular hydrogen. It would be convenient if hydrogen molecules, especially, had emissions at convenient wavelengths for ground-based observation, but no such luck. It turns out, however, that there is one molecule present in small amounts in interstellar gas which does have a convenient emission: CO (carbon monoxide). CO has a rotational emission at 870 μm (346 GHz). At the values of z of interest here (1.2 and 2.3) these fall into the 2 mm and 3 mm bands – which can be observed.

In a nutshell, then, what the research under discussion did was to measure the total flux from the selected galaxies at the appropriate wavelengths. This indicates that amount of cold CO gas present in the galaxies. Studies of nearby galaxies show that this accurately indicates the total amount of cold gas present. From this, and the estimated mass in the form of stars, one has fraction of total mass (stars + gas) represented by the cold gas available to form stars.

For the galaxies in the sample, this fraction was found to be 34% at z≈1.2 and 44% at z≈2.3. By contrast, contemporary large spiral galaxies have fractions in the 3% to 12% range – quite a difference.

For a summary of the results, here's the abstract:

High molecular gas fractions in normal massive star-forming galaxies in the young Universe
Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars, and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today’s massive spiral galaxies. The slow decrease between z ≈ 2 and z ≈ 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.

The results from this research about star formation rates (SFR) are especially interesting. From other research involving much larger samples, it's known that when SFR is plotted against galaxy stellar mass, the distribution can be fit by a power law:
SFR (M/year) = 150 (M*/1011M)0.8×([1+z]/3.2)2.7
In this equation, M* is the galactic stellar mass. Thus SFR depends on total stellar mass of a galaxy, which makes sense, because the larger the galaxy, the more cold molecular gas is available to make stars. Further, because of the factor involving 1+z (where z is redshift), the SFR curve is shifted upwards at larger z – the rate of star formation is greater, in a regular way, at earlier times in the universe.

This equation is pretty close even in the nearby universe, where z=0. For a galaxy the size of the Milky Way (which is not among the largest of spirals), M* is estimated as 5×1010 M, predicting SFR of about 3.7 M/year – which is surprisingly accurate. So SFR has continued to decline in a fairly regular way.

Interestingly enough, however, in the galaxies sampled in the present research, the percentage of cold gas in a galaxy does not appear to have any clear relationship to either the SFR or the total stellar mass of a galaxy. So almost all of the variation in SFR is related to the total stellar mass. This is what it means to say that the "efficiency" of star formation is not very dependent on percentage of cold gas or cosmic epoch. Instead, SFR is probably largely dependent on total available cold gas, which is proportional (at a given z) to a galaxy's stellar mass.

One additional interesting conclusion can be drawn from the research. Namely, given the SFR in sampled galaxies at z≈2.3, there ought to be much less cold gas in equivalent galaxies at the later time (about 2.3 billion years later) corresponding to z≈1.2 than is actually observed. Much of that cold gas should have been incorporated into stars. Yet the amount of cold gas actually observed at the later time is more than the original amount less what was converted to stars. And so there is apparently more cold gas added over time, even though, as a whole, galaxies really are "runnng out of gas".

Tacconi, L., Genzel, R., Neri, R., Cox, P., Cooper, M., Shapiro, K., Bolatto, A., Bouché, N., Bournaud, F., Burkert, A., Combes, F., Comerford, J., Davis, M., Schreiber, N., Garcia-Burillo, S., Gracia-Carpio, J., Lutz, D., Naab, T., Omont, A., Shapley, A., Sternberg, A., & Weiner, B. (2010). High molecular gas fractions in normal massive star-forming galaxies in the young Universe Nature, 463 (7282), 781-784 DOI: 10.1038/nature08773

Further reading:

Young galaxies gorge on gas (2/10/10)

Why Today's Galaxies Don't Make As Many Stars As They Once Did (2/11/10)

Early Galaxies Formed Stars Fast Because They Had More Gas (2/10/10)

Stellar Baby Boom of Early Universe Explained (2/11/10)

Ancient Galaxies Packed More Raw Material for Stellar Formation (2/10/10)

In the News this month: the molecular content of early galaxies (3/4/10)

Astrophysics: Less greedy galaxies gulp gas (2/11/10)

Selected readings 3/8/10

Monday, March 8, 2010

Interesting reading and news items.

These items are also bookmarked at my Diigo account.

What Is Time? One Physicist Hunts for the Ultimate Theory
I’m trying to understand cosmology, why the Big Bang had the properties it did. And it’s interesting to think that connects directly to our kitchens and how we can make eggs, how we can remember one direction of time, why causes precede effects, why we are born young and grow older. It’s all because of entropy increasing. It’s all because of conditions of the Big Bang. [Wired, 2/26/10]

Black holes and white slopes
It is an incredible feat of observational astronomy to make these movies. It requires adaptive optics on the largest telescopes in the world (the Keck telescopes on Mauna Kea). We used to think of the heavens as eternal and unchanging. Now we watch movies of stars orbiting black holes. [Cosmic Variance, 2/25/10]

The climate machine
For climatologists, models are not just tools that can give a glimpse of what the future holds; they are also an experimental playground – a replica world on which they can test their knowledge of the climate system. Without the ability to conduct global-scale experiments in the lab or in the field, models are the only tools they have. [Climate Feedback, 2/26/10]

Most European males 'descended from farmers'
Most men in Europe can trace a line of descent to early farmers who migrated from the Near East, a study says. However, other scientists subscribe to a different interpretation - that this common lineage arrived in Europe during or before the last Ice Age. [BBC News, 1/20/10]

Esa mission concepts vie for position
The competition to find the next great European space mission has seen three ideas move to the front of the field. ... The concepts include a satellite that would map the "dark Universe" (called Euclid), a probe to study the Sun up-close (Solar Orbiter), and a telescope to find distant planets (Plato). [BBC News, 1/20/10]

Mississippi Delta earthquake: America's Haiti waiting to happen?
One of the strongest series of earthquakes ever to hit the United States happened not in Alaska or along California's San Andreas fault, but in southeast Missouri along the Mississippi River. In 1811 and 1812, the New Madrid fault zone that zig zags through five states shook so violently that it shifted furniture in Washington, D.C., and rang church bells in Boston. The series of temblors changed the course of the Mississippi River near Memphis, and historical accounts claim the river even flowed backward briefly. Geologists consider the New Madrid fault line a major seismic zone and predict that an earthquake roughly the magnitude of the Haiti earthquake (7.0 on the Richter scale) could occur in the area during the next 50 years. [CSMonitor.com, 1/17/10]

Researchers find clues to evolution by studying genes of living people
Scientists seeking to unravel the evolutionary history of human beings are looking not to the past but into the genes of people living today. Researchers at Harvard University have developed a powerful method for identifying genes that have been favored by evolution and have spread rapidly among the population because of natural selection, the process by which organisms with beneficial traits survive in greater numbers and pass on their genes to more offspring than others. The hope is the new tool will cast a light on recent changes in human biology and provide insight into modern-day disease, since one of the most powerful evolutionary pressures human beings have faced are pathogens. [Boston Globe, 1/18/10]

Light and the Age of the Universe - the Cosmic Microwave Background
Our main window to understanding the universe is light and the electromagnetic spectrum. Trapped here on earth, there is very little of the universe that we can actually touch and test with our own hands, but light provides an amazing tool. The Cosmic Microwave Background is perhaps on of the best methods we have of finding the age of the universe. [The Light Side of Science, 3/3/10]

How Google’s Algorithm Rules the Web
Want to know how Google is about to change your life? Stop by the Ouagadougou conference room on a Thursday morning. It is here, at the Mountain View, California, headquarters of the world’s most powerful Internet company, that a room filled with three dozen engineers, product managers, and executives figure out how to make their search engine even smarter. This year, Google will introduce 550 or so improvements to its fabled algorithm, and each will be determined at a gathering just like this one. [Wired, 2/22/10]

Brain cells' new role defunct?
In the past couple of years, the idea that these non-neural brain cells, known as glial cells, participate in neurotransmission "had been widely accepted," Frank Kirchhoff, a cellular and molecular neurobiologist at the Max Planck Institute for Experimental Medicine, who did not participate in the research, wrote in an email to The Scientist. "Therefore, the scientific community was rather surprised to see" that calcium levels in glial cells have no affect on neurotransmission in the hippocampus, added Kirchhoff. [The Scientist, 3/4/10]

Female teachers transmit math anxiety to female students
Girls often believe themselves to be bad at math, in accordance with gender stereotyping, and often experience high levels of anxiety about the subject. That anxiety appears to be driven by social influences, and may be vanishing in early education. Still, identifying its causes could help eliminate it at later stages of education, and prevent it from making a reappearance in young girls. A new study suggests that elementary school may be a breeding ground for this anxiety. The study found that when elementary school teachers, who are primarily female, displayed a high level of anxiety about math, that skittishness was transmitted to their female students. [Nobel Intent, 1/25/10]

Speculating about the Universe as a quantum fluid
What really seems to turns Murayama on is the problem of explaining why some forces are long-range and some are short-range. Basically, gravity reaches out over huge distances. Electromagnetism would reach just as far, but because there are both negative and positive charges, forces due to one set of charges tend get screened out by opposite signed charges. This effectively limits the reach of electromagnetic forces. Nevertheless, the fundamental distance scaling for the two forces is the same. The strong and weak nuclear forces are very short range, extending no further than the width of a nucleus. [Nobel Intent, 1/25/10]

Understanding deep ocean circulation and climate modeling
For Europe, which may experience dramatic changes to its regional climates if the thermohaline circulation changes, this represents a very scary scenario, because we don't know where the point of instability is. We also don't know how we would get back to a circulating solution should the worst occur. And the measurements we presently have of the current are so noisy that we aren't likely to see it coming until it has already happened. [Nobel Intent, 1/27/10]

The amazing race for the cheapest and fastest DNA machine
In the past decade, the cost of sequencing an entire human genome has dropped from $1 billion to $10,000. As companies race to crack the $1,000 genome, the contending DNA machines in the marketplace suggest an end is near. [Smartplanet.com, 3/4/10]

Mammoth Achievement: Researchers at the forefront of molecular biology
By successfully sequencing the DNA of a long-extinct species, Stephan Schuster and Webb Miller have helped push back the boundaries of molecular biology. Stephan Schuster was never all that interested in ancient DNA. As a young genomicist at the Max Planck Institute for Developmental Biology in his native Germany, his forte had always been bacteria. By deciphering and comparing the genomes -- the genetic blueprints -- of various microbial species, he sought to unlock the secrets of these ubiquitous creatures: how they evolve and interact with the organisms that play them host. [Physorg.com, 1/26/10]

Why humans outlive apes
The same evolutionary genetic advantages that have helped increase human lifespans also make us uniquely susceptible to diseases of aging such as cancer, heart disease and dementia. [Physorg.com, 1/26/10]

Neuroscientists making computers smart enough to see connections between brain's neurons
Now a handful of researchers scattered across the globe are tackling a much more ambitious project: to find connectomes of brains more like our own. ... With these technologies, they intend to map the connectomes of our animal cousins, and eventually perhaps even those of humans. Their results could fundamentally alter our understanding of the brain. [Physorg.com, 1/28/10]

Why hasn't ET made contact yet?
He's absolutely convinced. Frank Drake has been scouring the sky for 50 years, looking for signs of extraterrestrial intelligence. He's heard nothing... but he's in no doubt they're out there. Drake was a founder-member of Seti, the Search for Extraterrestrial Intelligence. [BBC News, 1/25/10]

Channeling your inner alien? Maybe, scientists say
The idea that alien micro-organisms could be hiding on Earth has been discussed for a while, according to Jill Tarter, the director of Search for Extraterrestrial Intelligence, a U.S. project that listens for signals from civilizations based around distant stars. She said several of the scientists involved in the project were interested in pursuing the notion, which Davies laid out in a 2007 Scientific American article, "Are Aliens Among Us?" [Physorg.com, 1/26/10]

Physicists’ Dreams and Worries in Era of the Big Collider
“I want to set out the questions for the next nine decades,” Maria Spiropulu said on the eve of the conference, called the Physics of the Universe Summit. She was hoping that the meeting, organized with the help of Joseph D. Lykken of the Fermi National Accelerator Laboratory and Gordon Kane of the University of Michigan, would replicate the success of a speech by the mathematician David Hilbert, who in 1900 laid out an agenda of 23 math questions to be solved in the 20th century. [New York Times, 1/25/10]

Genetic tests give consumers hints about disease risk; critics have misgivings
Gall's experience illuminates the controversy around direct-to-consumer genotyping. Advocates say these services can guide people toward appropriate preventive medical care, help them choose medications and motivate them to make lifestyle changes. But others criticize the companies for overselling their supposed insights and producing reports that untrained consumers might easily misunderstand. The American Medical Association recommends that a physician always be involved in genetic testing. [Washington Post, 1/26/10]

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Gamma-ray bursts without the gamma rays?

Sunday, March 7, 2010

We discussed supernovae a bit in this recent post on gamma-ray bursts. There is now interesting new information on the connection between supernovae and gamma-ray bursts from two recently-described supernovae with atypical properties.

Let's first review a little. Gamma-ray bursts (GRBs) are identified by detection of relatively brief (usually less than a few minutes) but highly energetic emissions of gamma rays. Although there's a great deal of diversity, most events fall into one of two categories: "short", emitting strongly for less than two seconds, and "long", having strong gamma-ray emissions for more than a few seconds and higher total energy.

Short GRBs are less well understood, but they are commonly thought to result from the merger of two stellar-weight black holes or neutron stars. Short GRBs are not relevant for the present discussion.

Long GRBs are thought, with fairly general consensus, to result from certain types of supernovae (the "core-collapse" kind). One reason for this consensus is that the total energy output of a long GRB appears to be in the same neighborhood as that of a core-collapse supernova: around 1051 ergs (1044 joules, if you prefer). Also, if a GRB occurs sufficiently nearby, we can optically identify it with a bright supernova event, and there have been several instances of this.

Supernovae also come in several different kinds, which differ in their internal mechanisms. The main types are Type I, which have no evidence of hydrogen in their spectra, and Type II, which do have hydrogen lines in the spectrum.

There are further subdivisions of the Type I case: Type Ia supernovae are thought to be associated with progenitors that are white dwarfs. These are the end stage reached by most stars, which have less than about ten times the mass of the Sun (10 M). Such stars lose most of their mass when they pass through an earlier red giant stage. After they have burned (via fusion) all of their hydrogen they shrink down to become white dwarfs with a mass less than about 1 M.

Eventually such a star can no longer produce energy even by fusing heavier elements. The star produces energy only by gravitational contraction. Contraction stops only when degeneracy pressure (due to the Pauli exclusion principle) prevents further collapse. Degeneracy pressure, however, can support a stellar mass only if it's less than 1.38 M – the Chandrasekhar limit. If a star reaches this stage with a mass of more than 1.38 M, it will collapse further into a neutron star or black hole. If this doesn't happen, yet the star later accretes mass from an external source – such as a companion in a multiple star system – the star may wind up with enough additional matter to sustain an explosive fusion reaction – and then you get a Type Ia supernova. There's some controversy now whether this is more likely to happen due to the merger of two white dwarfs or gradual accretion from a companion, but that's irrelevant for the present discussion, because Type Ia supernovae are not associated with GRBs.

The problem with Type Ia supernovae is that there's no conceivable mechanism to create one particular hallmark of a GRB – namely, jets of matter ejected from the explosion in opposite directions at speeds near the speed of light. It takes a particular mechanism called a "central engine" to accelerate matter that dramatically. This mechanism is thought to be a rapidly spinning neutron star or black hole that is surrounded by a gaseous accretion disk. The matter is sucked in by the central object, but because it has such a large amount of angular momentum it is expelled outwards in jets along the axis of rotation, instead of falling into or onto the central object.

A "core collapse" supernova is required in order to produce the right conditions for a central engine of this sort to form. (And even in this case, only a small percentage of events seem to have just the right conditions.)

To have a core collapse supernova, it's necessary for the progenitor star to have a mass more than about 10 M. In this case, which is pretty rare, after a certain point, even though fusion of heavy elements may still be going on, the energy available from fusion becomes insufficient to support the entire mass of the star, and the whole thing then collapses very rapidly to a black hole or neutron star. If there is still some hydrogen remaining in the outer shell of the star immediately before collapse, you get a Type II supernova. Otherwise you get a Type Ib supernova, if there's still some helium remaining, or else a Type Ic (no helium).

If the progenitor star had a mass between about 10 M and 20 M, the end result is a neutron star. Above 20 M the result is a black hole. However, as noted, it's not known what conditions are required in order to have a central engine capable of producing a GRB. From the relative frequency of observed GRBs compared to Type Ib/c or Type II supernovae, the right conditions seem to occur only 1 or 2% of the time.

The new wrinkle that two recently described supernova events exhibit is that it is possible to have relativistic jets of matter from a supernova without detectable gamma ray activity. Just how fast are we talking about in terms of the ejected matter? Well, in an "ordinary" supernova matter is ejected at speeds, at most, only 2 or 3% of the speed of light (which is still plenty fast). Yet in the recent examples, the matter is accelerated to more than 50% of the speed of light.

How is the speed actually measured? Well, the interesting thing is that it's easy to measure at radio frequencies, using radio telescopes. Let's consider the case of the supernova known as SN 2009bb, to be specific, which was first observed on March 21, 2009. This one showed up in a galaxy called NGC 3278, which is about 130 million light-years away.

Astronomers didn't even have to act especially quickly to do the measurement – in fact it's best to do it several weeks after the event. Using radio interferometry it's straightforward to observe the size of the expanding sphere of radio emissions. Divide the radius of the sphere by the time since the initial event and you have (roughly) the rate of expansion.

Actually, it's slightly more complicated than that, because relativistic speeds are involved. It's necessary to apply equations of special relativity. Suppose R is the apparent radius of the sphere, and the time interval is Δt. Then the apparent velocity of expansion is R/Δt. It is even possible for this apparent velocity to exceed c (the speed of light).

However, this apparent velocity is not actually the rate at which the ejected matter is moving. Suppose that rate is denoted by v. Let β=v/c, the ratio of the actual speed to the speed of light. Define the quantity (Lorentz factor) γ=1/√(1-β2). Then what's actually true is that γβc=R/Δt. In the case of SN 2009bb the apparent velocity that was measured was ~0.85c. Solving for β you get β~0.65. That is, ejected matter was moving at about 65% of the speed of light. This matter was just lightweight electrons, but it still takes one honking explosion to move even electrons that fast – perhaps 30 times as fast as what one gets in an "ordinary" supernova.

Here's the research abstract:

A relativistic type Ibc supernova without a detected γ-ray burst
Long duration γ-ray bursts (GRBs) mark the explosive death of some massive stars and are a rare sub-class of type Ibc supernovae. They are distinguished by the production of an energetic and collimated relativistic outflow powered by a central engine (an accreting black hole or neutron star). Observationally, this outflow is manifested in the pulse of γ-rays and a long-lived radio afterglow. Until now, central-engine-driven supernovae have been discovered exclusively through their γ-ray emission, yet it is expected that a larger population goes undetected because of limited satellite sensitivity or beaming of the collimated emission away from our line of sight. In this framework, the recovery of undetected GRBs may be possible through radio searches for type Ibc supernovae with relativistic outflows. Here we report the discovery of luminous radio emission from the seemingly ordinary type Ibc SN 2009bb, which requires a substantial relativistic outflow powered by a central engine. A comparison with our radio survey of type Ibc supernovae reveals that the fraction harbouring central engines is low, about one per cent, measured independently from, but consistent with, the inferred rate of nearby GRBs.

There's a practical application of this discovery. It means that astronomers can use radio telescopes to identify supernova events in which a central engine is involved, even if no gamma rays are detectable. Eventually this will help figure out more precisely what conditions are necessary in order to create the central engine.

There are several possible reasons why gamma rays may not be detectable in such events. It could be that sufficiently powerful gamma rays simply are not produced in some cases. Or else they are produced, but quickly absorbed in the neighborhood of the event so that we never see them. Finally, since the gamma rays are presumably directed in a narrow beam, like the ejected matter, the beam simply is not along our line of sight.

A very similar result has also just been reported for the supernova SN 2007gr. This one is in a somewhat closer galaxy, NGC 1058, about 34 million light-years away. In this case, matter was ejected with β~0.52, corresponding to an apparent rate of 60% of the speed of light. Again, no associate gamma ray emission was detected.

The research report:

A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr
The class of type Ic supernovae have drawn increasing attention since 1998 owing to their sparse association (only four so far) with long duration γ-ray bursts (GRBs). Although both phenomena originate from the core collapse of a massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit mostly in soft γ-rays or hard X-rays. Though the GRB central engine generates ultra-relativistic jets, which beam the early emission into a narrow cone, no relativistic outflows have hitherto been found in type Ib/c supernovae explosions, despite theoretical expectations and searches. Here we report radio (interferometric) observations that reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days apart, we detect expansion of the source and establish a conservative lower limit for the average apparent expansion velocity of 0.6c.

Soderberg, A., Chakraborti, S., Pignata, G., Chevalier, R., Chandra, P., Ray, A., Wieringa, M., Copete, A., Chaplin, V., Connaughton, V., Barthelmy, S., Bietenholz, M., Chugai, N., Stritzinger, M., Hamuy, M., Fransson, C., Fox, O., Levesque, E., Grindlay, J., Challis, P., Foley, R., Kirshner, R., Milne, P., & Torres, M. (2010). A relativistic type Ibc supernova without a detected γ-ray burst Nature, 463 (7280), 513-515 DOI: 10.1038/nature08714

Paragi, Z., Taylor, G., Kouveliotou, C., Granot, J., Ramirez-Ruiz, E., Bietenholz, M., van der Horst, A., Pidopryhora, Y., van Langevelde, H., Garrett, M., Szomoru, A., Argo, M., Bourke, S., & Paczyński, B. (2010). A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr Nature, 463 (7280), 516-518 DOI: 10.1038/nature08713

Further reading:

Astronomers Find Rare Beast by New Means (1/27/10)

Oddball Cosmic Explosion Holds Clues to Universe's Biggest Bangs (1/27/09)

Newborn Black Holes May Add Power to Many Exploding Stars (1/27/10)

Astronomers in the Netherlands catch supernova, observe relativistic expansion (1/27/10)

Star Shoots out Material at Close to the Speed of Light (1/28/10)

Some baby black holes give boost, but no burst (1/29/10)

Supernovae linked to gamma ray bursts (1/28/10)

The Birth Place of the Type Ic Supernova 2007gr

Doctor Who and the Silver Spiral (1/27/10)

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