Posted on by Tammy Plotner

New Research Sheds Light On Black Hole Growth

The black hole that has grown the most can be found in the Sombrero galaxy . The researchers estimate that this black hole has been swallowing the equivalent of one Sun every twenty years and is now over 500 million times as heavy as the Sun. ESO Public Image Release

In a new study led by University of Central Lancashire astronomer Dr. Victor Debattista, researchers are looking into the mystery of how black holes grow and evolve. For many years, astronomers surmised black holes took on mass when their host galaxies merged, but now new modeling techniques show that black holes in spiral galaxies are forced to take on mass.

“Recent Hubble Space Telescope (HST) observations have revealed that a majority of active galactic nuclei (AGN) are resident in isolated disk galaxies, contrary to the usual expectation that AGN are triggered by mergers.” says Debattista. “Here we develop a new test of the cosmic evolution of supermassive black holes (SMBHs) in disk galaxies by considering the local population of SMBHs. We show that substantial SMBH growth in spiral galaxies is required as disks assemble.”

Weighing in a range of one million to one billion times that of the Sun, the black holes located at the core of most galaxies would appear to be gaining at much quicker rates than expected. These are not just exceptions – more like rules. Even the Milky Way’s quiescent black hole might be gaining as much mass as the Sun every 3,000 years. Past observations have shown growth during collision events, when huge amounts of gas around the black hole become intensely hot and shine as an active galactic nucleus. This is a process which can be spotted as far back as the first formations in our Universe. However, these new simulations are giving insight into large scale growth without the need for violence.

“The X-ray-selected sample of moderate luminosity AGN consists of more than 50% disk galaxies, with ongoing mergers evident no more frequently than in nonactive galaxies.” explains the research team. “Some show that even heavily obscured quasars are hosted largely by disks, not by mergers. Studies of star-formation using Herschel find that the specific star formation rates of X-ray selected AGN hosts are no different from those of inactive galaxies, also indicating that AGN hosts are not undergoing fundamentally different behaviors”

These modeling techniques, combined with current observations done with the Hubble Space Telescope, give credence to the theory that black holes can gain significant mass even in “quiet” spiral galaxies. As a matter of fact, there is a strong possibility that AGNs present in some spiral galaxies may even outnumber galaxy mergers. To make this concept even more exciting, astronomers are anticipating an event later this year in our own galaxy – an event where a gas cloud near the Milky Way’s nucleus will encounter our own central black hole. According to predictions, our black hole may take on as much as 15 Earth masses in a period of 10 years from this cloud.

This concept of black hole growth isn’t entirely new, though. According to other research done with the Hubble Space Telescope and led by Dr. Stelios Kazantzidis of Ohio State University and Professor Frank C. van den Bosch of Yale University, they had previously pinpointed mass properties of black holes – making size predictions which utilized the speed of stars residing in the galaxies. In this instance, the team disproved previous assumptions that black holes were unable to grow while the host galaxy grew. Their comparison of spiral and elliptical galaxies “found there is no mismatch between how big their black holes are.” This means black holes would be gaining in mass – growing along at the same rate as the galaxy itself.

“These simulations show that it is no longer possible to argue that black holes in spiral galaxies do not grow efficiently. ” comments Debattista on this new research. ” Our simulations will allow us to refine our understanding of how black holes grew in different types of galaxies.”

Posted on by Jason Major

A Star’s Dying Scream May Be a Beacon for Physics

When a star suffered an untimely demise at the hands of a hidden black hole, astronomers detected its doleful, ululating wail — in the key of D-sharp, no less — from 3.9 billion light-years away. The resulting ultraluminous X-ray blast revealed the supermassive black hole’s presence at the center of a distant galaxy in March of 2011, and now that information could be used to study the real-life workings of black holes, general relativity, and a concept first proposed by Einstein in 1915.

Within the centers of many spiral galaxies (including our own) lie the undisputed monsters of the Universe: incredibly dense supermassive black holes, containing the equivalent masses of millions of Suns packed into areas smaller than the diameter of Mercury’s orbit. While some supermassive black holes (SMBHs) surround themselves with enormous orbiting disks of superheated material that will eventually spiral inwards to feed their insatiable appetites — all the while emitting ostentatious amounts of high-energy radiation in the process — others lurk in the darkness, perfectly camouflaged against the blackness of space and lacking such brilliant banquet spreads. If any object should find itself too close to one of these so-called “inactive” stellar corpses, it would be ripped to shreds by the intense tidal forces created by the black hole’s gravity, its material becoming an X-ray-bright accretion disk and particle jet for a brief time.

Such an event occurred in March 2011, when scientists using NASA’s Swift telescope detected a sudden flare of X-rays from a source located nearly 4 billion light-years away in the constellation Draco. The flare, called Swift J1644+57, showed the likely location of a supermassive black hole in a distant galaxy, a black hole that had until then remained hidden until a star ventured too close and became an easy meal.

See an animation of the event below:

The resulting particle jet, created by material from the star that got caught up in the black hole’s intense magnetic field lines and was blown out into space in our direction (at 80-90% the speed of light!) is what initially attracted astronomers’ attention. But further research on Swift J1644+57 with other telescopes has revealed new information about the black hole and what happens when a star meets its end.

(Read: The Black Hole that Swallowed a Screaming Star)

In particular, researchers have identified what’s called a quasi-periodic oscillation (QPO) embedded inside the accretion disk of Swift J1644+57. Warbling at 5 mhz, in effect it’s the low-frequency cry of a murdered star. Created by fluctuations in the frequencies of X-ray emissions, such a source near the event horizon of a supermassive black hole can provide clues to what’s happening in that poorly-understood region close to a black hole’s point-of-no-return.

Einstein’s theory of general relativity proposes that space itself around a massive rotating object — like a planet, star, or, in an extreme instance, a supermassive black hole — is dragged along for the ride (the Lense-Thirring effect.) While this is difficult to detect around less massive bodies a rapidly-rotating black hole would create a much more pronounced effect… and with a QPO as a benchmark within the SMBH’s disk the resulting precession of the Lense-Thirring effect could, theoretically, be measured.

If anything, further investigations of Swift J1644+57 could provide insight to the mechanics of general relativity in distant parts of the Universe, as well as billions of years in the past.

See the team’s original paper here, lead authored by R.C. Reis of the University of Michigan.

Thanks to Justin Vasel for his article on Astrobites.

Image: NASA. Video: NASA/GSFC

Posted on by Tammy Plotner

Dodging Black Hole Bullets

This 327-MHz radio view of the center of our galaxy highlights the position of the black hole system H1743-322, as well as other features. (Credit: J. Miller-Jones, ICRAR-Curtin Univ.; C. Brogan, NRAO)

[/caption]

In mid-2009 a binary star system cataloged as H H1743–322 shot off something very unusual. Poised about 28,000 light years distant in the direction of the constellation of Scorpius, this rather ordinary system made up of a normal star and unknown mass black hole was busy exchanging mass. The pair orbits in mere days with a stream of material flowing continuously between them. This gas causes a flat accretion disk measuring millions of miles across to form and it is centered on the black hole. As the matter twirls toward the center, it becomes compressed and heats to tens of millions of degrees, spitting out X-rays… and bullets.

Utilizing data from NASA’s Rossi X-ray Timing Explorer (RXTE) satellite and the National Science Foundation’s (NSF) Very Long Baseline Array (VLBA) radio telescope, an international team of astronomers were able to confirm the moment a black hole located within our galaxy fired a super speedy clump of gas into surrounding space. Blasting forth at about one-quarter the speed of light, these “bullets” of ionized gas are hypothesized to have originated from an area just outside the black hole’s event horizon.

“Like a referee at a sports game, we essentially rewound the footage on the bullets’ progress, pinpointing when they were launched,” said Gregory Sivakoff of the University of Alberta in Canada. He presented the findings today at the American Astronomical Society meeting in Austin, Texas. “With the unique capabilities of RXTE and the VLBA, we can associate their ejection with changes that likely signaled the start of the process.”

As we have learned, some of the matter headed toward the center of a black hole can be ejected from the accretion disk as opposing twin jets. For the most part, these jets are a constant stream of particles, but can sometimes form into strong “outflows” which get spit out – rapid fire – as gaseous blobs. In early June 2009, H1743–322 did just that… and astronomers were on hand observing with RXTE, the VLBA, the Very Large Array near Socorro, N.M., and the Australia Telescope Compact Array (ATCA) near Narrabri in New South Wales. During this time they were able to confirm the happenings through X-ray and radio data. From May 28 to June 2, things were nominal “though RXTE data show that cyclic X-ray variations, known as quasi-periodic oscillations or QPOs, gradually increased in frequency over the same period” and by June 4th, ATCA verified that activity had pretty much sloughed off. By June 5th, even the QPOs were gone.

Then it happened…

On the same day that everything went totally quiet, H1743–322 fired off a bullet! Radio emissions jumped and a highly accurate and detailed VLBA image disclosed a energetic missile of gas blasting forth along a jet trajectory. The very next day a second bullet took out in the opposite direction. But this wasn’t the curious part of the event… It was the timing. Up to this point, researchers speculated that a radio outburst accompanied the firing of the gas bullet, but VLBA information showed they were launched around 48 hours in advance of the major radio flare. This information will be published in the Monthly Notices of the Royal Astronomical Society.

Radio imaging by the Very Long Baseline Array (top row), combined with simultaneous X-ray observations by NASA's RXTE (middle), captured the transient ejection of massive gas "bullets" by the black hole binary H1743-322 during its 2009 outburst. By tracking the motion of these bullets with the VLBA, astronomers were able to link the ejection event to the disappearance of X-ray signals seen in RXTE data. These signals, called quasi-periodic oscillations (QPOs), vanished two days earlier than the onset of the radio flare that astronomers previously had assumed signaled the ejection. (Credit: NRAO and NASA's Goddard Space Flight Center)

“This research provides new clues about the conditions needed to initiate a jet and can guide our thinking about how it happens,” said Chris Done, an astrophysicist at the University of Durham, England, who was not involved in the study.

These are just mini-ammo compared to what happens in the center of an active galaxy. They don’t just fire bullets – they blast off cannons. A massive black hole weighing in a millions to billions of times the mass of the Sun can shoot off its load across millions of light years!

“Black hole jets in binary star systems act as fast-forwarded versions of their galactic-scale cousins, giving us insights into how they work and how their enormous energy output can influence the growth of galaxies and clusters of galaxies,” said lead researcher James Miller-Jones at the International Center for Radio Astronomy Research at Curtin University in Perth, Australia.

Original Story Source: NASA News Feature.

Posted on by Jason Major

First Look at a Black Hole’s Feast


A true heart of darkness lies at the center of our galaxy: Sagittarius A* (pronounced “A-star”) is a supermassive black hole with the mass of four million suns packed into an area only as wide as the distance between Earth and the Sun. Itself invisible to direct observation, Sgr A* makes its presence known through its effect on nearby stars, sending them hurtling through space in complex orbits at speeds upwards of 600 miles a second. And it emits a dull but steady glow in x-ray radiation, the last cries of its most recent meals. Gas, dust, stars… solar systems… anything in Sgr A*’s vicinity will be drawn inexorably towards it, getting stretched, shredded and ultimately absorbed (for lack of a better term) by the dark behemoth, just adding to its mass and further strengthening its gravitational pull.

Now, for the first time, a team of researchers led by Reinhard Genzel from the Max-Planck Institute for Extraterrestrial Physics in Germany will have a chance to watch a supermassive black hole’s repast take place.

Continue reading “First Look at a Black Hole’s Feast”

Posted on by Tammy Plotner

Hubble Telescope Directly Observes Quasar Accretion Disc Surrounding Black Hole

A team of scientists has used the NASA/ESA Hubble Space Telescope to observe a quasar accretion disc — a brightly glowing disc of matter that is slowly being sucked into its galaxy’s central black hole. Their study makes use of a novel technique that uses gravitational lensing to give an immense boost to the power of the telescope. The incredible precision of the method has allowed astronomers to directly measure the disc’s size and plot the temperature across different parts of the disc. Image credit: NASA, ESA, J.A. Munoz (University of Valencia)

[/caption]

Thanks to the magic of the NASA/ESA Hubble Space Telescope, a team of international astronomers have made an incredible observation – a quasar accretion disc surrounding a black hole. By employing a technique known as gravitation lensing, the researchers have been able to get an accurate size measurement and spectral data. While you might not think this exciting at first, know that this type of observation is akin to spotting individual grains of sand on the Moon!

Of course, we know we can’t see a black hole – but we’ve learned a lot about them with time. One of their properties is a bright, visible phenomenon called a quasar. These glowing discs of matter are engaged in orbit around the black hole, much like a coil on an electric stove. As energy is applied, the “coil” heats up and unleashes bright radiation.

“A quasar accretion disc has a typical size of a few light-days, or around 100 billion kilometres across, but they lie billions of light-years away. This means their apparent size when viewed from Earth is so small that we will probably never have a telescope powerful enough to see their structure directly,” explains Jose Munoz, the lead scientist in this study.

Because of the diminutive size of the quasar, most of our understanding of how they work has been based on theory… but great minds have found a way to directly observe their effects. By employing the gravity of stars in an intervening galaxy like a scanning microscope, astronomers have been able to observe the quasar’s light as the stars move. While most of these types of features would be too small to see, the gravitation lensing effect ramps up the strength of the quasar’s light and allows study of the spectra as it cruises across the accretion disc.

This diagram shows how Hubble is able to observe a quasar, a glowing disc of matter around a distant black hole, even though the black hole would ordinarily be too far away to see clearly. Credit: NASA and ESA

By observing a group of gravitationally lensed quasars, the team was able to paint a vivid color portrait of the activity. They were able to pick out small changes between single images and spectral shifts over a period of time. What causes these kaleidoscopic variances? For the most part, it’s the different properties in the gases and dust of the lensing galaxies. Because they travel at different angles to the quasar’s light, scientists were even able to distinguish extinction laws at work.

But there was something special about one of the quasars. Says the Hubble Team, “There were clear signs that stars in the intervening galaxy were passing through the path of the light from the quasar. Just as the gravitational effect due to the whole intervening galaxy can bend and amplify the quasar’s light, so can that of the stars within the intervening galaxy subtly bend and amplify the light from different parts of the accretion disc as they pass through the path of the quasar’s light.”

By documenting these color changes, the team could build a profile of the accretion disc. Unlike our Earthly electric stove coil which glows red as it heats up, the accretion disc of a black hole turns blue as it gets closer to the event horizon. By measuring the blue hue, the team was able to measure the disc diameter and the various tints gave them an indicator of distances from its center. In this case, they found that the disc is between four and eleven light-days across (approximately 100 to 300 billion kilometres). Of course, these are only rough estimates, but considering just how far away we’re looking at such a small object gives these types of observations great potential for future studies… and even improvements on accuracy.

“This result is very relevant because it implies we are now able to obtain observational data on the structure of these systems, rather than relying on theory alone,” says Munoz. “Quasars’ physical properties are not yet well understood. This new ability to obtain observational measurements is therefore opening a new window to help understand the nature of these objects.”

Original Story Source: ESA/Hubble News Release. For Further Reading: A Study of Gravitational Lens Chromaticity With the Hubble Space Telescope.

Posted on by Tammy Plotner

AGNs As A New Standard Candle?

Hubble Space Telescope image of a 5000 light-year (1.5 kiloparsec) long jet being ejected from the active nucleus of the active galaxy M87, a radio galaxy. The blue synchrotron radiation of the jet contrasts with the yellow starlight from the host galaxy.

[/caption]

Nope. A standard candle isn’t the same red, green, blue, yellow and omni-present pink wax sticks that decorate your every day birthday cake. Until now a standard candle meant a Cepheid variable star – or more recently – a Type 1a supernova. But something new happens almost every day in astronomy, doesn’t it? So start thinking about how an active galactic nucleus could be used to determine distance…

“Accurate distances to celestial objects are key to establishing the age and energy density of the Universe and the nature of dark energy.” says Darach Watson (et al). “A distance measure using active galactic nuclei (AGN) has been sought for more than forty years, as they are extremely luminous and can be observed at very large distances.”

So how is it done? As we know, active galactic nuclei are home to supermassive black holes which unleash powerful radiation. When this radiation ionizes nearby gas clouds, they also emit their own light signature. With both emissions in range of data gathering telescopes, all that’s needed is a way to measure the time it takes between the radiation signal and the ionization point. The process is called reverberation mapping.

“We use the tight relationship between the luminosity of an AGN and the radius of its broad line region established via reverberation mapping to determine the luminosity distances to a sample of 38 AGN.” says Watson. “All reliable distance measures up to now have been limited to moderate redshift — AGN will, for the first time, allow distances to be estimated to z~4, where variations of dark energy and alternate gravity theories can be probed.”

The AGN Hubble diagram. The luminosity distance indicator =pF is plotted as a function of redshift for 38 AGN with H lag measurements. On the right axis the luminosity distance and distance modulus (m-M) are shown using the surface brightness fluctuations distance to NGC3227 as a calibrator. The current best cosmology is plotted as a solid line. The line is not fit to the data but clearly follows the data well. Cosmologies with no dark energy components are plotted as dashed and dotted lines. The lower panel shows the logarithm of the ratio of the data compared to the current cosmology on the left axis, with the same values but in magnitudes on the right. The red arrow indicates the correction for internal extinction for NGC3516. The green arrow shows where NGC7469 would lie using the revised lag estimate. NGC7469 is our largest outlier and is believed to be an example of an object with a misidentified lag.

The team hasn’t taken their research “lightly”. It means careful calculations using known factors and repeating the results with other variables thrown into the mix. Even uncertainty…

“The scatter due to observational uncertainty can be reduced significantly. A major advantage held by AGN is that they can be observed repeatedly and the distance to any given object substantially refined.” explains Watson. “The ultimate limit of the accuracy of the method will rely on how the BLR (broad-line emitting region) responds to changes in the luminosity of the central source. The current tight radius-luminosity relationship indicates that the ionisation parameter and the gas density are both close to constant across our sample.”

At the first standard candle we discovered the Universe was expanding. At the second we learned it was accelerating. Now we’re looking back to just 750 million years after the Big Bang. What will tomorrow bring?

Maybe a new kind of cake…

Original Story Source: A New Cosmological Distance Measure Using AGN.

Posted on by Mike Simonsen

J-E-T-S, Jets, Jets, Jets!

Bipolar jet from a young stellar object (YSO). Credit: Gemini Observatory, artwork by Lynette Cook

[/caption]

It seems oddly appropriate to be writing about astrophysical jets on Thanksgiving Day, when the New York football Jets will be featured on television. In the most recent issue of Science, Carlos Carrasco-Gonzalez and collaborators write about how their observations of radio emissions from young stellar objects (YSOs) shed light one of the unsolved problems in astrophysics; what are the mechanisms that form the streams of plasma known as polar jets? Although we are still early in the game, Carrasco-Gonzalez et al have moved us closer to the goal line with their discovery.

Astronomers see polar jets in many places in the Universe. The largest polar jets are those seen in active galaxies such as quasars. They are also found in gamma-ray bursters, cataclysmic variable stars, X-ray binaries and protostars in the process of becoming main sequence stars. All these objects have several features in common: a central gravitational source, such as a black hole or white dwarf, an accretion disk, diffuse matter orbiting around the central mass, and a strong magnetic field.

Relativistic jet from an AGN. Credit: Pearson Education, Inc., Upper Saddle River, New Jersey

When matter is emitted at speeds approaching the speed of light, these jets are called relativistic jets. These are normally the jets produced by supermassive black holes in active galaxies. These jets emit energy in the form of radio waves produced by electrons as they spiral around magnetic fields, a process called synchrotron emission. Extremely distant active galactic nuclei (AGN) have been mapped out in great detail using radio interferometers like the Very Large Array in New Mexico. These emissions can be used to estimate the direction and intensity of AGNs magnetic fields, but other basic information, such as the velocity and amount of mass loss, are not well known.

On the other hand, astronomers know a great deal about the polar jets emitted by young stars through the emission lines in their spectra. The density, temperature and radial velocity of nearby stellar jets can be measured very well. The only thing missing from the recipe is the strength of the magnetic field. Ironically, this is the one thing that we can measure well in distant AGN. It seemed unlikely that stellar jets would produce synchrotron emissions since the temperatures in these jets are usually only a few thousand degrees. The exciting news from Carrasco-Gonzalez et al is that jets from young stars do emit synchrotron radiation, which allowed them to measure the strength and direction of the magnetic field in the massive Herbig-Haro object, HH 80-81, a protostar 10 times as massive and 17,000 times more luminous than our Sun.

Finally obtaining data related to the intensity and orientation of the magnetic field lines in YSO’s and their similarity to the characteristics of AGN suggests we may be that much closer to understanding the common origin of all astrophysical jets. Yet another thing to be thankful for on this day.

Posted on by Jean Tate

Supermassive Black Holes Spinning Backwards Create Death Ray Jets?

Centaurus A. Image credit: NASA

[/caption]
Why do some of the supermassive black holes in active galactic nuclei create back-to-back jets that can vaporize entire solar systems, while others have no jets at all?

Dan Evans, a postdoctoral researcher at MIT Kavli Institute for Astrophysics and Space Research (MKI) thinks he knows why; it’s because the jet-producing supermassive black holes are spinning backwards, relative to their accretion disks.

Radio image of a typical DRAGN, showing the main features (Image credit:C. L. Carilli)

For two years, Evans has been comparing several dozen galaxies whose black holes host powerful jets (these galaxies are known as radio-loud active galactic nuclei, or AGN, and are often DRAGNs – double radio source associated with galactic nucleus) to those galaxies with supermassive black holes that do not eject jets. All black holes – those with and without jets – feature accretion disks, the clumps of dust and gas rotating just outside the event horizon. By examining the light reflected in the accretion disk of an AGN black hole, he concluded that jets may form right outside black holes that have a retrograde spin – or which spin in the opposite direction from their accretion disk. Although Evans and a colleague recently hypothesized that the gravitational effects of black hole spin may have something to do with why some have jets, Evans now has observational results to support the theory in a paper published in the Feb. 10 issue of the Astrophysical Journal.

Although Evans has suspected for nearly five years that retrograde black holes with jets are missing the innermost portion of their accretion disk, it wasn’t until last year that computational advances meant that he could analyze data collected between late 2007 and early 2008 by the Suzaku observatory, a Japanese satellite launched in 2005 with collaboration from NASA, to provide an example to support the theory. With these data, Evans and colleagues from the Harvard-Smithsonian Center for Astrophysics, Yale University, Keele University and the University of Hertfordshire in the United Kingdom analyzed the spectra of the active galactic nucleus with a pair of jets located about 800 million light years away in an AGN named 3C 33.

1477 MHz image of 3C 33 (Credit: Leahy & Perley (1991))

“It’s the first convincing galaxy of this type seen at this angle where the result is pretty robust,” said Patrick Ogle, an assistant research scientist at the California Institute of Technology, who studies AGN. Ogle believes Evans’s theory regarding retrograde spin is among the best explanations he has heard for why some AGN contain a supermassive black hole with a jet and others don’t.

Astrophysicists can see the signatures of x-ray emission from the inner regions of the accretion disk, which is located close to the edge of a black hole, as a result of a super hot atmospheric ring called a corona that lies above the disk and emits light (electromagnetic radiation) that an observatory like Suzaku can detect. In addition to this direct light, a fraction of light passes down from the corona onto the black hole’s accretion disk and is reflected from the disk’s surface, resulting in a spectral signature pattern called the Compton reflection hump, also detected by Suzaku.

But Evans’ team never found a Compton reflection hump in the x-ray emission given off by 3C 33, a finding the researchers believe provides crucial evidence that the accretion disk for a black hole with a jet is truncated, meaning it doesn’t extend as close to the center of the black hole with a jet as it does for a black hole that does not have a jet. The absence of this innermost portion of the disk means that nothing can reflect the light from the corona, which explains why observers only see a direct spectrum of x-ray light.

The researchers believe the absence may result from retrograde spin, which pushes out the orbit of the innermost portion of accretion material as a result of general relativity, or the gravitational pull between masses. This absence creates a gap between the disk and the center of the black hole that leads to the piling of magnetic fields that provide the force to fuel a jet.

While Ogle believes that the retrograde spin theory is a good explanation for Evans’ observations, he said it is far from being confirmed, and that it will take more examples with consistent results to convince the astrophysical community.

The field of research will expand considerably in August 2011 with the planned launch of NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) satellite, which is 10 to 50 times more sensitive to spectra and the Compton reflection hump than current technology. NuSTAR will help researchers conduct a “giant census” of supermassive black holes that “will absolutely revolutionize the way we look at X-ray spectra of AGN,” Evans explained. He plans to spend another two years comparing black holes with and without jets, hoping to learn more about the properties of AGN. His goal over the next decade is to determine how the spin of a supermassive black hole evolves over time.

Sources: MITnews, Evans’ Astrophysical Journal paper (preprint is arXiv:1001.0588)