SAO: Weekly Science Updates 2018

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SAO: Weekly Science Updates 2018

Post by bystander » Mon Jan 08, 2018 8:03 pm

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Gravitational Waves Measure the Universe

Post by bystander » Mon Jan 08, 2018 8:28 pm

Gravitational Waves Measure the Universe
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Jan 05
su201801[1].jpg
NGC4993, the galaxy hosting the gravitational wave event GW170817 that
has been used to measure the age of the universe. The source of the event
is the red dot to the upper left of the galaxy's center; it was not there in
earlier images. Credit: NASA/ESA

The direct detection of gravitational waves from at least five sources during the past two years offers spectacular confirmation of Einstein's model of gravity and space-time. Modeling of these events has also provided information on massive star formation, gamma-ray bursts, neutron star characteristics, and (for the first time) verification of theoretical ideas about how the very heavy elements, like gold, are produced.

Astronomers have now used a single gravitational wave event (GW170817) to measure the age of the universe. CfA astronomers Peter Blanchard, Tarreneh Eftekhari, Victoria Villar, and Peter Williams were members of a team of 1314 scientists from around the world who contributed to the detection of gravitational waves from a merging pair of binary neutron stars, followed by the detection of gamma-rays, and then the identification of the origin of the cataclysm in a source in the galaxy NGC 4993 spotted in images taken with various time delays at wavelengths from the X-ray to the radio.

An analysis of the gravitational waves from this event infers their intrinsic strength. The observed strength is less, implying (because the strength diminishes with distance from the source) that the source is about 140 million light-years away. NGC 4993, its host galaxy, has an outward velocity due to the expansion of the universe that can be measured from its spectral lines. Knowing how far away it is and how fast the galaxy is moving from us allows scientists to calculate the time since the expansion began – the age of the universe: between about 11.9 and 15.7 billion years given the experimental uncertainties.

The age derived from this single event is consistent with estimates from decades of observations relying on statistical methods using two other sources: the cosmic microwave background radiation (CMBR) and the motions of galaxies. The former relies on mapping the very faint distribution of light dating from a time about four hundred thousand years after the big bang; the latter involves a statistical analysis of the distances and motions of tens of thousands of galaxies in relatively recent times. The fact that this one single gravitational-wave event was able to determine an age for the universe is remarkable, and not possible with every gravity wave detection. In this case there was an optical identification of the source (so that a velocity could be measured) and the source was neither too distant or too faint. With a large statistical sample of gravitational wave events of all types, the current range of values for the age will narrow.

The new result is intriguing for another reason. Although both the CMBR and the galaxy measurements are each quite precise, they seem to disagree with each other at roughly the ten percent level. This disagreement could just be observational error, but some astronomers suspect it might be a real difference reflecting something currently missing from our picture of the cosmic expansion process, perhaps connected with the fact that the CMBR arises from a vastly different epoch of cosmic time than does the galaxy data. This third method, gravitational wave events, may help solve the puzzle.

A Gravitational-Wave Standard Siren Measurement of the Hubble Constant - B. P. Abbott et al
http://asterisk.apod.com/viewtopic.php?t=37665
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A New Bound on Axions

Post by bystander » Fri Jan 19, 2018 5:03 pm

A New Bound on Axions
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Jan 19
An axion is a hypothetical elementary particle whose existence was postulated in order to explain why certain subatomic reactions appear to violate basic symmetry constraints, in particular symmetry in time. The 1980 Nobel Prize in Physics went for the discovery of time-asymmetric reactions. Meanwhile, during the following decades, astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize that most of the matter in the universe was not visible. It was dubbed dark matter, and today's best measurements find that about 84% of matter in the cosmos is dark. This component is dark not only because it does not emit light -- it is not composed of atoms or their usual constituents, like electrons and protons, and its nature is mysterious. Axions have been suggested as one possible solution. Particle physicists, however, have so far not been able to detect directly axions, leaving their existence in doubt and reinvigorating the puzzles they were supposed to resolve.

CfA astronomer Paul Nulsen and his colleagues used a novel method to investigate the nature of axions. Quantum mechanics constrain axions, if they exist, to interact with light in the presence of a magnetic field. As they propagate along a strong field, axions and photons should transmute from one to the other other in an oscillatory manner. Because the strength of any possible effect depends in part on the energy of the photons, the astronomers used the Chandra X-ray Observatory to monitor bright X-ray emission from galaxies. They observed X-rays from the nucleus of the galaxy M87, which is known to have strong magnetic fields, and which (at a distance of only fifty-three million light-years) is close enough to enable precise measurements of variations in the X-ray flux. Moreover, M87 lies in a cluster of galaxies, the Virgo cluster, which should insure the magnetic fields extend over very large scales and also facilitate the interpretation. Not least, M87 has been carefully studied for decades and its properties are relatively well known.

The search did not find the signature of axions. It does, however, set an important new limit on the strength of the coupling between axions and photons, and is able to rule out a substantial fraction of the possible future experiments that might be undertaken to detect axions. The scientists note that their research highlights the power of X-ray astronomy to probe some basic issues in particle physics, and point to complementary research activities that can be undertaken on other bright X-ray emitting galaxies.

A New Bound on Axion-Like Particles - M.C. David Marsh et al
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Exocomets

Post by bystander » Fri Jan 26, 2018 4:11 pm

Exocomets
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Jan 26
su201804.jpg
An image of Halley's comet. Astronomers have detected around other stars
exocomets with masses comparable to Halley's comet. Credit: W. Liller,
The International Halley Watch Large Scale Phenomena Network

There are currently over 3500 confirmed exoplanets known thanks to the remarkable sensitivity of the Kepler spacecraft and to technological advances in space and ground-based methods made over the past dozen years. Relatively little is known, however, about the minor bodies that might orbit within these systems, asteroids and comets for example. Planet-formation theories predict that such minor bodies should be common, but their low masses and small radii present extreme detection challenges. Methods that rely on solid body transits or velocity variations are generally orders-of-magnitude too weak to spot such small objects. The smallest solid body that has been detected so far via the transit method is an object about one-quarter the size of the Earth, while pulsar timing measurements have spotted a lunar-mass object orbiting a pulsar.

In a tour de force analysis of the Kepler data sets spanning 201250 target stars, CfA astronomers Andrew Vanderburg, Dave Latham, and Allyson Bieryla joined eight of their colleagues in discovering and modeling a likely set of six transiting comets around one star, with another comet possible around a second star. The physical characteristic that made these detections possible was unexpected: the comets have large, extended dust tails that can block enough starlight to make themselves recognizable via unique, asymmetrically shaped absorption dips in their transit lightcurves. (The paper reports, in press, finding a prediction of just such an effect published in 1999). The astronomers systematically consider other explanations for the dips, including starspots, as well as possible inconsistencies in their cometary model, like orbital behavior, but reject them all.

The scientists can estimate the mass of the comets from the observed transit properties and simple assumptions, and they conclude that the bodies are probably similar in mass to Halley's Comet. The scientists also conclude that exocomets are probably not rare given that these seven were spotted without using sophisticated computer tools, although deeper searches will need to be undertaken to find them. Since the two stars hosting exocomets in their study are quite similar in type, they conclude by wondering whether comet transits happen preferentially around certain kinds of stars, although why this might be is not known.

Likely Transiting Exocomets Detected by Kepler - S. Rappaport et al
  • Monthly Notices of the RAS 474(2):1453 (Feb 2018) DOI: 10.1093/mnras/stx2735
    arXiv.org > astro-ph > arXiv:1708.06069 > 21 Aug 2017 (v1), 26 Oct 2017 (v2)

viewtopic.php?p=274246#p274246
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Massive Galaxies in the Early Universe

Post by bystander » Fri Feb 02, 2018 9:11 pm

Massive Galaxies in the Early Universe
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Feb 02
The South Pole Telescope (SPT) is a 10-meter-diameter telescope in the Antarctic that has been operating at millimeter- and submillimeter-waves for a decade; the CfA is an institutional member of the collaboration. For the past six years it has been surveying the sky in a search for galaxies in the first few billion years of cosmic history; they are thought to be preferentially detectable at these wavelengths because their dust has been heated by the ultraviolet light of young stars. One of SPT discoveries, the galaxy SPT0311–58, has upon further investigation turned out to date from an epoch a mere 780 million years after the big bang. It is the most distant known case of this postulated but previously undetected population of optically dim but infrared luminous clusters.

CfA astronomers Chris Hayward, Matt Ashby and Tony Stark are members of the SPT team that made the discovery and then followed up with the Spitzer Space Telescope, the ALMA array, the Hubble Space Telescope, and the Gemini optical/infrared telescope. The scientists were able to determine the cluster's distance and epoch from the redshift of its spectral features, including a line of ionized carbon, and to characterize the overall emission properties across a wider range of wavelengths. The Spitzer and Hubble images of the source revealed the presence of a foreground galaxy that is acting as a gravitational lens to magnify SPT0311-58 and thus greatly facilitated its detection. The ALMA measurements at high spatial resolution found that the original source is actually two galaxies less than twenty-five thousand light-years apart. The implication is that these two galaxies are in the midst of colliding.

The masses of the two galaxies are nearly one hundred billion and ten billion solar masses, respectively. The larger one is more massive than any other known galaxy at this early time in cosmic evolution, a period during which many galaxies are thought to be just forming, and is very bright, making new stars at a rate of about 2900 solar masses per year (thousands of times faster than the Milky Way). Although current models of cosmic evolution do not preclude such giant systems from existing at such early times, the observation does push the models to its limits. The results also imply that there should be a dark-matter halo present with more than 400 billion solar masses, among the rarest dark-matter haloes that should exist in the early universe.

Galaxy Growth in a Massive Halo in the First Billion Years of Cosmic History - D. P. Marrone et al
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The Extreme Nucleus of the Galaxy Arp 220

Post by bystander » Sun Feb 11, 2018 5:34 pm

The Extreme Nucleus of the Galaxy Arp 220
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Feb 09
The galaxy Arp 220 is ultraluminous (defined as having more than about 300 times the luminosity of our own galaxy) and, at a distance of only about 260 million light-years, is the closest ultraluminous galaxy to our Milky Way. Even more dramatic galaxies can have luminosities as much as ten times brighter, and astronomers are still piecing together the reasons for these huge energy outputs. The two primary suspects for the energetics are bursts of star formation that produce many hot young stars, or the accretion of material onto the supermassive black hole at a galaxy's nucleus - an active galactic nucleus (AGN). As the closest example, Arp 220 is one of the best places to probe these different scenarios; however, observations are difficult because whatever is powering the activity in Arp220 is heavily shrouded in dust and the nuclear region is invisible at optical wavelengths.

The starburst explanation should produce many hot young stars with abundant ultraviolet light and supernovae resulting from the deaths of the most massive and short-lived stars. The AGN explanation will produce hotter gas with more X-ray emission and characteristic spectral features. So far signs of both processes have been detected. Astronomers generally have concluded that stars are being made at a rate of about ten thousand solar-masses per year, dominating the luminosity, and that the AGN contributes only modestly to the output, less than 25%.

Adding to the appeal and mystery of Arp 220, however, is the fact that it is a merger of two galaxies and its two constituent galactic nuclei are approaching coalescence, currently being only about one thousand light-years apart. This makes the relatively small AGN output puzzling: Simulations of galaxy mergers suggest that as the nuclei get close together their accretion soars and their luminosity dominates the emission, even exceeding 80% of the total. Moreover, observations of supermassive black hole nuclei in general find that they are systematically larger in larger galaxies, something that would be expected if, as galaxies grow in a merger, their black holes also grow from accreting matter and radiate as they do so. ...

X-Ray Emission from the Nuclear Region of Arp 220 - Alessandro Paggi et al
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Magnetic Reconnection in the Sun

Post by bystander » Sat Feb 17, 2018 7:36 pm

Magnetic Reconnection in the Sun
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Feb 16
The Sun glows with a surface temperature of about 5500 degrees Celsius. On the other hand its hot outer layer, the corona, has a temperature of over a million degrees and ejects a wind of charged particles at a rate equivalent to about one-millionth of the moon's mass each year. Some of these particles bombard the Earth, producing auroral glows and occasionally disrupting global communications. In between these two regions of the Sun is the chromosphere. Within this complex interface zone, only a few thousand kilometers deep, the density of the gas drops with height by a factor of about one million and the temperature increases. Almost all of the mechanical energy that drives solar activity is converted into heat and radiation within this interface zone.

Charged particles are produced by the high temperatures of the gas, and their motions produce powerful, dynamic magnetic fields. Those field lines can sometimes break apart forcefully, but movement of the underlying charged particles often leads them to reconnect. There are two important, longstanding, and related questions about the hot solar wind: how is it heated, and how does the corona produce the wind? Astronomers suspect that magnetic reconnection in the chromosphere plays a key role. ...

Magnetic Reconnection in Strongly Magnetized Regions of the Low Solar Chromosphere - Lei Ni et al
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High Pressure Star Formation in the Galactic Center

Post by bystander » Sun Feb 25, 2018 5:20 pm

High Pressure Star Formation in the Galactic Center
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Feb 23
su201808[1].jpg
A false color Spitzer infrared image of the Milky Way's Central Molecular
Zone (CMZ). Star formation in the CMZ is surprisingly low, suppressed
by gas turbulence. CfA astronomers, in a comprehensive study of the CMZ
using the SMA, have detected two new young, massive stars and another
thirteen dense cores. Credit: Spitzer/NASA/JPL-Caltech/CfA

Some galaxies in the universe are as much as a thousand times more luminous than our Milky Way galaxy, with most of their light being emitted in the infrared. Astronomers attribute that ultra-intense luminosity to warm dust heated by massive bursts of star formation that are often concentrated in the galaxy's center, near the supermassive black hole. The Milky Way also has a supermassive black hole, and its inner region (called the Central Molecular Zone, CMZ) has plenty of the gas needed to form new stars. But the star formation rate there is not only not intense, it is less than average given the amount of mass present. There are several notable exceptions, like the dramatic Arches Cluster, but these serve to highlight the strange inactivity everywhere else.

The low rate of star formation in the CMZ has puzzled astronomers for decades. Because the physical conditions there differ from those in normal giant molecular clouds, astronomers generally have concluded that the responsibility must lie with some combination of their properties, in particular the high values of gas density, temperature, pressure, motion, and magnetic field strength. CfA astronomers Qizhou Zhang, Cara Battersby, and Eric Keto and their colleagues used the Submillimeter Array (SMA) to undertake a large and comprehensive study of the CMZ in search of answers. They discovered a sample of thirteen high mass cores with between 50-2200 solar-masses of material that could be incipient young stars, and two objects that appear to be previously unknown young, massive stars. The SMA also observed the spectral lines of the molecules formaldehyde and methyl cyanide to measure the temperature of the gas and its kinematics, including turbulent motions. The scientists conclude, in agreement with previous speculation, that the turbulent environment of the CMZ is responsible for inhibiting star formation there.

Star Formation in a High-Pressure Environment:
An SMA View of the Galactic Centre Dust Ridge
- D. L. Walker et al
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A Stellar System with Three Super Earths

Post by bystander » Sat Mar 03, 2018 5:07 pm

A Stellar System with Three Super Earths
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Mar 02
Over 3500 extra-solar planets have been confirmed to date. Most of them were discovered using the transit method, and astronomers can combine the transit light curves with velocity wobble observations to determine the planet's mass and radius, and thereby constrain its interior structure. The atmosphere can also be studied in a transit by using the fact that the chemical composition of the atmosphere means its opacity varies with wavelength. By measuring the depth of the transit at different wavelengths, it is possible to infer the composition and temperature of the planet's atmosphere.

CfA astronomers Joseph Rodriguez, Andrew Vanderburg, Jason Eastman, David Latham, and Samuel Quinn and their team of scientists discovered three small transiting planets orbiting the star GJ9827 which lies at the relatively close distance of 100 light-years. The three exoplanets have radii of about 1.6, 1.3, and 2.1 Earth-radii respectively. All of them are categorized as super-Earths, that is, with masses that are larger than Earth’s but less than Neptune's. (Radial velocity measurements of the exoplanets, not included in this paper, have just been separately published and confirm this conclusion.)

GJ9827 is one of the few known stars to have multiple transiting terrestrial-sized exoplanets that are suited for atmospheric characterization. In fact, its three exoplanets are particularly interesting because two of them have radii between 1.5 and 2.0 Earth-radii. Across this range in radii, the composition of planets is expected to change from rocky to gaseous; moreover, there are relatively few such candidates for study. These planets orbit very close to the star, with periods of 1.2, 3.6 and 6.2 days respectively, and at these close distances they have fairly hot temperatures, estimated at 1172, 811 and 680 degrees kelvin. Future observations will probe their atmospheres and provide a much more detailed picture of this unusual family of super-Earths.

A System of Three Super Earths Transiting the Late K-Dwarf GJ 9827 at 30 pc - Joseph E. Rodriguez et al
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Imaging a Galaxy's Molecular Outflow

Post by bystander » Fri Mar 09, 2018 4:52 pm

Imaging a Galaxy's Molecular Outflow
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Mar 09
A merger between galaxies can trigger can intense radiation from bursts star formation and from the accretion of gas onto the two supermassive black holes at their centers. Astronomers have observed a strong statistical correlation between the masses of these black holes and other properties of the galaxies like their velocity structure or luminosity, and have concluded that there must be a connection. Feedback of some kind seems most likely to explain these correlations, and astronomers have been working to identify its source and nature. One prominent suggestion for feedback is an outflow of molecular gas; once turned on, it would deplete the galaxy of the raw material needed for making new stars and from further enhancing the black hole's mass. Evidence for molecular outflows has been reported in far infrared lines of molecules, but these spectral results lack the convincing spatial information needed to associate the activity with the nuclei themselves.

CfA astronomer Junko Ueda is a member of a team of fifteen astronomers who used the ALMA submillimeter telescope facility, with its superb spatial imaging capabilities, to study the outflow in the luminous galaxy NGC6240, known to be a luminous merger in its late stages. Its double nuclei, separated by a modest two thousand light-years, has already been seen at wavelengths from the X-ray to the radio. The astronomers used one of the spectral lines from the abundant molecule carbon monoxide to probe the inner region of the galaxy. The line also revealed the presence of gas motions of up to two thousand kilometers per second, consistent with a powerful wind driving a massive flow of material out of the galaxy.

The new images were able to identify for the first time several regions of outflow activity located only a few thousand light-years from the black holes and aligned as though they were driven by processes associated with the nuclear black holes. Moreover, these regions are spatially coincident with other indicators of general activity like shocked gas and X-rays. The new results are one of the first demonstrations that the widely seen molecular outflow activity does originate from black hole feedback mechanisms.

Imaging the Molecular Outflows of the Prototypical ULIRG NGC 6240 with ALMA - T. Saito et al
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Measuring White Dwarf Masses with Gravitational Lensing

Post by bystander » Fri Mar 30, 2018 3:57 pm

Measuring White Dwarf Masses with Gravitational Lensing
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Mar 16
Measuring the mass of a celestial body is one of the most challenging tasks in observational astronomy. The most successful method uses binary systems because the orbital parameters of the system depend on the two masses. In the case of black holes, neutron stars, and white dwarfs, the end states of stellar evolution, many are isolated objects, and most of them are also very faint. As a result, astronomers still do not know the distribution of their masses. They are of great interest, however, because they participate in dramatic events like the accretion of material and emission of energetic radiation, or in mergers that can result in gravitational waves, gamma-ray bursts, or Type Ia supernovae, all of which depend on an object's mass.

CfA astronomers Alexander Harding, Rosanne Di Stefano, and Claire Baker and three colleagues propose a new method for determining the masses of isolated compact objects: gravitational lensing. The path of a light beam will be bent by the presence of mass, an effect calculated by General Relativity. A massive body will act like a lens to distort the image of an object seen behind it when the two are close to being aligned along our line-of-sight, and the specifics of the image distortions will depend on the body's mass. The astronomers describe the prospects for predicting lensing events generated by nearby compact objects as their motions take them across the field of background stars.

The team estimates that the nearby population of compact objects contains about 250 neutron stars, 5 black holes, and about 35,000 white dwarf stars suitable for this study. Knowing the general motions of the white dwarfs across the sky, they obtain a statistical estimate of about 30-50 lensing events per decade that could be spotted by Hubble, ESA's Gaia mission, or NASA's new JWST telescope. The next step in this effort is to use ongoing stellar surveys like that of Gaia to refine the bodies' positions and motions to be able to predict specifically which objects to monitor for lensing.

Predicting Gravitational Lensing by Stellar Remnants - Alexander J Harding et al
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Ionized Molecules Trace Galactic Outflows

Post by bystander » Fri Mar 30, 2018 4:08 pm

Ionized Molecules Trace Galactic Outflows
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Mar 23
There is a process at work in most galaxies that affects both the central black hole mass as well as the galaxy's global velocity structure and luminosity. Astronomers suspect that feedback of some kind is involved, and one popular mechanism is outflowing gas. The outflow would deplete a galaxy of the raw material needed both for making new stars and for enhancing the black hole's mass. The first evidence for molecular outflows was discovered by an infrared satellite about twenty years ago: the molecule OH showed outflowing motions of thousands of kilometers a second in its far infrared emission lines. The Herschel Space Observatory recently followed up those detections in great detail, finding that in some extreme cases powerful outflows carry over a thousand solar-masses per year and have the power of a hundred billion Suns - a few percent of the total luminous energy of the galaxy.

CfA astronomers Eduardo Gonzalez-Alfonso, Matt Ashby, and Howard Smith have now discovered that the ionized molecule OH+ traces hot gas in these outflows and also (probably) from the torus of material thought to ring the black hole. The scientists led a team that reduced and modeled three far infrared lines of OH+ and one of the ionized water molecule H2O+ in the galaxy Markarian 231. The lines confirm much of the diagnostics from the neutral molecular gas analyses; the most curious result, however, was the huge abundance of the ionized material, nearly 10% of the neutral gas. The scientists are unable to explain the presence of so much ionized material either with hot, ultraviolet-emitting stars or with X-rays – it requires ten thousand times the excitation that is present in the Milky Way galaxy. They argue instead that cosmic rays are responsible, energized by repeated acceleration in shock fronts from star formation or similar processes. One additional implication is that strong shocks must be active in the galaxy and should have be responsible for other observable phenomena like the heating of other gas.

Outflowing OH+ in Markarian 231: The Ionization Rate of the Molecular Gas - E. González-Alfonso et al
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Rings and Gaps in a Developing Planetary System

Post by bystander » Fri Mar 30, 2018 4:21 pm

Rings and Gaps in a Developing Planetary System
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Mar 30
The discovery of an exoplanet has most often resulted from the monitoring of a star's flicker (the transiting method) or its wobble (the radial velocity method). Discovery by direct imaging is rare because it is so difficult to spot a faint exoplanet hidden in the glare of its host star. The advent of the new generation of radio interferometers (as well as improvements in near-infrared imaging), however, has enabled the imaging of protoplanetary discs and, in the disc substructures, the inference of orbiting exoplanets. Gaps and ring-like structures are particularly fascinating clues to the presence or ongoing formation of planets.

Rings of dust have already been identified in many protoplanetary systems from their infrared and submillimeter emission. The origin of these rings is debated. They might have formed from dust "pile-up," dust settling, gravitational instabilities, or even from variations in the optical properties of the dust. Alternatively, the rings could result dynamically from the orbital motions of planets that have already developed or that are well on their way. Planets will induce waves in the dusty discs which, as they dissipate, can produce gaps or rings. Key to solving the problem is recognizing that different sized dust grains behave differently, with small grains being strongly coupled to the gas and so track the gas mass, whereas larger grains (millimeter-sized or larger) tend to follow pressure gradients and concentrate near gap edges.

CfA astronomers Sean Andrews and David Wilner were members of a team of scientists who used the ALMA facility to image the dust around the young star Elias 24 with a resolution of about 28 au (one astronomical unit being about the average distance of the Earth from the Sun). The astronomers find evidence for gaps and rings and, assuming these are produced by an orbiting planet, they model the system allowing both the planet’s mass and location and the dust's density distribution to evolve. Their best model explains the observations quite well: after about forty-four thousand years the inferred planet has a mass 70% of Jupiter's mass and is located 61.7 au from the star. The result reinforces the conclusion that both gaps and rings are prevalent in a wide variety of young circumstellar disks, and signal the presence of orbiting planets.

Rings and Gaps in the Disc Around Elias 24 Revealed by ALMA - G. Dipierro et al
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Exotic Binary Stars

Post by bystander » Wed Apr 11, 2018 5:23 pm

Exotic Binary Stars
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Apr 06
Cataclysmic variable stars (CVs) are white dwarf stars that are accreting from an orbiting, low mass binary companion star. The accretion is facilitated by the proximity of the stars; typical orbital periods range from about one to ten hours. Although the family of these exotic CV binaries is heterogeneous, there are, roughly speaking, four classes characterized by the accretion physics, eruptions caused by occasional accretion events, flaring from activity on the white dwarf's surface, and the appearance of hydrogen lines in the companion star.

CVs are found in many galactic environments, but their presence in globular clusters, whose distances and populations are well characterized, allows a more precise comparative study of their properties. CVs can affect the evolution of the cluster while themselves being influenced by the dense stellar environment in a cluster. Evolutionary models of globular cluster evolution imply that after about ten billion years, a cluster with a million stars should have about two hundred CVs – many more than have been seen so far in any cluster. Identifying them, however, is not easy because they can be faint and because they exist in such crowded environments.

CfA astronomers Maureen van den Berg and Josh Grindlay and their colleagues detected twenty-two new CVs in the nearby globular cluster 47 Tucanae (47 Tuc) using Chandra X-ray Observatory and Hubble measurements, bringing the total known to forty-three, the largest sample in any globular cluster so far. The scientists find that 47 Tuc has fewer bright CVs than had been expected. Many globular clusters show a steep increase in stellar density near their centers (the so-called “core collapse” scenario). The scientists argue that the high central densities in these core-collapsed clusters has led to frequent close encounters between stars, which in turn has resulted in the formation of younger and brighter CVs. The cluster 47 Tuc has not experienced core collapse, which could explain the relative lack of such bright CVs. These new results imply that the CV population in 47 Tuc is therefore a combination of primordial CVs and others formed dynamically early in the evolution of the cluster.

New Cataclysmic Variables and Other Exotic Binaries in the Globular Cluster 47 Tucanae - L.E. Rivera Sandoval et al
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The Gamma Ray Burst – Supernova Connection

Post by bystander » Fri Apr 13, 2018 2:32 pm

The Gamma Ray Burst – Supernova Connection
Smithsonian Astrophysical Observatory
Weekly Science Update | 2018 Apr 13
A "core-collapse" supernova occurs when the iron core of a massive star collapses under the force of gravity and then rebounds, generating pressure waves and shocks that propagate outward. A superluminous supernovae is a rare class of core collapse supernovae whose luminosity, equal to 10-1000 billion suns, is too high to be powered by the usual process that drives supernovae, the radioactive decay of nickel (there is not enough nickel present to do it). The source of the energetics has been hotly contested, with suggestions including shocks from the ejected material or pulsating instabilities interacting with surrounding material. The most favored model, however, is the sustained injection of energy from a source like a spinning compact remnant: a neutron star or an accreting black hole.

Long-duration gamma-ray bursts are those that last for a few seconds up to several minutes, unlike the more common gamma-ray bursts that last for under a few seconds. The long-duration bursts are suspected of being sustained by the rotational energy of a spinning compact object left behind from a supernova. Superluminous supernovae seem to be associated with these kinds of long-duration bursts, lending support to the idea that they too are powered by a spinning remnant. CfA astronomer Matt Nicholl and four colleagues have proposed a unifying model for superluminous supernovae and long-duration gamma-ray bursts in which a spinning neutron star has a slight misalignment between its spin axis and its magnetic axis. The consequence is that substantial fractions of the spinning power are supplied both to the supernova and to a jet of particles moving at speeds close to the speed of light that enabled the long burst. Moreever, the scientists are able to predict the radio emission and thermal wind effects, and to address some of the transient effects that appear in these dramatic events.

The GRB–SLSN connection: misaligned magnetars, weak jet emergence, and observational signatures - Ben Margalit et al
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Is Dark Matter Made of Primordial Black Holes?

Post by bystander » Mon Apr 23, 2018 2:59 pm

Is Dark Matter Made of Primordial Black Holes?
SAO Weekly Science Update | 2018 Apr 20
IC_1613_UV.jpg
The dwarf irregular galaxy IC1613. Astronomers wondering whether primordial
black holes might compose the dark matter in the universe suggest that the
shapes of faint dwarf galaxies with dark matter halos might reveal the answer.
(Credit: NASA/JPL-Caltech/SSC/GALEX)

Astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize in the last century that most of the matter in the universe was not visible. About 84% of the matter in the cosmos is dark matter, much of it located in halos around galaxies. It was dubbed dark matter because it does not emit light, but it is also mysterious: it is not composed of atoms or their usual constituents like electrons and protons.

Meanwhile, astronomers have observed the effects of black holes and recently even detected gravitational waves from a pair of merging black holes. Black holes usually are formed in the explosive death of massive stars, a process that can take many hundreds of millions of years as a star coalesces from ambient gas, evolves and finally dies. Some black holes are inferred to exist in the early universe, but there is probably is not enough time in the early universe for the normal formation process to occur. Some alternative methods have been proposed, like the direct collapse of primordial gas or processes associated with cosmic inflation, and many of these primordial black holes could have been made.

CfA astronomer Qirong Zhu led a group of four scientists investigating the possibility that today's dark matter is composed of primordial black holes, following up on previously published suggestions. If galaxy halos are made of black holes, they should have a different density distribution than halos made of exotic particles. There are some other differences as well - black hole halos are expected to form earlier in a galaxy’s evolution than do some other kinds of halos. The scientists suggest that looking at the stars in the halos of faint dwarf galaxies can probe these effects because dwarf galaxies are small and faint (they shine with a mere few thousand solar luminosities) where slight effects can be more easily spotted. The team ran a set of computer simulations to test whether dwarf galaxy halos might reveal the presence of primordial black holes, and they find that they could: interactions between stars and primordial halo black holes should slightly alter the sizes of the stellar distributions. The astronomers also conclude that such black holes would need to have masses between about two and fourteen solar masses, right in the expected range for these exotic objects (although smaller than the black holes recently spotted by gravitational wave detectors) and comparable to the conclusions of other studies. The team emphasizes, however, that all the models are still inconclusive and the nature of dark matter remains elusive.

Primordial Black Holes as Dark Matter: Constraints from Compact Ultra-Faint Dwarfs - Qirong Zhu et al
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Finding Galaxies with Active Nuclei

Post by bystander » Fri Apr 27, 2018 3:08 pm

Finding Galaxies with Active Nuclei
SAO Weekly Science Update | 2018 Apr 27
Image
The Hubble image of a galaxy spotted by Spitzer's IRAC.
Credit: NASA/ESA/Hubble; M. Polimera et al MNRAS 2018

The nuclei of most galaxies host supermassive black holes with millions or even billions of solar-masses of material. Material in the vicinity of such black holes can accrete onto a torus of dust and gas around the black hole, and when that happens the nuclei radiate powerfully across the full spectrum. These active galactic nuclei (AGN) are among the most dramatic and interesting phenomena in extragalactic astronomy, and puzzling as well. Exactly what turns the accretion on or off is not understood, nor is how the associated processes produce the emission, generate jets of particles, or influence star formation in the galaxy.

Because AGN play an important role in the evolution of galaxies, astronomers are studying galaxies with AGN at cosmological distances. It is in earlier epochs of the universe, about ten billion years after the big bang, when the most significant AGN fueling is thought to occur. But AGN at these distances are also faint and more difficult to find. Historically, they have been spotted by their having very red colors due to heavy dust obscuration, characteristic emission lines (signaling very hot gas), and/or their variability.

CfA astronomers Matt Ashby, Steve Willner and Giovanni Fazio and two colleagues used deep infrared extragalactic surveys taken over 14 years by the IRAC instrument on the Spitzer Space Telescope to search for distant AGN. The various surveys in the archive repeatedly scanned different portions of the sky over as many as eleven epochs in their efforts to peer deeper and farther into the cosmos, and the multiple observations allow spotting variable sources. The astronomers found almost a thousand infrared-variable galaxies in these surveys, about one percent of all the galaxies recorded. They estimate that about eighty percent of these variable sources are AGN, the others being due either to supernovae or spurious data. The variability had not been seen in studies at other wavelengths because of the heavy obscuration around the nuclei and/or the weakness of X-ray emission; the infrared can peer through the obscuring dust. The team examined Hubble images of the sources and finds that a majority show indication of disruption, perhaps from a galaxy-galaxy collision. Their results suggest that mid-infrared variability identifies a unique population of galaxies with AGN.

Morphologies of mid-IR Variability-Selected AGN Host Galaxies - Mugdha Polimera et al
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A Massive Protocluster of Galaxies in the Early Universe

Post by bystander » Fri May 04, 2018 2:14 pm

A Massive Protocluster of Galaxies in the Early Universe
SAO Weekly Science Update | 2018 May 04
Astronomers have detected massive clusters of galaxies dating from as early as only about three billion years after the big bang. The stars in these clusters presumably had to form at an even earlier period and those progenitor systems, dubbed 'protoclusters,' are now the subject of intense searches. They have been identified in simulations of cosmological evolution, so they are expected to be present. Active star formation underway in them should be bright enough to allow their detection, but so far the evidence for any has been slight.

In the new issue of Nature, CfA astronomers Matt Ashby, Chris Hayward, and Tony Stark join with thirty-five of their colleagues to report discovering a massive protocluster dating back to an epoch less than two billion years after the big bang. The team has been using the South Pole Telescope to identify distant galaxy clusters at millimeter wavelengths. In that survey, which covers almost 6% of the sky, the brightest object discovered was one called SPT2349-56. The astronomers performed follow-up studies on this object using several other facilities including the ALMA array of telescopes and the APEX telescope. The source turned out to be a clump with multiple sources (in fact the team found evidence for fourteen components in the cluster), and spectral information showed the objects to be located in the epoch about 1.4 billion years after the big bang.

The scientists compared their data with results of simulations of galaxy clusters in the early universe. They conclude that most likely SPT2349-56 is a cluster with a mass about one thousand times larger than the Milky Way’s mass, and that each of its major component galaxies is forming stars at a rate of between fifty to one thousand times faster than our galaxy. Moreover, the cluster seems to be compact in scale, stretching across only about five hundred thousand light-years. The modest dimensions, large mass, and relatively modest velocities seen in the object suggest that many of the components are likely to coalesce as the system evolves. The paper concludes that this system represents the detection of a galaxy cluster core that is already at an advanced stage of formation when the Universe was only 1.4 billion years old. ...

A Massive Core for a Cluster of Galaxies at a Redshift of 4.3 - T. B. Miller et al
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Helium in an Exoplanet Atmosphere

Post by bystander » Sun May 13, 2018 2:56 pm

Helium in an Exoplanet Atmosphere
SAO Weekly Science Update | 2018 May 11
Helium is the second most abundant element in the universe, after hydrogen, and is a major constituent of the gas-giant planets in our Solar System. By contrast, helium is rare in the Earth's atmosphere, comprising less than about a thousandth of a percent of air. It is ten times less abundant even than neon, which cosmically is only the fifth most abundant element. The reason for the different relative planetary abundances of helium is thought to lie in their formation process: the gas giant planets formed in the colder outer part region of the young solar system which was rich in primordial gas, whereas the Earth formed closer in from rocky material, and to the extent that the Earth has any helium at all it comes from the radioactive decay of heavy elements in the Earth's rocks.

Helium is predicted to be among the most readily-detectable species in the atmospheres of exoplanets, especially in extended and escaping atmospheres. Searches for helium, however, have until now been unsuccessful. CfA astronomers Antonija Oklopcic, Laura Kreidberg, Jonathan Irwin, and David Charbonneau were among a team of astronomers reporting in this week’s Nature the first convincing detection of helium in an exoplanet, the warm gas giant WASP-107b. This object orbits its star every 5.7 days and is one of the lowest density planets known with a radius similar to that of Jupiter (about 94% of Jupiter’s size) but a much lower mass (only about 12%).

The team used the Hubble Space Telescope to detect a narrow absorption feature of helium in the infrared spectrum of WASP-107b during one its transits. The strength of the absorption signal suggests that this exoplanet has an extended atmosphere, and that it is evaporating at a rate that could be high enough to generate a comet-like tail. There is evidence that atmospheric mass loss can substantially alter the bulk composition of an exoplanet, and calibrating such possible effects is important for understanding the observations and the implications for its formation. The new measurements on WASP-107b provide important empirical evidence for these processes. The results also represent the first time an atmosphere was detected via an infrared spectral line.

Helium in the Eroding Atmosphere of an Exoplanet - J. J. Spake et al
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A New KInd of Supernova

Post by bystander » Sun May 20, 2018 6:24 pm

A New KInd of Supernova
SAO Weekly Science Update | 2018 May 18
Image
Before and after images show
the detection of a new kind of
rapidly brightening supernova
that took only 2.2 days to reach
peak brightness. (Credit:
DECam/CTIO 4-meter and
A. Rest et al. 2018)

The K2 Mission, an extension of the immensely successful NASA Kepler mission to search for exoplanets, has itself discovered nearly one hundred new exoplanets so far. K2 monitors stars for variability, the sign of a transiting exoplanet, but in the course of searching it makes many other variable star discoveries. CfA astronomers David James and Victoria Villar were members of a team of astronomers that discovered evidence in K2 observations for the most extreme case known of a rapidly brightening supernova. Their results appeared in Nature last week.

Normal supernova brighten dramatically (and then dim) over periods of weeks. A few recent supernova searches using faster cadences, however, have spotted a handful of luminous transients that peak more quickly, in only ten days, before fading in month. The K2 mission, with its frequent monitoring of stars, has now found an extreme case: a supernova that brightened in only 2.2 days and then dimmed in roughly a week. The process that powers this rapid, dramatic rise cannot be the same one that powers normal supernova emission, namely the radioactive decay of elements produced in the explosive event. The rise time for radioactive decay is well-understood and set by the time it takes for light to propagate through the remnant material, which in turn depends on the mass of the material. The short rise time in this object implies too little material to explain the energetics.

The scientists consider a variety of alternative scenarios, for example the brightening being driven directly by accretion processes around a black hole. They conclude however that the exploding stellar debris has run into external material around the remnant, presumably gas expelled from the star during a pre-explosion event. The existence of this new class of rapid supernovae not only expands our knowledge of how supernova look and behave, it also illustrates the serendipitous power of astronomical survey missions.

A Fast-Evolving Luminous Transient Discovered by K2/Kepler - A. Rest et al
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X-Ray Binary Stars at the Galactic Center

Post by bystander » Sat May 26, 2018 2:26 pm

X-Ray Binary Stars at the Galactic Center
SAO Weekly Science Update | 2018 May 25
The center of our Milky Way galaxy is about twenty-five thousand light years from Earth, in the direction of the constellation Sagittarius. At the core of the galaxy is a supermassive black hole with about four million solar-masses of material and around it, within a volume just a few light-years in radius, orbit hundreds of massive stars and probably hundreds of thousands of smaller, harder to detect stars. The whole region is invisible to us in optical light because of the extensive amounts of absorbing, intervening dust. Other wavelengths, however, including the infrared, radio, and energetic X-rays, can penetrate the veiling material and enable us to study this unique environment.

The supermassive black hole at a galaxy’s center is expected gradually to accumulate many small, stellar-mass black holes around it. In the case of our own galaxy, as many as 20,000 black holes may have settled around the central few light-years. So far, however, no such density cusp has been reported. One of the best ways to look for such black holes is via binary stars in which one member is stellar-mass black hole, because accretion around the black hole would generate detectable X-rays.

CfA astronomer Jaesub Hong was a member of a six-person team that used the Chandra X-ray Observatory to search for such binaries. They examined the equivalent of several weeks worth of archival Chandra observations obtained over twelve years, in an area corresponding to a volume that stretches to about sixty light-years from the galactic nucleus. In this region, thousands of X-ray point sources are seen, produced by a range of processes including hot gas, stellar atmospheres, binaries with white dwarf star members, neutron stars, and black holes. The innermost region itself, out to about twelve light-years, has hundreds of sources. (For comparison, the nearest star to the Sun is four light-years away.) The X-ray energies of the sources can be used to diagnose their character, but in this dense complex source confusion was a challenge. To minimize the confusion, the team focused on relatively bright sources, about one hundred of them, and also used simulations as a reality check. They found that twelve of the sources in the central dozen light-years had relatively "soft" X-ray spectra consistent with these sources being black-hole binaries. Although some alternative explanations cannot be ruled out (for example, a class of pulsars), the observed X-ray properties of these sources are the first strong evidence for the population of black hole binaries predicted to settle near the galactic center. The results suggest there are a larger number of (still undetected) isolated BHs present, and not least emphasize the complex and fascinating nature of this unique location in our galaxy.

A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy - Charles J. Hailey et al
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Outflowing Gas from Galaxy Supermassive Black Hole Nuclei

Post by bystander » Sat Jun 09, 2018 4:15 pm

Outflowing Gas from Galaxy Supermassive Black Hole Nuclei
SAO Weekly Science Update | 2018 Jun 01
Mrk348.jpg
An image of the galaxy Markarian 348 in the ultraviolet. Its active nucleus powers
outflowing atomic gas, and new observations of it and four similar galaxies have been
able to image the outflow as well as a rotating gas component. (Credit: NASA/Galex)

Supermassive black holes at the nuclei of most galaxies, including our Milky Way, develop gradually as material accretes onto the seed black hole. The physical processes that drive this growth – the so-called feeding and feedback processes – occur in the vicinity of the galaxy nucleus. When the accretion becomes active, radiation is emitted that illuminates and ionizes the gas in the vicinity of the nucleus. Accretion disc winds can interact with the gas to produce outflowing gas that is observed to reach velocities of hundreds of km/sec. Relativistic jets of particles emanating from the black hole can also interact with his material. These various kinds of feedback are essential to avoid producing overly massive galaxies.

Clear evidence for all these processes has been detected in their optical emission lines of ionized atoms, whose velocities can be measured. However it has been much hard to obtain spatial information about the geometry of the excited gas. CfA astronomer Martin Elvis and nine colleagues used the Gemini eight-meter telescope and a powerful new instrument that records both high-resolution spatial (as small as a few hundred light-years in size) and velocity information. The team studied five relatively nearby galaxies known to have active black hole nuclei with bright atomic emission. They discovered that in all cases the gas has two major components, one rotating and one outflowing. But otherwise the galaxies are all somewhat different: in one the gas rotates opposite to its stars, in another only one lobe of the outflow can be seen, and there are other differences as well. The new paper is just the first in a series expected to probe and model in detail how nuclear black holes grow.

Outflows in the Narrow Line Region of Bright Seyfert Galaxies - I: GMOS-IFU Data - I. C. Freitas et al
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Unconfirmed Near Earth Objects

Post by bystander » Sat Jun 23, 2018 3:49 pm

Unconfirmed Near Earth Objects
SAO Weekly Science Update | 2018 Jun 15
Near Earth Objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth, potentially threatening a collision. NEOs are tracers of the composition, dynamics and environmental conditions throughout the solar system and of the history of our planetary system. Most meteorites come from NEOs, which are thus one of our key sources of knowledge about the solar system’s development. Because some of them are easier to reach with spacecraft than the Moon or planets, NEOs are potential targets for NASA missions. The total number of known NEOs exceeds 18000. The discovery rate has risen rapidly recently, driven by in part the 1998 mandate of Congress to identify 90% of NEOs larger than 1 km (in 2005 Congress, recognizing the danger posed even by smaller NEOs, extended the mandate to sizes as small as 140 meters.)

The importance of NEOs for science and safety has emphasized the need for accurate statistics of the population – but there is a problem. The discovery process for NEOs requires distinguishing between known and unknown targets, and then following up previously unknown targets to measure their orbits. The catalog of orbital elements of known NEOs, their size-frequency distribution, as well as the region of sky visited by telescopes, all serve as inputs for deriving debiased population models. But many NEOs are spotted and reported, but follow-up observations are not done. ...

Unconfirmed Near-Earth Objects - Peter Vereš et al
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Planetary Nebula Lasers

Post by bystander » Sat Jun 23, 2018 4:04 pm

Planetary Nebula Lasers
SAO Weekly Science Update | 2018 Jun 22
Astronomical masers (the radio wavelength analogs of lasers) were first identified in space over fifty years ago and have since been seen in many locations; astronomical lasers have since been seen as well. Some of the most spectacular masers are found in regions of active star formation; in one case the region radiates as much energy in a single spectral line as does our Sun in its entire visible spectrum. Typically the maser radiation comes from molecules like water or OH that are excited by collisions and the radiation environment around young stars. In 1989, maser emission from atoms of atomic hydrogen gas was discovered around the star MWC349.

This remarkable source has since been found to emit lines at infrared wavelengths short enough to qualify them as being genuine lasers (not just masers). The object has been carefully modeled and the detailed conditions producing the lasers and masers have been determined: the lines arise predominantly in a dense disk of ionized gas seen nearly edge-on. Since the initial discovery, despite many searches, no other source has been found that is as complex and dramatic in its emission as is MWC349, although several other cases of weak hydrogen masers have been found. ...

Herschel Planetary Nebula Survey (HerPlaNS): Hydrogen Recombination Laser Lines in Mz 3 - Isabel Aleman et al
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A Cluster of X-Ray Binary Stars Near the Galaxy's Center

Post by bystander » Fri Jun 29, 2018 3:33 pm

A Cluster of X-Ray Binary Stars Near the Galaxy's Center
SAO Weekly Science Update | 2018 Jun 29
A binary star system is one that contains a pair of stars orbiting each other. A black hole X-ray binary is a binary system in which one of the stars is a black hole and the other is a normal star. When matter from the normal star accretes onto the black hole, charged particles are ejected that emit X-ray radiation, making identification of these objects possible. At the other extreme, the most massive known black holes, with millions or even billions of solar-mass, exist at the centers of most galaxies including our own, and one of the basic predictions of galaxy evolution is that the gravitational potency of a nuclear supermassive black hole should lead to a dense accumulation of black holes in its vicinity. The models predict that as many as twenty thousand black holes should have settled into the central few light-years of our Milky Way. (For a sense of how extreme this scenario is, consider that the closest star to our Sun, Proxima Centauri, is 4.2 light-years away.) Until now, however, no such density cusp of black holes has been detected.

The most straightforward way to detect a black hole is to find it in an X-ray binary system. Writing in last week's issue of the journal, Nature, CfA astronomer Jaesub Hong and five colleagues used the Chandra X-ray Observatory archive to identify a dozen X-ray binaries in a density cusp within three light-years of the Galactic Center. Over the past twelve years, Chandra has observed the region for over 380 hours. The scientists analyzed X-ray images of the region between 0.65 and 12.4 light-years from the supermassive black hole (the innermost region had too many complicating sources). They found 415 X-ray point sources, and after carefully considering other possible origins for each one (novae, unusual stars, extragalactic background sources, among other possibilities) they conclude that these dozen are black hole X-ray binaries. The authors then consider the sensitivity limits, the expected abundance of single black holes relative to binaries, and other uncertainties to conclude that the observed number is in good agreement with general model expectations for black holes. They add, however, that alternative models of galaxy evolution cannot be entirely excluded. Some of the observed binaries could be the result of unique events, for example if a globular cluster and its black holes fell into the region.

A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy - Charles J. Hailey et al
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