SAO: Science Updates 2020

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SAO: Science Updates 2020

<< Science Updates 2019

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Early Galaxies
SAO Weekly Science Update | 2020 Jan 03

An image of the sky with a rare, massive, 'dusty star-forming galaxy' dating from about one billion years after the big bang. The Submillimeter Array results at 850 microns are shown in greyscale and with blue contours, and at 1.1 millimeters with red contours; the SCUBA-2 submillimeter contours (having much lower spatial resolution) are shown in yellow. Credit: Greenslate et al. 2019 MNRAS

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Galaxies that are very luminous in the infrared are generally active in making new stars whose ultraviolet radiation heats the dust. The energy, re-radiated by the dust at infrared wavelengths, is characterized by having a broad spectral shape with a distinct emission peak. As the universe expands, and as the observed spectra of galaxies shift to the red, light at the wavelength of this peak moves into the submillimeter band leaving the levels of observed infrared flux deficient. Star-forming galaxies in the very distant universe are thus fainter in the infrared than in the submillimeter.

Thousands of galaxies have been discovered dating from epochs only a few billion years after the big bang. Most of them are small, low-mass galaxies that are faint and relatively difficult to study. Although more luminous, massive star-forming galaxies should also be present, these large objects are difficult to assemble at early cosmic times and there are not as many of them. One type of such luminous early galaxy is called a dusty-star-forming galaxy. They contain so much obscuring dust that they are invisible at optical wavelengths, and (based on their luminosities) have star-formation rates exceeding a thousand solar-masses per year; for comparison, the Milky Way produces about one star per year.

Dusty star-forming galaxies in the earliest epochs, less than two billion years after the big bang, are particularly rare and hard to find, but they are extremely valuable in helping understand how the first galaxies develop. CfA astronomer Glen Petitpas was a member of a team of astronomers who used the SCUBA-2 camera (Submillimeter Common User Bolometer Array-2) and the far-infrared Herschel SPIRE instrument to discover and characterize a dusty star-forming galaxy. They serendipitously detected the unusual galaxy in a SCUBA-2 survey. When they realized that the object was not detected by Herschel - or by any other optical or infrared survey, suggesting that its infrared peak had moved very far to the red - they concluded that it likely was from an extremely early epoch.

The team then used the Submillimeter Array, with a spatial resolution about ten times finer than SCUBA-2, to confirm the detection and study the source. Since a firm distance measurement requires detecting a spectral line and measuring its redshift, the scientists also tried using other submillimeter facilities suitable for line searches, but without success. Nevertheless, from the flux limits at various wavelengths they were able to make a strong case that this object is a massive, dusty star-forming galaxy, among the first generation of massive galaxies in the universe and dating from between roughly seven hundred million and one billion years after the big bang.

A SCUBA-2 Selected Herschel-SPIRE Dropout and the Nature of this Population ~ J. Greenslade et al

• Monthly Notices of the RAS 490(4):5317 (Dec 2019) DOI: 10.1093/mnras/stz2850
• arXiv.org > astro-ph > arXiv:1910.03512 > 08 Oct 2019

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The Interiors of Stars
SAO Weekly Science Update | 2020 Jan 10

An illustration of vibration modes in the Sun. Astronomers have used the TESS mission to study for the first time stellar oscillations in intermeidate mass stars. Image Credit: Kosovichev et al., 1997 Solar Physics

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The interiors of stars are largely mysterious regions because they are so difficult to observe directly. Our lack of understanding about the physical processes there, like rotation and the mixing of hot gas, introduces considerable ambiguity about how stars shine and how they evolve. Stellar oscillations, detected through brightness fluctuations, offer one way to probe these subsurface regions. In the Sun, these vibrations are due to pressure waves generated by turbulence in its upper layers (the layers dominated by convective gas motions). Helioseismology is the name given to the study of these oscillations in the Sun, and astroseismology is the term used for other stars.

Astronomers have long detected strong brightness variations in other stars, for example the class of Cepheid variable stars used to calibrate the cosmic distance scale, but the small, solar-like oscillations driven by convection near the star's surface are much harder to see. Over the past few decades, space telescopes have successfully applied astroseismology to solar-type stars spanning many stages of stellar life. CfA astronomer Dave Latham was a member of a large team of astronomers who used the new TESS (Transiting Exoplanet Survey Satellite) datasets to study the interiors of the class of intermediate mass stars known as δ Sct and γ Dor stars. These stars are more massive than the Sun but not large enough to burn through their hydrogen fuel very rapidly and die as supernovae. Pulsations generally arise principally from one of two processes, those dominated by pressure (where the gas pressure restores perturbations) or by gravity (where buoyancy does). In these intermediate-mass stars both of these processes can be important, with pulsations having typical periods of roughly about six hours. The complexity of the combined processes, among other things, results in these intermediate-mass stars coming in a veritable zoo of variability types, and this variety offers astronomers more ways to test models of stellar interiors.

The astronomers analyzed TESS data on 117 of these stars using observations taken every two minutes; accurate distances to the stars (and hence accurate luminosities) were obtained from Gaia satellite measurements. The team was able for the first time to test and successfully refine models of pulsation for these stars. They found, for example, that gas mixing in the outer envelope plays an important role. They also spotted many higher-frequency pulsators, thereby identifying promising targets for future studies. Not least, they showed that the TESS mission has unprecedented potential not just for studying exoplanets, but also for improving our understanding of intermediate mass stars.

The First View of δ Scuti and γ Doradus Stars with the TESS Mission ~ V. Antoci et al

• Monthly Notices of the RAS 490(3):4040 (Dec 2019) DOI: 10.1093/mnras/stz2787
• arXiv.org > astro-ph > arXiv:1909.12018 > 26 Sep 2019

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First Results from the Dark Energy Survey
SAO Weekly Science Update | 2020 Jan 17

The Dark Energy Survey uses the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile, seen here. A paper analyzing the first data release finds that cosmic voids have environments whose properties are in good agreement with models, being relatively simple and with emitted light that scales linearly with mass. Credit: Reidar Hahn/Fermilab

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The Dark Energy Survey (DES) program uses the patterns of cosmic structure as seen in the spatial distribution of hundreds of millions of galaxies to reveal the nature of "dark energy," the source of cosmic acceleration. Since it began in 2013, DES has mapped over ten percent of the sky with a digital camera containing 570 million pixels and five optical filters that provide galaxy colors to estimates redshift distances. CfA astronomers are part of a team of over 400 scientists in seven countries working on DES, and last year it released the first set of data.

Cosmic voids occupy most of the volume of the universe. Unlike clusters of galaxies and other dense structures which are strongly affected by gravitational effects, not to mention processes associated with galaxy formation, these voids are the most underdense regions of the universe and have relatively simple dynamics. This makes them particularly straightforward probes for constraining cosmological parameters.

CfA astronomer David James is a member of the DES Collaboration and one of the co-authors on a new paper analyzing the first data release, with the aim of describing the relationship between the mass and light around cosmic voids. The scientists use statistical modeling to analyze both the 2-D distribution of galaxies and their 3-D distribution, the latter obtained from calculating galaxy distances from their photometrically determined redshifts. They find the two methods agree well with each other, and with models in which the physics of void environments is very simple, and in which the amount of emitted light scales directly with the mass. Voids with diameters between about one hundred and six hundred million light-years fit well enough to enable tests of the mass-light relationship to better than ten percent. With future observations, the improved statistics should enable useful new consistency tests of gravity and General Relativity and dark-matter scenarios.

Dark Energy Survey Year 1 Results: The Relationship
between Mass and Light around Cosmic Voids ~ Y. Fang et al

• Monthly Notices of the RAS 490(3):3573 (Dec 2019) DOI: 10.1093/mnras/stz2805
• arXiv.org > astro-ph > arXiv:1909.01386 > 03 Sep 2019 (v1), 11 Nov 2019 (v2)

viewtopic.php?p=273864#p273864

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X-Ray Detection of a Cepheid Binary
SAO Weekly Science Update | 2020 Jan 24

An artist's depiction of XMM-Newton. Astronomers have used this mission to monitor the X-ray light-curve of a peculiar Cepheid variable star, V473 Lyr, and discovered that it has a low mass binary companion star. (Credit: ESA/D. Ducros)

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A Cepheid variable star is one whose particular mass and age (along with some of its other physical parameters) result in brightness oscillations with a precise period proportional to the star's intrinsic luminosity. This extraordinarily useful property of Cepheid variables, discovered and roughly calibrated at Harvard by Henrietta Leavitt starting in 1908, allows them to be used as cosmic distance calibrators. By comparing the intrinsic brightness as determined from the period (which is easily measured) with the measured brightness, the period-luminosity relationship, a precise distance can in principle be obtained. Cepheids in nearby galaxies that are receding from us provide the basis for the famous distance-velocity relationship of galaxies that underpins the expanding model of the universe (the "big bang" model). Cepheids are so important that they have also become benchmarks for testing our understanding of stellar evolution.

Precise, long-term photometry of Cepheids from new satellites like Kepler and CoRoT has alerted astronomers to the complexity of Cepheid behaviors, and in particular to the subset that have multiple intensity periodicities. The pulsation cycle in these stars results in disturbances both in the stellar photosphere and in the hot layer just above it, the chromosphere, producing intensity fluctuations with more than one period.

The Cepheid variable star V473 Lyr, in addition to having the usual, very regular Cepheid variability (in its case, a period of 1.49 days) also has long-term variations whose approximate period is 3.3 years. Its variations, however, do not conform to the patterns found in other Cepheids. CfA astronomers Nancy Evans, Scott Wolk, Sofia Moschou, Jeremy Drake, and Vinay Kashyap and their colleagues used the XMM-Newton X-ray satellite to monitor V473 Lyr. They find that its X-ray brightness seems to be relatively stable over the cycle. They conclude that the long-term X-ray variations are not from the Cepheid itself, but most likely are caused by an orbiting, low-mass companion star, about one solar mass in size at an estimated separation of 30-300 AU (one AU is about the average distance of the Earth from the Sun). They note that X-ray discoveries of binaries like this one will help provide a more complete statistical accounting of Cepheid binary stars. In the case of V473 Lyr, the result also helps confirm that it is sun-like in its origins, being relatively rich in heavy elements like other stars in the galaxy's spiral arms.

X-ray Observations of the Peculiar Cepheid V473 Lyr Identify A Low-Mass Companion ~ Nancy Remage Evans et al

• arXiv.org > astro-ph > arXiv:2001.02253 > 07 Jan 2020

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Driving Massive Galaxy Outflows with Supermassive Blackholes
SAO Weekly Science Update | 2020 Jan 31

A Chandra X-ray Observatory image of the center of the Perseus cluster of galaxies. Astronomers used the ALMA millimeter facility to study AGN quenching mechanisms and the molecular filament structures in twelve galaxy clusters. Credit: NASA/CXC/IoA/A.Fabian et al.

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Active galactic nuclei (AGN) are supermassive black holes at the centers of galaxies that are accreting material onto their hot circumnuclear disks, releasing the energy in bursts of radiation or as particle jets moving at close to the speed of light. These energetic outbursts in turn drive outflows of ionized, neutral, and molecular gas that can extend over thousands of light-years and move at speeds of hundreds of kilometers per second. The gas flows can be launched directly from the hot accretion disc, though radiation pressure on the dust that is mixed in with the gas, by hot thermal winds, or other mechanisms that generate hot bubbles of gas. By driving the gas out of the galaxy, an active nucleus restricts the fuel available for further star formation and slows down the galaxy’s growth. The mechanism is also self-limiting, since it ultimately suppresses gas accreting onto the black hole. Astronomers tracking the rate of star formation across cosmic time believe this process, called quenching, is responsible for the dramatic decline in star formation since the peak of star-formation activity about ten billion years ago.

CfA astronomer Paul Nulsen and his colleagues used new and archival data from the ALMA millimeter facility to study molecular gas outflows in twelve massive galaxies at the centers of galaxy clusters. The hot gas surrounding the galaxies in these massive clusters should cool, fall back onto the galaxies, and produce more new stars, continuing the feedback cycle. The high spatial resolution of the ALMA images, taken in the emission line of carbon monoxide gas, enabled the scientist to investigate the processes in detail, in particular the filamentary structures that characterize most of the gas in these central cluster galaxies. They find that giant molecular filaments and clouds apparently form as the hot bubbles of escaping gas begin to cool, and that these outflows will eventually stall and recirculate in the galaxy. They also identify a trend between the mass of the molecular gas directly around the central AGN and the power of the jet.

Driving Massive Molecular Gas Flows in Central Cluster Galaxies with AGN Feedback ~ H.R. Russell et al

• Monthly Notices of the RAS 490(3):3025 (Dec 2019) DOI: 10.1093/mnras/stz2719
• arXiv.org > astro-ph > arXiv:1902.09227 > 25 Feb 2019 (v1), 25 Sep 2019 (v2)

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The Cosmic Confusion of the Microwave Background
SAO Weekly Science Update | 2020 Feb 07

An image of the South Pole Telescope's 'SPTpol' instruments core, containing 768 pixels and 1536 detectors capable of measuring the polarization of incoming millimeter radiation. The SPT team used SPTpol to determine that the combined polarized radiation from distant galaxies is not strong enough to obscure the search for polarization effects in the cosmic microwave background radiation. Credit: SPT collaboration; DOE

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Roughly 380,000 years after the big bang, about 13.7 billion years ago, matter (mostly hydrogen) cooled enough for neutral atoms to form, and light was able to traverse space freely. That light, the cosmic microwave background radiation (CMBR), comes to us from every direction in the sky, uniform except for faint ripples and bumps at brightness levels of only a few part in one hundred thousand, the seeds of future structures like galaxies.

Astronomers have conjectured that these ripples also contain traces of an initial burst of expansion -- the so-called inflation – which swelled the new universe by thirty-three orders of magnitude in a mere ten-to-the-power-minus-thirty-three seconds. Clues about the inflation should be faintly present in the way the cosmic ripples are curled, an effect that is expected to be perhaps one hundred times fainter than the ripples themselves. CfA astronomers and their colleagues, working at the South Pole, have been working to find evidence for such curling, the "B-mode polarization."

Traces of this tiny effect are not only difficult to measure, they may be obscured by unrelated phenomena that can confuse or even mask it. CfA astronomer Tony Stark is a member of the large South Pole Telescope (SPT) consortium, a collaboration that has been studying galaxies and galaxy clusters in the distant universe at microwave wavelengths. Individual cosmic sources are in general dominated either by active supermassive black hole nuclei and emit radiation from the charged particle jets ejected from the regions around them, or by star formation whose radiation comes from warm dust. The emission is also probably polarized and could complicate the positive identification of CMBR B-mode radiation signals. The SPT team used a new analysis method to study the combined polarization strength of all the millimeter emission sources they find in a 500 square degree field in the sky, about four thousand objects. They conclude - good news for CMBR researchers - that the extragalactic foreground effects should be smaller than any expected B-mode signals, at least over a wide range of spatial scales.

Fractional Polarisation of Extragalactic Sources in the 500 deg² SPTpol Survey ~ N. Gupta et al

• Monthly Notices of the RAS 490(4):5712 (Dec 2019) DOI: 10.1093/mnras/stz2905
• arXiv.org > astro-ph > arXiv:1907.02156 > 03 Jul 2019 (v1), 17 Jan 2020 (v2)

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A Submillimeter Survey of Protostars
SAO Weekly Science Update | 2020 Feb 14

An infrared image of the young star forming complex NGC 1333 in Perseus. A new Submillimeter Array study of protostars in Perseus is the largest and most complete spectral imaging survey of protostars, including six extremely young objects known as first cores. Credit: IRAC/ NASA/JPL-Caltech/R. A. Gutermuth/Harvard-Smithsonian CfA

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The formation of stars involves the complex interactions of many phenomena, including gravitational collapse, magnetic fields, turbulence, stellar feedback, and cloud rotation. The balance between these effects varies significantly between sources, and astronomers have adopted a statistical approach to understand the typical, early-stage star formation sequence. The earliest stage is called the protostellar stage. For low-mass stars (those with masses about that of the sun) this stage is usually separated into two subclasses as the star grows by accreting material from a massive envelope whose size can extend between five hundred and ten thousand astronomical units (AU) in a process that can last roughly half a million years. There are considerable uncertainties, however: some gas is ejected back into the medium in strong outflows, for example.

The lack of a large, systematic survey of such sources has made it hard for astronomers to sort out the multiple processes at play. CfA astronomers Ian Stephens, Tyler Bourke, Mike Dunham, Phil Myers, Sarah Sadavoy, Katherine Lee, Mark Gurwell, and Alyssa Goodman led a team using the Submillimeter Array to compile and publish the largest public, high resolution submillimeter spectral line survey of young protostars. The team observed seventy-four young objects in the Perseus molecular cloud located about one thousand light-years away. The program, called MASSES (Mass Assembly of Stellar Systems and Their Evolution with the SMA), observed the protostars with both high and low spatial resolution, sampling scales from about three hundred AU to more than nine thousand AU in as many as forty molecular lines (although not every source had all lines).

This region had been studied before and was known to have many bipolar protostellar outflows, but the new high-resolution images reveal a wealth of outflow properties, mostly as seen in carbon monoxide gas. The study examined six of these objects that are so young they are not yet hot enough to dissociate their primary constituent gas, molecular hydrogen. These protostars are known as "first cores" and the MASSES program detected outflows in four of them, identifying one as being the most promising example of its type because of its compact nature and slow outflow velocity. This new study, the largest and most complete public survey of its kind, offers astronomers a new database for studying low-mass star formation in its earliest stages.

Mass Assembly of Stellar Systems and Their Evolution with
the SMA (MASSES)—Full Data Release ~ Ian W. Stephens et al

• Astrophysical Journal Supplement 245(2):21 (2019 Dec) DOI: 10.3847/1538-4365/ab5181
• arXiv.org > astro-ph > arXiv:1911.08496 > 19 Nov 2019

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The Radcliffe Wave
SAO Weekly Science Update | 2020 Feb 21

A graphic of the extended molecular cloud structure discovered in the solar neighborhood, as viewed from a position in the Milky Way looking at it sideways. This long sinusoidal feature stretches over about nine thousand light-years, and replaces an older view of a Gould Belt ring of star formation. The red points are the objects that were measured and fitted to a new three-dimensional algorithm. Credit: Alves et al. 2020 Nature

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The Gould Belt is an expanding ring of young stars, gas and dust situated about a thousand light-years from the Sun - which is to say, in our solar neighborhood. It stretches over a few thousand light-years and is thought to subsume many famous nearby nebulae like the one in Orion. The physical relationships between its many gas clouds, if any, have been unknown because the uncertainties in their distances are comparable to or even larger than their dimensions.

CfA astronomers Catherine Zucker, Alyssa Goodman, Josh Speagle, and Doug Finkbeiner and their colleagues used new astrometric surveys like the Gaia mission, coupled with detailed analyses of stellar brightenesses, colors, and distances to construct three-dimensional maps of the Milky Way around the Gold Belt to higher accuracies than ever before. The new distance information reveals for the first time the three-dimensional structure of the clouds in the solar neighborhood, including the presumptive Gould Belt clouds and others like the North American Nebula that had not been previously associated with the Gould Belt. They discovered that the clouds are not quasi-randomly distributed in a ring, as had been thought, but rather form a dramatic, elongated structure stretching about nine thousand light-years in length. The formation includes the majority of nearby star-forming regions, has an aspect ratio of about twenty-to-one, and contains about three million solar masses of gas. Most remarkably, this structure appears to be undulating, with a three-dimensional shape well described by a damped sinusoidal wave on the plane of the Milky Way with an average period of about seven thousand light-years and a maximum amplitude of about five hundred light-years.

The astronomers have named the structure the Radcliffe Wave in honor of both the early-20th-century female astronomers from Radcliffe College and the interdisciplinary spirit of the current Radcliffe Institute, which contributed to the discovery. The origin of the Radcliffe Wave is unclear but the scientists speculate that it could be the outcome of a large-scale galactic process, perhaps a shock front in the galaxy's nearby spiral arm. Why it should be such a well-defined wave, however, is even harder to explain; possibly it implies some sort of collision. The Radcliffe Wave provides a new framework for understanding molecular cloud formation and evolution. The team concludes by calling for a revision in the architecture of the gas in the solar neighborhood and a re-interpretation of other local structural phenomena.

A Galactic-Scale Gas Wave in the Solar Neighbourhood ~ João Alves et al

• Nature 578(7794):237 (13 Feb 2020) DOI: s41586-019-1874-z

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Ultrared, Dusty Star-Forming Galaxies in the Early Universe
SAO Weekly Science Update | 2020 Feb 28

An IRAC/Spitzer infrared image of a field containing an ultrared luminous galaxy
whose light has been traveling towards us for about 12 billion years. Astronomers
have completed detailed studies of three hundred ultrared luminous star-forming
galaxies in the early universe, twenty-three of them gravitationally lensed galaxies.
Credit: J. Ma et al., 2020 ApJ

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Star formation takes place within natal clouds of dust and gas that absorb much of the emitted ultraviolet and optical radiation but which also block these regions from optical view. In recent decades, however, infrared space-based observatories like Herschel and Spitzer have revolutionized our understanding of obscured star formation in dusty galaxies because infrared light can penetrate the dust clouds to reveal the stars being formed. Herschel and Spitzer have discovered large numbers of very dusty, very red star-forming galaxies that are immensely luminous in the infrared (exceeding one trillion solar-luminosities) yet are not seen at shorter wavelengths. In fact, these dusty galaxies are responsible for most of the infrared background light in the cosmos. Some of these objects display the most extreme kinds of starbursts known, with star formation rates exceeding a thousand per year, but which are also exceedingly rare with on average only one of them in a volume of a few hundred thousand million cubic light-years.

The Herschel mission, exploring the sky in far-infrared wavelength bands where the dust emission peaks, discovered thousands of candidate dusty galaxies. CfA astronomer Matt Ashby was a member of a large team of astronomers who helped to characterize these galaxies more fully. The team identified a set of three hundred "ultrared" galaxies (that is, brightest at the longest infrared wavelengths) and that had also been observed at shorter infrared wavelengths by the IRAC camera on Spitzer. The team gathered additional submillimeter and millimeter data to fully assess these galaxies' output, and spectra to determine their distances and luminosities. The most distant galaxy they found is from the epoch about one billion years after the big bang (a redshift of 6.02); it is one of twenty-three sources in the study that are confirmed to be gravitationally lensed.

The astronomers conclude that these ultrared galaxies, although they include some of the most luminous and massive galaxies known, are too rare to represent the star-forming progenitors of local, quiescent galaxies; other kinds of galaxies will have to fill that role. But the new study has identified the most extreme cases, and further investigations of these monsters will help determine how extreme star formation in the universe operates.

Spitzer Catalog of Herschel-Selected Ultrared Dusty Star-Forming Galaxies ~ Jingzhe Ma et al

• Astrophysical Journal Supplement 244(2):30 (2019 Oct) DOI: 10.3847/1538-4365/ab4194
• arXiv.org > astro-ph > arXiv:1908.08043 > 21 Aug 2019

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Dark Matter and Massive Galaxies
SAO Weekly Science Update | 2020 Mar 06

A dark matter map, created by Japanese astronomers using weak lensing. The background image of a wide field of galaxies was analyzed for weak lensing effects and the inferred dark matter distribution is indicated with the contours. Credit: Satoshi Miyazaki

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About eighty-five percent of the matter in the universe is in the form of dark matter, whose nature remains a mystery, and the rest is of the kind found in atoms. Dark matter exhibits gravity but otherwise does not interact with normal matter, nor does it emit light. Astronomers studying the evolution of galaxies find that because it is so abundant dark matter does, however, dominate the formation in the universe of large-scale structures like clusters of galaxies.

Despite being hard to detect directly, dark matter can be traced by modeling sensitive observations of the distributions of galaxies across a range of scales. Galaxies generally reside at the centers of vast clumps of dark matter called haloes because they surround the galaxies. Gravitational lensing of more distant galaxies by foreground dark matter haloes offers a particularly unique and powerful probe of the detailed distribution of dark matter. "Weak lensing" results in modestly yet systematically deforming shapes of background galaxies and can provide robust constraints on the distribution of dark matter within the clusters; "strong lensing," in contrast, creates highly distorted, magnified and occasionally multiple images of a single source.

In the past decade, observations and hydrodynamic simulations have significantly furthered our understanding of how massive galaxies develop, with a two-phase scenario now favored. In the first step, the massive cores of today’s galaxies form at cosmological times from the gravitational collapse of matter into a galaxy, together with their surrounding dark matter halo. Star-formation then boosts the stellar mass of the galaxy. The most massive galaxies, however, have a second phase in which they capture stars from the outer regions of other galaxies, and once their own star formation subsides this phase dominates their assembly. Computer models and some observational results appear to confirm this scenario. ...

Weak Lensing Reveals a Tight Connection between Dark Matter Halo Mass
and the Distribution of Stellar Mass in Massive Galaxies ~ Song Huang et al

• Monthly Notices of the RAS 492(3):3685 (Mar 2020) DOI: 10.1093/mnras/stz3314
• arXiv.org > astro-ph > arXiv:1811.01139 > 03 Nov 2018

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Free-Floating Stars in the Milky Way's Bulge
SAO Weekly Science Update | 2020 Mar 27

Hubble Space Telescope images of a microlens system. The image on the left was
taken 3.7 years after an observed microlensing event; the one on the right was
taken 8.9 years later after the moving foreground (lensing) source had changed
position. The lens and source components (A and B) are clearly resolved in the
later image. Credit: NASA/Hubble

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The path of a light beam is bent by the presence of mass, as explained by General Relativity. A massive body can therefore act like a lens - a so called "gravitational lens" – to distort the image of an object seen behind it. Microlensing is a related phenomenon: a short flash of light is produced when a moving cosmic body, acting as a gravitational lens, modulates the intensity of light from a background star as it fortuitously passes in front of it. About fifty years ago scientists predicted that if it ever became possible to observe a microlensing flash from two well-separated vantage points, a parallax measurement would pin down the distance of the dark object. The Spitzer Space Telescope, orbiting the Sun at the distance of the Earth but trailing behind the Earth by about one-quarter of the orbital path, had been working with ground-based telescopes to do just that until it was shut down last month by NASA as a cost-savings measure.

CfA astronomer Jennifer Yee is a member of a large international team of astronomers making parallax microlensing measurements of small stellar objects. The technique is a powerful tool for probing isolated objects like free-floating planets, brown dwarfs, low-mass stars, and black holes. At the low-mass end, microlensing has already detected several free-floating planet candidates including several possible Earth-mass objects. Such discoveries are crucial for testing theories about the origin and evolution of free-floating planets. Similarly, microlensing observations of more massive objects like like isolated brown dwarf stars have identified some objects orbiting in a sense opposite to that of normal disk stars. Stellar-mass sized objects found via microlensing reveal stellar-mass black holes and neutron stars. ...

Spitzer Microlensing Parallax Reveals Two Isolated Stars in the Galactic Bulge ~ Weicheng Zang et al

• Astrophysical Journal 891(1):3 (2020 Mar 01) DOI: 10.3847/1538-4357/ab6ff8
• arXiv.org > astro-ph > arXiv:1904.11204 > 25 Apr 2019

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Gas Motions in Interstellar Cores Forming Low-Massive Stars
SAO Weekly Science Update | 2020 Mar 20 (???)

A IRAC infrared image of the massive star forming complex NGC 6334, with an arrow pointing to a cold, dark filament in which forty-nine dense, pre-stellar cores have been observed. Astronomers have found that, contrary to some expectations and earlier results, in these low-mass cores the turbulent gas velocities are not supersonic and not able to prevent gravitational collapse. Credit: NASA/JPL-Caltech/IRAC/SpItzer

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Astronomers have long realized that the process of making new stars is very inefficient. In the Milky Way the overall efficiency is only about 5% as measured by the mass in stars compared to the total mass of the galaxy. Most simple models of star formation predict that gravity should be much more effective in forming stars as it compresses the gas in molecular clouds. Various solutions have been proposed, including the disruption of molecular clouds by newly formed massive stars with powerful winds and jets, and by supersonic gas motions (generated, for example, in intercloud collisions) that can help support the cloud against collapse. Indeed, numerous observational results appear to have confirmed the presence of supersonic gas motions, at least in regions forming high-mass stars.

CfA astronomers Shanghuo Li, Qizhou Zhang, Howard Smith, and Joe Hora led a team that used the ALMA facility to study star formation in a massive, dark filamentary cloud in the complex known as NGC 6334. They observed forty-nine small, dense cores along the filament – putative sites of future low mass stars – in the emission from two molecular species that sample relatively high density, low temperature regions, H¹³CO⁺ and NH₂D. The cores themselves ranged in mass from about 0.17 to 14 solar-masses. Key to the success of the project was ALMA's excellent spatial resolution, which at the distance of these clouds is 0.07 light-years, much smaller than typical in this research area, enabling the scientists to carefully measure the individual cores without contamination from adjacent material.

Images of the dark, filamentary cloud in the light of these two molecules revealed the presence of the string of dense cores, and velocity measurements revealed that the gas motions with in them were not supersonic. The authors conclude that the majority of the structures are gravitationally unstable and likely to collapse into stars. They caution that the earlier conclusions about supersonic turbulence may have been biased by using larger telescope beams that included the motions of gas not associated with individual star forming cores. ...

ALMA observations of NGC 6334S − I: Forming massive stars and cluster
in subsonic and transonic filamentary clouds ~ Shanghuo Li et al

• arXiv.org > astro-ph > arXiv:2003.13534 > 30 Mar 2020

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Forming the TRAPPIST-1 Exoplanets
SAO Weekly Science Update | 2020 Apr 03

An artist's schematic of the TRAPPIST-1 system of exoplanets with a comparison to the inner solar system planets. Astronomers looking for evidence of a TRAPPIST-1 Kuiper Belt and clues to whether or not its comets might have been available to carry water to the inner set of planets found no evidence for one but were able to set a limit to its size and mass. Credit: NASA/JPL-Caltech

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TRAPPIST-1 is a system of seven Earth-sized worlds orbiting an ultra-cool dwarf star about forty light-years away. The IRAC camera on Spitzer was used to help discover these seven Earth-sized planets orbiting the star, at least three of them lying in the star's habitable zone. The star, and hence its system of planets, is thought to be about eight billion years old, almost twice as old as our own solar system. For scientists seeking evidence for life elsewhere, the advanced age provides more time for chemistry and evolution to operate than the Earth had. On the other hand, the planets are all close to the star (in fact they are probably tidally locked to the star with one side always facing it), and consequently would have soaked up billions more year's-worth of high energy radiation from the star’s winds, perhaps adversely affecting any atmospheres they host.

Nevertheless, the three planets in the habitable zone could have liquid water if they formed with the right composition and/or if water was subsequently deposited on their surfaces. The Kuiper Belt in our solar system is an orbiting disk of comets and small objects that extends roughly from Neptune out to fifty AU from the Sun (one AU being the average Earth-Sun distance). It is thought that comets brought water to the Earth during its youth, and comets in TRAPPIST-1's Kuiper belt – if there are any – might provide a way to deposit water onto its seven planets. With the right atmospheric conditions, the three planets in the habitable zone might even have liquid water on their surfaces. ...

Searching for a Dusty Cometary Belt around TRAPPIST-1 with ALMA ~ S. Marino et al

• Monthly Notices of the RAS 492(4):6067 (Mar 2020) DOI: 10.1093/mnras/staa266
• arXiv.org > astro-ph > arXiv:1909.09158 > 19 Sep 2019 (v1), 27 Jan 2020 (v2)

[bystander]

VLASS, A Survey of the Radio Sky
SAO Weekly Science Update | 2020 Apr 10

The radio source 3C402. The greyscale background is an optical image of the field while the contours show earlier radio imaging results. The insets are new radio images from VLASS that show the previous radio source is actually two separate galaxies. Credit: VLASS; Lacy et al. 2020 PASP

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Technological advances in recent years have increased the sensitivity of radio interferometers like the Karl G. Jansky Very Large Array (VLA) to the radio emission from astronomical sources in their continuum (not only in their lines) by factors of several, enabling them to see fainter and more distant objects. Radio interferometers obtain high spatial resolution details of astronomical sources, and the new VLA, in addition to its sensitivity and high resolution, can provide information about the polarization of the emission, enable more reliable large-scale mosaic images, and with repeating observations monitor temporal variations. Not least, a series of recent sensitive sky surveys at optical and infrared wavelengths justify completing a corresponding radio survey. When combined, these multi-wavelength all-sky surveys will permit astronomers to characterize stellar and galaxy populations in unprecedented detail.

CfA astronomers Edo Berger, Atish Kamble, and Peter Williams are members of the VLASS (The Very Large Array Sky Survey) team, a large group working on a unique radio all-sky survey having all the aforementioned capabilities and able to cover all of the sky visible from the VLA location in New Mexico. VLASS science has four themes: finding otherwise hidden explosions and/or transient events, probing astrophysical magnetic fields, imaging galaxies both near and distant, and using radio wavelengths to peer through dust obscuration effects to study the Milky Way. Each theme contains numerous subtopics. Hidden explosions, for example, will probe the explosive death throes of massive stars including supernovae, their role in cosmological studies, gamma-ray bursts; signs of mergers between black holes and neutron stars will have implications for gravitational wave detections.

VLASS observations, begun in September 2017, are expected to be completed in 2024. In a new paper, the team reviews the VLASS goals and first-look results from early observations, showing how the data successfully demonstrate the ability of the project to achieve all its proposed goals. VLASS includes an integral education and outreach component with two workshops on data visualization held in the first year to train users to produce images that are aesthetic as well as scientifically accurate. The first preliminary data and materials are now available to scientists and the public.

The Karl G. Jansky Very Large Array Sky Survey (VLASS).
Science Case and Survey Design ~ M. Lacy et al

• Publications of the ASP 132(1009):035001 (Mar 2020) DOI: 10.1088/1538-3873/ab63eb
• arXiv.org > astro-ph > arXiv:1907.01981 > 03 Jul 2019 (v1), 30 Dec 2019 (v3)

[bystander]

The Masses of Supermassive Black Holes
SAO Weekly Science Update | 2020 Apr 17

The galaxy NGC 5548 has a supermassive black hole at its nucleus (a supermassive black hole has a mass exceeding a million solar-masses). Astronomers had reported that the masses of nuclear black holes seem to correlate with the stellar masses of their host galaxies, suggesting that they somehow evolve together, but these studies were based primarily on galaxy averages. A new analysis of the X-ray emission of individual galaxies concludes that that nuclear black holes and stellar masses do indeed evolve together, and that some earlier conclusions were biased because of selection effects. Credit: ESA/Hubble and NASA. Acknowledgement: Davide de Martin

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Most galaxies are thought to host at its nucleus a supermassive black hole (SMBH), an object with a mass exceeding a million solar-masses. Our Milky Way, for example, has a four million solar-mass black hole at its center, and the most extreme examples are estimated to have as much as ten billion solar-masses. Both active galaxies and inactive galaxies have SMBHs, but the former are actively accreting material and radiating from the hot environment. The masses of these monsters are usually measured directly from the kinematics of gas or stars moving under the strong gravitational presence of the nucleus. Indirect measurements can also be made from ancillary correlations that have been found; for example, the masses of SMBHs appear to be tightly correlated with galaxies' stellar masses and with the spread of motions ("velocity dispersion") seen in the galactic hosts. Since black holes usually grow over time, these correlations suggest that there is some kind of co-evolution with the galaxy but what that is and how it evolves is not understood. Inactive galaxies, for example, sometimes show a different correlation than active ones; perhaps this is due to selection biases. Some scientists have argued that the correlation with stellar mass is just a by-product of the more fundamental connections with velocity dispersion.

CfA astronomer Francesca Civano is a member of a team of astronomers that used data from the Chandra X-Ray Observatory and other X-ray missions to probe the key issue of whether observational selection effects result in the appearance of a correlation. The limited capabilities of modern telescopes, for example, inevitably favor galaxies whose gas and stars have the largest motions, and computer simulations have shown that this effect alone could account for the appearance of a correlation. The scientists instead looked at the X-ray luminosity of a sample of galaxies, a measure of the accretion onto their SMBHs, which in turn is a measure of their masses and their efficiency of producing radiation. The astronmers' technique uses X-ray results from individual galaxies to obtain masses and is more reliable than similar, older attempts that used combined X-ray averages.

The astronomers find that the stellar masses of galaxies and their nuclear SMBHs do appear to grow together, and that this relation is nearly independent of galaxy epoch as far back as about ten billion years. This result provides independent evidence that earlier correlations were biased by selection effects, at least for those derived from kinematics. The team reports one surprise: the efficiency of accretion radiation is apparently about fifteen percent, nearly ten times higher than expected from theory, and implies that the black holes are spinning rapidly since spining black holes should be more efficent radiators.

Probing black hole accretion tracks, scaling relations, and radiative
efficiencies from stacked X-ray active galactic nuclei ~ Francesco Shankar et al

• Monthly Notices of the RAS 493(1):1500 (Mar 2020) DOI: 10.1093/mnras/stz3522
• arXiv.org > astro-ph > arXiv:1912.06153 > 12 Dec 2019

[bystander]

Ionizing the Universe with Oligarchs
SAO Weekly Science Update | 2020 Apr 24

A Hubble image of the starburst galaxy M82. Astronomers have concluded that during the early universe, the reionization of the gas in the intergalactic medium was probably done by ultraviolet light emitted by the star formation in very massive starburst galaxies. Credit: NASA, ESA, Hubble Heritage (STScI/AURA)

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The sparsely distributed hot gas found today between galaxies, the intergalactic medium (IGM), is ionized. The early universe started off hot, but then it rapidly expanded and cooled allowing its main constituent, hydrogen, to combine to form neutral atoms. When and how did these neutral atoms become reionized to compose the IGM we see today? Astronomers think that ultraviolet radiation emitted by massive young stars did this work once stars began to form and shine during the cosmic era named after this activity, the "era of reionization."

One of the key steps in the reionizing of the IGM it the ultraviolet radiation’s escape from galaxies into the IGM, but this is not well understood. Astronomers know only that it would have had to have been efficient because only if the fraction escaping were high enough could starlight have done the job. Star forming galaxies, however, are rich in dense molecular gas and dust, and that dust also absorbs much of the uv radiation. That suggests that some other significant source of ionizing radiation is required, and speculation has included the possible existence of exotic objects like faint quasars, X-ray binary stars, or perhaps even decaying/annihilating particles. There is, however, little evidence so far that any of these are abundant enough or capable of doing the job.

CfA astronomers Rohan Naidu, Sandro Tacchella, Charlotte Mason, Sownak Bose, and Charlie Conroy led an effort to better estimate the most uncertain parameter in this puzzle (and the one most difficult to measure directly): the escape fraction of ionizing photons. They compare measurements and models of the two other key processes involved, the star formation rate in galaxies and the number of uv photons produced. They apply these to constrain what the escape fraction would have to have beeen in order to make the modeling consistent. The measurements are uncontroversial, but the models differ and the scientists selected from two types: those in which the escape fraction is constant during the epoch of reionization and those in which it depends on the star formation rate. ...

Rapid Reionization by the Oligarchs: The Case for Massive, UV-bright,
Star-forming Galaxies with High Escape Fractions ~ Rohan P. Naidu et al

• Astrophysical Journal 892(2):109 (2020 Apr 01) DOI: 10.3847/1538-4357/ab7cc9
• arXiv.org > astro-ph > arXiv:1907.13130 > 30 Jul 2019 (v1), 03 Apr 2020 (v2)

[bystander]

Potentially Active Asteroids
SAO Weekly Science Update | 2020 May 01

An IRAC/Spitzer image of extended emission from the dormant comet Don Quixote, showing a diffuse, elongated feature centered on the saturated object (the saturated bright core produces the well-known bright column artifact seen). Astronomers have studied eighty-three candidate NEOs, none of which showed extended emission. Credit: NASA/IRAC/Spitzer, Mommert et al. 2014 ApJ

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Small objects in the solar system, including both comets and asteroids, have a wide range of dynamical parameters and physical properties. Comets, for example, originate in the outer reaches of the solar system and have icy surfaces that evaporate when approaching the Sun, producing characteristic tails and comas. Asteroids on the other hand lack these volatiles and do not show such behavior. In recent decades, however, this previously strict division between active and inert bodies has blurred as sensitive infrared and other detectors have discovered that objects nominally categorized as asteroids can show signs of comet-like activity.

CfA astronomers Joe Hora and Howard Smith are members of a team that has been using the IRAC infrared camera on Spitzer to study Near Earth Objects, small solar system bodies whose orbits sometimes bring them close to the Earth. Besides being able to measure the surface properties of NEOs, the infrared sensors are also able to spot faint emission from evaporating volatiles. Several years ago the team discovered, for example, that the NEO named Don Quixote (the third largest known NEO), previously thought to be quiescent, was in fact a dormant comet, faintly active with a coma and a tail detectable in the infrared. The result had direct implications for the origins and evolution of the population of NEOs, including objects that follow asteroidal (not cometary) orbits, a group referred to as “Active Asteroids”.

Astronomers are trying to understand whether or not, despite their very different dynamical properties, this class has extended emission as a result of the same mechanisms active in comets; such processes include not only the sublimation of volatiles but also mass shedding through spin-up, impacts, and/or thermal fracturing. The variety of possibilities makes it hard to predict which NEOs might show activity without further information. ...

Systematic Characterization of and Search for Activity in Potentially Active Asteroids ~ Michael Mommert et al

• Planetary Science Journal 1(1):10 (2020 June) DOI: 10.3847/PSJ/ab8191

[bystander]

Measuring the Central Gas in a Cosmic Cooling Flow
SAO Weekly Science Update | 2020 May 08

The NGC 5044 galaxy group, the brightest X-ray group in the sky, viewed in a variety of wavebands from low-frequency radio to X-ray. Astronomers studying the 'cooling flows' that develop in galaxy clusters as the very hot X-ray-emitting gas cools have used a variety of telescopes and spatial resolutions to trace accurately cooling flows through the distribution and motion of carbon monoxide. Credit: E. O'Sullivan and ESA/XMM-Newton

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Our Milky Way is a member of a cluster of galaxies, the co-called "Local Group," about fifty galaxies whose other large member is Andromeda about 2.3 million light-years away. Most galaxies lie in clusters, and the closest large cluster of galaxies to us is the Virgo Cluster with about 2000 members. The space between galaxies is not empty, but is filled with hot ionized gas, X-ray emitting material whose temperature is of order ten million kelvin, or even higher.

The development of clusters is a key feature of galaxy evolution but our understanding remains remarkably incomplete. One problem relates to the fate of its hot, X-ray emitting gas. In the cores of clusters the gas is heated to high temperatures as it falls toward the cluster center and by some other processes. The mystery is why this hot gas does not cool more efficiently and sink towards the center of the cluster where it might trigger more star formation. The common surmise has been that outflowing jets from supermassive black holes, or perhaps other kinds of feedback, inhibit the formation of such "cooling flows," but establishing the details of this mechanism has been elusive.

The galaxy cluster NGC 5044 is one of the brightest X-ray groups in the sky, and has been extensively studied. Its central region is known to contain about one hundred million solar-masses of cool molecular gas as determined from observations of carbon monoxide (CO) gas. These CO results, however, were obtained either with very low spatial resolution (“single dish”) telescopes that measured the total gas content or with the very high spatial resolution ALMA array that was primarily effective for seeing small substructures (dimensions on the order of fifty light-years at the distance of NGC 5044). These earlier results not only missed the gas in intermediate scale structures, essential to understanding how cooling flows behave, but also flagged a problem: the cold gas measured overall was more than double that seen in small substructures, implying that a large fraction was unaccounted for. ...

Atacama Compact Array Measurements of the Molecular Mass
in the NGC 5044 Cooling Flow Group ~ Gerrit Schellenberger et al

• Astrophysical Journal 894(1):72 (2020 May 01) DOI: 10.3847/1538-4357/ab879c
• arXiv.org > astro-ph > arXiv:2004.01717 > 03 Apr 2020

[bystander]

An Eccentric Hot Neptune
SAO Weekly Science Update | 2020 May 15

The MEarth-South array of eight 40 cm telescopes with cameras sensitive to optical and
near-infrared light. Observations of the hot Neptune, K2-25, with MEarth, IRAC/Spitzer,
and Kepler were used to try to confirm whether or not this exoplanet migrated in to its
present location after being born in the cold, outer regions of the system.
Credit: The MEarth Project

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Of the roughly 4300 exoplanets confirmed to date, about ten percent of them are classified as "hot Jupiters." These are planets with masses between about 0.4 and 12 Jupiter-masses and orbital periods less than about 110 days (implying that they orbit close to their star – usually much closer than Mercury is to the Sun - and have hot surface temperatures). A "hot Neptune" has a smaller mass, closer to that of Neptune which is about twenty times less than Jupiter, and which also orbits close to its star. Astronomers study not only the properties of exoplanets but also how they evolved within their planetary systems. Hot Jupiters and hot Neptunes are puzzles. They are expected to have formed much farther out in the cold reaches of their systems as did the giant planets in our Solar System and then to have migrated inward to their current, close locations. Evidence supporting this evolutionary history should be found in the planets’ orbital eccentricities and other clues, but is difficult to obtain.

CfA astronomers Jonathan Irwin, David Charbonneau and Jennifer Winters were members of a team that probed the evolution of the hot Neptune K2-25, a transiting exoplanet with an orbital period of only 3.48 days, an estimated mass of roughly about seven Earth-masses, and a highly eccentric orbit (value of 0.27; its maximum distance from the star exceeds its minimum distance by about 70%). K2-25 has the advantage of being in a young stellar cluster whose age is well-constrained at about 650 million years. This young age tests whether there is time for the migration mechanism to work, whether or not such a process could leave the planet with its large observed eccentricity, and not least, whether such a young host star might be active enough to have complicated the dataset with starspots (the star itself is seen to rotate in 1.88 days). ...

The Young Planetary System K2-25: Constraints on Companions and Starspots ~ Isabel J. Kain et al

• Astronomical Journal 159(3):83 (2020 Mar) DOI: 10.3847/1538-3881/ab655b
• arXiv.org > astro-ph > arXiv:1912.05552 > 11 Dec 2019

[bystander]

Ultraviolet Emitting Galaxies in the Early Universe
SAO Weekly Science Update | 2020 May 22

An multiwavength image of a Lyman alpha galaxy. Astronomers have studied 3700 such galaxies in the early universe to determine how much of the hydrogen emission is produced by star formation and how much by an accreting black hole. In this image yellow is the hydrogen gas glowing in Lyman-alpha; white shows the main galaxy; red is infrared as seen with the IRAC camera; and blue is X-ray emission showing evidence for a supermassive black hole. X-ray (NASA/CXC/Durham Univ./D.Alexander et al.); Optical (NASA/ESA/STScI/IoA/S.Chapman et al.); Lyman-alpha Optical (NAOJ/Subaru/Tohoku Univ./T.Hayashino et al.); Infrared (NASA/JPL-Caltech/Durham Univ./J.Geach et al.

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Massive galaxies in the local universe, in order to be large today, probably began forming their stars in the early universe. Astronomers do indeed see significantly enhanced star-formation activity in distant galaxies and find that the peak star formation rate occurred when the universe was only about two billion years old. Galaxies actively making stars naturally produce many that are massive and hot. They emit ultraviolet radiation that ionizes hydrogen gas which in turn radiates in characteristic spectral lines; the most energetic of these hydrogen features, the Lyman alpha line, itself lies in the ultraviolet. Since galaxies are expanding away from us, however, their apparent spectrum is shifted to the red and their Lyman alpha line is shifted to visible wavelengths or longer, where optical instruments can detect it.

Lyman alpha emitters ("LAEs") are galaxies or clusters of galaxies that are bright in this hydrogen feature, and astronomers piecing together the star formation history of the universe use LAEs to trace the cosmic evolution of matter. There is one major complicating factor however: supermassive black holes at galaxy centers accrete matter (AGN: active galactic nuclei) and also produce copious amounts of ultraviolet radiation and hence Lyman alpha light. An accurate accounting of star formation using ultraviolet light has to account for the effects of black hole accretion.

CfA astronomer Andra Stroe was a member of a team that examined the X-ray and radio activity of about 3700 LAEs to search for and quantify their AGN contributions to the radiation using the distinctive character of accretion radiation at these wavelengths. Their set of objects dates from the epochs between one and two billion years after the big bang, and was selected from deep optical and near infrared images, Chandra X-ray observations, and radio data from a Very Large Array survey. They found that 6.8% of the LAEs were also detected in X-rays and hence could be classified as AGN, and that the luminosity of the X-rays correlated with the Lyman alpha strength in these objects suggesting that the hydrogen emission was AGN produced. ...

The X-ray and Radio Activity of Typical and Luminous Ly α Emitters
from z ∼ 2 to z ∼ 6: Evidence for a Diverse, Evolving Population ~ João Calhau et al

• Monthly Notices of the RAS 493(3):4331 (Apr 2020) DOI: 10.1093/mnras/staa476
• arXiv.org > astro-ph > arXiv:1909.11672 > 25 Sep 2019 (v1), 24 Feb 2020 (v2)

viewtopic.php?t=17042

[bystander]

Galactic Star Formation and Supermassive Black Hole Masses
SAO Weekly Science Update | 2020 May 29

A simulation of the stellar content of the universe today seen across one hundred million light-years. Astronomers used this simulation to investigate how accretion onto a supermassive black hole quenches galaxy star formation. Credit: IllustrisTNG

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Astronomers studying how star formation evolved over cosmic time have discovered that quiescent galaxies (galaxies that are currently not making many new stars) frequently have active galactic nuclei. These AGN accrete material onto hot circumnuclear disks, and the resultant energy is released in bursts of radiation, or as jets of particles moving at close to the speed of light. The suspicion is that these outbursts drive gas outflows over thousands of light-years, disrupting and dispersing potential star forming material in a process called quenching. The quenching mechanism is in addition a self-limiting one since the dispersion ultimately suppresses the gas accretion onto the black hole itself. There are other proposed mechanisms for quenching however: supernovae produced during star formation could be responsible (or at least an important contributor) as could strong stellar winds. Verifying these various alternatives is hence a key goal of galactic research.

CfA astronomers Bryan Terrazas, Rainer Weinberger and Lars Hernquist and their colleagues used the large-scale hydrodynamic simulation called IllustrisTNG to trace the development of galaxies and their black holes, in particular to investigate the correlations between black hole feedback and the suppression of star formation. Although the details of black hole accretion are still only sketchily understood, the simulation allows scientists to vary many input parameters of the simulation to test a range of alternatives. ...

The Relationship between Black Hole Mass and Galaxy Properties:
Examining the Black Hole Beedback Model in IllustrisTNG ~ Bryan A. Terrazas et al

• Monthly Notices of the RAS 493(2):1888 (Apr 2020) DOI: 10.1093/mnras/staa374
• arXiv.org > astro-ph > arXiv:1906.02747 > 06 Jun 2019

[bystander]

A New Catalog of Infrared Dark Clouds
SAO Weekly Science Update | 2020 Jun 05

A false-color infrared image of the Infrared Dark Cloud called 'the Snake' as seen
by the IRAC camera on the Spitzer Space Telescope. Astronomers have produced
a new catalog of IRDCs from IRAC's sky survey images using a new computational
search-and-detection algorithm. (Blue dots are stars relatively undimmed by dust,
while the red dots are young stars embedded in the cloud.)
Credit: NASA, JPL-Caltech/S. Carey (SSC/Caltech)

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Infrared dark clouds (IRDCs) are dark patches of cold dust and gas seen in the sky against the bright diffuse infrared glow of warm dust in our galaxy. These IRDCs, massive and rich in molecules, are natural sites for star birth - one of the main reasons why astronomers are actively studying them. IRDCs were first detected by two early space infrared missions, the Infrared Space Observatory and the Midcourse Space Experiment, but the IRAC camera on Spitzer revolutionized the field with its dramatically enhanced sensitivity and spatial resolution. IRAC had completed several surveys of the Milky Way before being shut off last February, and astronomers have been using the infrared images to identify and analyze the characteristics of IRDCs. The Submillimeter Array and ALMA facilities, operating with high sensitivity and resolution at submillimeter wavelengths where the cold molecular gas can be characterized, have enabled astronomers to follow up on these newly discovered sources and to determine the gas temperatures, densities and motions, leading to advances in our understanding of the earliest stages of star formation in these stellar nurseries.

One issue constraining research has been the lack of an up-to-date catalog of IRDCs. There are three main difficulties: IRDC sizes can vary greatly, from those that are very extended (more than a light-year in size) to others that are over one hundred times smaller (and of course their distances are key to their angular appearances), their shapes are usually irregular and ill defined, and not least they often are located in complex regions with hundreds of other sources. Searching the Milky Way for them has been a daunting task. ...

A Semi-Automated Computational Approach for Infrared Dark Cloud
Localization: A Catalog of Infrared Dark Clouds ~ Jyothish Pari, Joseph L. Hora

• Publications of the ASP 132(1011):054301 (May 2020) DOI: 10.1088/1538-3873/ab7b39
• arXiv.org > astro-ph > arXiv:2003.01122 > 02 Mar 2020

[Ann]

The 'Snake' among dust clouds in the Milky Way. The red dot in the 'belly' of the snake is a still-developing young dust-choked star 20 to 50 times as massive as the Sun. Photo: Credit: NASA, JPL-Caltech/S. Carey (SSC/Caltech).

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Jet Propulsion Laboratory wrote:

This infrared image from NASA's Spitzer Space Telescope shows what astronomers are referring to as a "snake" (upper left) and its surrounding stormy environment. The sinuous object is actually the core of a thick, sooty cloud large enough to swallow dozens of solar systems. In fact, astronomers say the "snake's belly" may be harboring beastly stars in the process of forming.
...
Yellow and orange dots throughout the image are monstrous developing stars; the red star on the "belly" of the snake is 20 to 50 times as massive as our sun.

Ann

[bystander]

Measuring the Spin of a Black Hole
SAO Weekly Science Update | 2020 Jun 12

An artist's impression of a spinning black hole surrounded by an accretion disc. Astromers have use X-ray spectroscopy to measure the spin of a solar-mass black hole in the Milky Way. Credit: ESO, ESA/Hubble, M. Kornmesser

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A black hole, at least in our current understanding, is characterized by having "no hair," that is, it is so simple that it can be completely described by just three parameters, its mass, its spin, and its electric charge. Even though it may have formed out of a complex mix of matter and energy, all other details are lost when the black hole forms. Its powerful gravitational field creates a surrounding surface, a "horizon," and anything that crosses that horizon (even light) cannot escape. Hence the singularity appears black, and any details about the infalling material are also lost and digested into the three knowable parameters.

Astronomers are able to measure the masses of black holes in a relatively straightforward way: watching how matter moves in their vicinity (including other black holes), affected by the gravitational field. The charges of black holes are thought to be insignificant since positive and negative infalling charges are typically comparable in number. The spins of black holes are more difficult to determine, and both rely on interpreting the X-ray emission from the hot inner edge of the accretion disk around the black hole. One method models the shape of the X-ray continuum, and it relies on good estimates of the mass, distance, and viewing angle. The other models the X-ray spectrum, including observed atomic emission lines that are often seen in reflection from the hot gas. It does not depend on knowing as many other parameters. The two methods have in general yielded comparable results. ...

The spin measurement of the black hole in 4U 1543-47
constrained with the X-ray reflected emission ~ Yanting Dong et al

• Monthly Notices of the RAS 493(3):4409 (Apr 2020) DOI: 10.1093/mnras/staa606
• arXiv.org > astro-ph > arXiv:2002.11922 > 27 Feb 2020