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  • richardmitnick 11:50 am on March 19, 2019 Permalink | Reply
    Tags: , AGN's - Active galactic nuclei, , , “Missing Gamma-Ray Halos and the Need for New Physics in the Gamma-Ray Sky”, , , High-energy gamma-ray photons,   

    From AAS NOVA: ” Missing Halos in the High-Energy Sky” 

    AASNOVA

    From AAS NOVA

    18 March 2019
    Susanna Kohler

    1
    This composite image reveals Centaurus A, a galaxy with an active nucleus spewing fast-moving jets into its surroundings. Active galactic nuclei like this one produce extremely high-energy photons. [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)]

    Wide Field Imager on the 2.2 meter MPG/ESO telescope at Cerro LaSilla

    MPG/ESO 2.2 meter telescope at Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres

    ESO/MPIfR APEX high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft)

    NASA/Chandra X-ray Telescope

    What’s going on in our high-energy sky? Powerful phenomena abound in our universe, and they can produce photons with tremendous energies. A new study explores a high-energy mystery from one of these sources: active galactic nuclei, or AGN.

    3
    Gamma rays span a broad range of energies in the most energetic part of the electromagnetic spectrum. Very high-energy gamma rays initially emitted from AGN have energies above 100 GeV, but these are reprocessed by interactions with background photons to energies of 1–100 GeV. [Ulflund]

    Where Are the Gamma Rays?

    Active galactic nuclei — the accreting supermassive black holes lurking at the centers of some galaxies — dot our universal landscape, spewing out very high-energy gamma-ray photons within jets moving at nearly the speed of light. These energetic photons speed across the sky — but they don’t travel unencumbered.

    Theory predicts that this energetic emission should be effectively reprocessed as it slams into the cosmic microwave background, generating a compact sheath of gamma-ray emission in the 1–100 GeV range, beamed forward in the direction of the jets emitted from each AGN. But there’s a problem: we don’t see this expected flux.

    3
    Galactic coordinates of the sources used to generate the authors’ stacked analysis. Two types of AGN-containing galaxies are included: FR I and FR II galaxies. [Broderick et al. 2019]

    One possible explanation for the missing light is that these traveling photons could be deflected from their path by a strong, large-scale magnetic field threading through intergalactic space. This would convert the compact, forward-beamed sheath into a more diffuse, harder-to-spot gamma-ray halo around each AGN. In a new study, a team of scientists led by Avery Broderick (University of Waterloo and the Perimeter Institute for Theoretical Physics, Canada) has gone on the hunt for these missing gamma-ray halos.

    Perimeter Institute in Waterloo, Canada


    Stacks of Galaxies

    Though the proposed gamma-ray halos may be too faint to spot individually, Broderick and collaborators suggest that by stacking a bunch of gamma-ray observations of off-axis AGN on top of one another, we should easily be able to detect their combined halo — if it exists.

    5
    The process of aligning the jets in two different radio images: an FR I galaxy (top) and an FR II galaxy (bottom). [Broderick et al. 2019]

    To do this, the AGN must first be oriented in the same direction. Broderick and collaborators use radio observations of AGN jets pointed off our line of sight to identify each jet’s orientation. They determine the transformations needed to align each of the radio jets, and then apply this transformation to corresponding Fermi-telescope gamma-ray observations of the active galaxies. The result is a sample of nearly 9,000 gamma-ray observations of AGN, all oriented in the same direction.

    Broderick and collaborators then stack these observations and compare their results to a model of what we would expect to see if an intergalactic magnetic field were deflecting the gamma-ray photons, generating a faint halo around the AGN.

    Still No Halos

    6
    Top: the authors’ stacked gamma-ray observations for FR I (left) and FR II (right) galaxies. Bottom: the expected signals if gamma-ray halos were present. The observations clearly rule out the presence of faint halos. [Broderick et al. 2019]

    Intriguingly, the authors find no hint of a combined gamma-ray halo. Their non-detection places strict limits on the strength of the intergalactic magnetic field allowed in this picture, and it rules out magnetic fields as an explanation for why we don’t see the gamma rays we expect from AGN.

    What does this mean? Broderick and collaborators argue that this requires us to consider brand new physics in high-energy processes. There must be some unexpected mechanism that prevents the creation of the expected gamma-ray halos, either because the highest-energy emission is suppressed in gamma-ray bright AGN, or because some process affects this emission before it can lead to the generation of halos. The mystery deepens!

    Citation

    “Missing Gamma-Ray Halos and the Need for New Physics in the Gamma-Ray Sky,” Avery E. Broderick et al 2018 ApJ 868 87.
    https://iopscience.iop.org/article/10.3847/1538-4357/aae5f2/meta

    See the full article here .


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    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 5:15 pm on September 8, 2018 Permalink | Reply
    Tags: AGN's - Active galactic nuclei, , QUEST-La Silla AGN Variability Survey, ,   

    From Discover Magazine: “Black Holes Flicker as They Stop Gorging Themselves on Matter” 

    DiscoverMag

    From Discover Magazine

    September 7, 2018
    Alison Klesman

    1
    This artistically enhanced image shows a Hubble Space Telescope view of the active galaxy Arp 220, which houses a feeding supermassive black hole at its center. (Credit: NASA/JPL-Caltech)

    NASA/ESA Hubble Telescope

    Black holes are by nature difficult to study directly. Because even light cannot escape these massive objects, astronomers must turn to other methods to spot and study them. While information is lost once it crosses a black hole’s event horizon, outside that boundary, it can still escape. A recent study, led by a graduate student in the Department of Astronomy of the Universidad de Chile, has now found that the amount of light emitted from around a black hole is determined by one thing, and one thing only: the rate at which matter is falling into the black hole.

    The research, published September 4 in The Astrophysical Journal, was aimed at determining the physical mechanism behind the variability observed from the active black holes at the centers of galaxies (known as active galactic nuclei, or AGN), which are supermassive black holes currently sucking in matter.

    In astronomy, this process is known as accretion. Such black holes have accretion disks, which are disks of matter swirling around them as it is funneled inward, like water going down a drain. Outside the event horizon, these disks shine brightly as the material inside is heated by friction, giving off visible light and even more energetic light, such as X-rays. These disks are also variable — astronomers aren’t exactly sure why, but the current understanding is that as clumps of matter interact in the disk or fall into the black hole, it causes changes in the light the disk emits.

    The team combined data from the Sloan Digital Sky Survey and the QUEST-La Silla AGN Variability Survey to combine physical properties —the mass and the accretion rate, or the speed at which a black hole is eating — of about 2,000 AGN with information about their variability.

    SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft)

    ESO/Cerro LaSilla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

    What they found was surprising: “Contrary to what was believed, the only important physical property to explain the amplitude of the variability is the AGN accretion rate,” said Paula Sánchez-Sáez, the student who led the study and first author of the paper, in a press release.

    Out With The Old

    Why is this surprising? “The results obtained in this study challenge the old paradigm that the amplitude of the AGN variability depended mainly on the luminosity of the AGN,” Sánchez-Sáez said. What this means is that previously, astronomers assumed that more luminous (brighter) AGN varied more, while less luminous (dimmer) AGN varied less. This study instead discovered that the rate at which a black hole is eating is the only thing that affects the amount its light varies, regardless of whether it is bright or dim.

    But the challenge to previous thinking makes sense, Sánchez-Sáez said, because in the past, it’s been difficult to accurately measure a black hole’s mass, and thus its accretion rate. Only with newer data provided by large surveys can astronomers begin to build up the numbers they need to test their assumptions.

    With Black Holes, Less is More

    Furthermore, the study revealed a relationship that may seem backwards: “What we detect is that the less they [black holes] swallow, the more they vary,” said Paulina Lira of the Universidad de Chile and the CATA Center for Excellence in Astrophysics, and a co-author on the paper. In scientific terms, the amplitude (amount) of variability is inversely proportional to the accretion rate, or the amount of food a black hole is consuming at any given time.

    This initial study was based on variability information from the QUEST-La Silla AGN Variability Survey spanning about five years. Now, the researchers are looking to study the variability of these objects in greater detail, for which they’ll need more data. That means staring at these AGN for longer periods of time — at least 10 years or more. For that, they’ll need to wait for future surveys, such as those proposed with the Large Synoptic Survey Telescope, which is expected to reach full science operations by 2023. This will “extend our light curves to an order of 20 years,” said Lira, providing an even more accurate picture of the black hole’s behavior over longer periods of time.

    See the full article here .

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  • richardmitnick 2:42 pm on April 28, 2018 Permalink | Reply
    Tags: AGN's - Active galactic nuclei, , , , ,   

    From Center For Astrophysics: “Finding Galaxies with Active Nuclei” 

    Harvard Smithsonian Center for Astrophysics


    Center For Astrophysics

    1
    The Hubble image of a galaxy spotted by Spitzer’s IRAC infrared camera to be variable, probably because it harbors an active galactic nucleus. IRAC infrared surveys taken over a decade have spotted about 800 previously unknown AGN. NASA/Hubble; Polimera et al. 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.

    1
    Inner Structure of an Active Galaxy. 8 February 2016. Author Original: Unknown; Vectorization: Wikipedia user-Rothwild

    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.

    Science paper:
    Morphologies of mid-IR variability-selected AGN host galaxies . MNRAS

    See the full article here .

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    The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

     
  • richardmitnick 9:02 am on February 14, 2018 Permalink | Reply
    Tags: AGN's - Active galactic nuclei, , , , , , , ,   

    From ALMA: “ALMA Observes a Rotating Dust and Gas Donut around a Supermassive Black Hole” 

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    ALMA

    14 February, 2018

    Nicolás Lira
    Education and Public Outreach Coordinator
    Joint ALMA Observatory, Santiago – Chile
    Phone: +56 2 2467 6519
    Cell phone: +56 9 9445 7726
    Email: nicolas.lira@alma.cl

    Masaaki Hiramatsu
    Education and Public Outreach Officer, NAOJ Chile
    Observatory
, Tokyo – Japan
    Phone: +81 422 34 3630
    Email: hiramatsu.masaaki@nao.ac.jp

    Charles E. Blue
    Public Information Officer
    National Radio Astronomy Observatory Charlottesville, Virginia – USA
    Phone: +1 434 296 0314
    Cell phone: +1 202 236 6324
    Email: cblue@nrao.edu

    Richard Hook
    Public Information Officer, ESO
    Garching bei München, Germany
    Phone: +49 89 3200 6655
    Cell phone: +49 151 1537 3591
    Email: rhook@eso.org

    1
    Artist’s impression of the dusty gaseous torus around an active supermassive black hole. ALMA revealed the rotation of the torus very clearly for the first time. Credit: ALMA (ESO/NAOJ/NRAO)

    High resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) imaged a rotating dusty gas torus around an active supermassive black hole. The existence of such rotating donuts-shape structures was first suggested decades ago, but this is the first time one has been confirmed so clearly. This is an important step in understanding the co-evolution of supermassive black holes and their host galaxies.

    2
    The central region of the spiral galaxy M77. The NASA/ESA Hubble Space Telescope imaged the distribution of stars. ALMA revealed the distribution of gas in the very center of the galaxy. ALMA imaged a horseshoe-like structure with a radius of 700 light-years and a central compact component with a radius of 20 light-years. The latter is the gaseous torus around the AGN. Red indicates emission from formyl ions (HCO+) and green indicates hydrogen cyanide emission. Credit: ALMA (ESO/NAOJ/NRAO), Imanishi et al., NASA/ESA Hubble Space Telescope and A. van der Hoeven

    NASA/ESA Hubble Telescope

    Almost all galaxies hold concealed monstrous black holes in their centers. Researchers have known for a long time that the more massive the galaxy is, the more massive the central black hole is. This sounds reasonable at first, but host galaxies are 10 billion times bigger than the central black holes; it should be difficult for two objects of such vastly different scales to directly affect each other. So how could such a relation develop?

    Aiming to solve this shadowy problem, a team of astronomers utilized the high resolution of ALMA to observe the center of spiral galaxy M77. The central region of M77 is an “active galactic nucleus,” or AGN, which means that matter is vigorously falling toward the central supermassive black hole and emitting intense light. AGNs can strongly affect the surrounding environment, therefore they are important objects for solving the mystery of the co-evolution of galaxies and black holes.

    The team imaged the area around the supermassive black hole in M77 and resolved a compact gaseous structure with a radius of 20 light-years. And, the astronomers found that the compact structure is rotating around the black hole, as expected.2
    Motion of gas around the supermassive black hole in the center of M77. The gas moving toward us is shown in blue and that moving away from us is in red. The gas’s rotation is centered around the black hole. Credit: ALMA (ESO/NAOJ/NRAO), Imanishi et al.

    “To interpret various observational features of AGNs, astronomers have assumed rotating donut-like structures of dusty gas around active supermassive black holes. This is called the ‘unified model’ of AGN,” explained Masatoshi Imanishi, from the National Astronomical Observatory of Japan (NAOJ), the lead author on a paper published in the Astrophysical Journal Letters. “However, the dusty gaseous donut is very tiny in appearance. With the high resolution of ALMA, now we can directly see the structure.”

    Many astronomers have observed the center of M77 before, but never has the rotation of the gas donut around the black hole been seen so clearly. Besides the superior resolution of ALMA, the selection of molecular emission lines to observe was key to revealing the structure. The team observed specific microwave emission from hydrogen cyanide molecules (HCN) and formyl ions (HCO+). These molecules emit microwaves only in dense gas, whereas the more frequently observed carbon monoxide (CO) emits microwaves under a variety of conditions [1]. The torus around the AGN is assumed to be very dense, and the team’s strategy was right on the mark.

    “Previous observations have revealed the east-west elongation of the dusty gaseous torus. The dynamics revealed from our ALMA data agrees exactly with the expected rotational orientation of the torus,” said Imanishi.

    Interestingly, the distribution of gas around the supermassive black hole is much more complicated than what a simple unified model suggests. The torus seems to have an asymmetry and the rotation is not just following the gravity of the black hole but also contains highly random motion. These facts could indicate the AGN had a violent history, possibly including a merger with a small galaxy [2]. Nevertheless, the identification of the rotating torus is an important step.

    The Milky Way Galaxy, where we live, also has a supermassive black hole at its center.

    Milky Way Galaxy Credits: NASA/JPL-Caltech/R. Hurt

    SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory

    This black hole is, however, in a very quiet state. Only a tiny amount of gas is accreting onto it. Therefore, to investigate an AGN in detail, astronomers need to observe the centers of distant galaxies. M77 is one of the nearest AGN and a suitable object for peering into the very center in detail.

    These observation results were published as Imanishi et al. ALMA Reveals an Inhomogeneous Compact Rotating Dense Molecular Torus at the NGC 1068 Nucleus in the Astrophysical Journal Letters (2018 February 1 issue, 853, L25).

    The research team members are:

    Masatoshi Imanishi (National Astronomical Observatory of Japan/SOKENDAI), Kouichiro Nakanishi (National Astronomical Observatory of Japan/SOKENDAI), Takuma Izumi (National Astronomical Observatory of Japan), and Keiichi Wada (Kagoshima University).

    Notes

    [1] García-Burillo et al. (2016) observed the distribution and motion of CO with ALMA and did not find clear rotation along the east-west torus direction. Their interpretation is that the turbulent motion is so intense that the east-west oriented rotating motion is not clear. Gallimore et al. (2016) also observed CO emission and found gas motion in the north-south direction. They interpret this as outflowing gas from the black hole.

    [2] Recently, astronomers used the Subaru Telescope to observe M77 and revealed signatures of a merger with a small galaxy billions of years ago. For details, please read the press release Minor Merger Kicks Supermassive Black Hole into High Gear issued in October 2017 from the Subaru Telescope.

    See the full article here .

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    The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan.

    ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

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  • richardmitnick 8:45 am on February 8, 2018 Permalink | Reply
    Tags: A New Look at Speeding Outflows, , AGN's - Active galactic nuclei, , , , , , UFOs- ultra-fast outflows   

    From AAS NOVA: ” A New Look at Speeding Outflows” 

    AASNOVA

    AAS NOVA

    7 February 2018
    Susanna Kohler

    1
    Artist’s impression of a galaxy that is releasing material via two strongly collimated jets (shown in red/orange) as well as via wide-angle, ultra-fast outflows (shown in gray/blue). The inset shows a closeup of the accretion disk and central supermassive black hole at the galaxy’s core. [ESA/AOES Medialab].

    The compact centers of active galaxies — known as active galactic nuclei, or AGN — are known for the dynamic behavior they exhibit as the supermassive black holes at their centers accrete matter. New observations of outflows from a nearby AGN provide a more detailed look at what happens in these extreme environments.

    Outflows from Giants

    2
    The powerful radio jets of Cygnus A, which extend far beyond the galaxy. [NRAO/AUI].

    AGN consist of a supermassive black hole of millions to tens of billions of solar masses surrounded by an accretion disk of in-falling matter. But not all the material falling toward the black hole accretes! Some of it is flung from the AGN via various types of outflows.

    The most well-known of these outflows are powerful radio jets — collimated and incredibly fast-moving streams of particles that blast their way out of the host galaxy and into space. Only around 10% of AGN are observed to host such jets, however — and there’s another outflow that’s more ubiquitous.

    Fast-Moving Absorbers

    Perhaps 30% of AGN — both those with and without observed radio jets — host wider-angle, highly ionized gaseous outflows known as ultra-fast outflows (UFOs). Ultraviolet and X-ray radiation emitted from the AGN is absorbed by the UFO, revealing the outflow’s presence: absorption lines appear in the ultraviolet and X-ray spectra of the AGN, blue-shifted due to the high speeds of the absorbing gas in the outflow.

    3
    Quasar PG 1211+143, indicated by the crosshairs at the center of the image, in the color context of its surroundings. [SDSS/S. Karge]

    SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft)

    But what is the nature of UFOs? Are they disk winds? Or are they somehow related to the radio jets? And what impact do they have on the AGN’s host galaxy?

    X-ray and Ultraviolet Cooperation

    New observations are now providing fresh information about one particular UFO. A team of scientists led by Ashkbiz Danehkar (Harvard-Smithsonian Center for Astrophysics [CfA]) recently used the Chandra and Hubble space telescopes to make the first simultaneous observations of the same outflow — a UFO in quasar PG 1211+143 — in both X-rays and in ultraviolet.

    Danehkar and collaborators found absorption lines in both sets of data revealing an outflow moving at ~17,000 km/s (for reference, that’s ~5.6% of the speed of light, and more than 1,500 times faster than Elon Musk’s roadster will be traveling at its maximum speed in the orbit it was launched onto yesterday by the Falcon Heavy). Having the information both from the X-ray and the ultraviolet data provides the opportunity to better asses the UFO’s physical characteristics.

    A Link Between Black Holes and Galaxies?

    4
    The X-ray spectrum for PG 1211+143 was obtained by Chandra HETGS (top); the ultraviolet spectrum was obtained by HST-COS G130M (bottom). [Adapted from Danehkar et al. 2018]

    NASA/Chandra Telescope

    NASA/ESA Hubble Telescope

    The authors use models of the data to demonstrate the plausibility of a scenario in which a shock driven by the radio jet gives rise to the fast bulk outflows detected in the X-ray and ultraviolet spectra.

    They also estimate the impact that the outflows might have on the AGN’s host galaxy, demonstrating that the energy injected into the galaxy could be somewhere between 0.02% and 0.6% of the AGN’s total luminosity. At the higher end of this range, this could have an evolutionary impact on the host galaxy, suggesting a possible link between the black hole’s behavior and how its host galaxy evolves.

    In order to draw definitive conclusions, we will need higher-resolution observations that can determine the total size and extent of these outflows. For that, we may need to wait for 2023, when a proposed X-ray spectrometer that might fit the bill, Arcus, may be launched.

    Citation

    Ashkbiz Danehkar et al 2018 ApJ http://iopscience.iop.org/article/10.3847/1538-4357/aaa427/meta

    See the full article here .

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    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 5:09 pm on December 26, 2017 Permalink | Reply
    Tags: 'Direct Collapse' Black Holes May Explain Our Universe's Mysterious Quasars, AGN's - Active galactic nuclei, , , , , , , , , , Star formation is a violent process, ,   

    From Ethan Siegel: “‘Direct Collapse’ Black Holes May Explain Our Universe’s Mysterious Quasars” 

    From Ethan Siegel
    Dec 26, 2017

    1
    The most distant X-ray jet in the Universe, from quasar GB 1428, is approximately the same distance and age, as viewed from Earth, as quasar S5 0014+81; both are over 12 billion light years away. X-ray: NASA/CXC/NRC/C.Cheung et al; Optical: NASA/STScI; Radio: NSF/NRAO/VLA

    NASA/Chandra Telescope


    NASA/ESA Hubble Telescope


    NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

    There’s a big problem when we look at the brightest, most energetic objects we can see in the early stages of the Universe. Shortly after the first stars and galaxies form, we find the first quasars: extremely luminous sources of radiation that span the electromagnetic spectrum, from radio up through the X-ray. Only a supermassive black hole could possibly serve as the engine for one of these cosmic behemoths, and the study of active objects like quasars, blazars, and AGNs all support this idea. But there’s a problem: it may not be possible to make a black hole so large, so quickly, to explain these young quasars that we see. Unless, that is, there’s a new way to make black holes beyond what we previously thought. This year, we found the first evidence for a direct collapse black hole, and it may lead to the solution we’ve sought for so long.

    2
    While distant host galaxies for quasars and active galactic nuclei can often be imaged in visible/infrared light, the jets themselves and the surrounding emission is best viewed in both the X-ray and the radio, as illustrated here for the galaxy Hercules A. It takes a black hole to power an engine such as this. NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA).

    Generically known as ‘active galaxies,’ almost all galaxies posses supermassive black holes at their center, but only a few emit the intense radiation associated with quasars or AGNs. The leading idea is that supermassive black holes will feed on matter, accelerating and heating it, which causes it to ionize and give off light. Based on the light we observe, we can successfully infer the mass of the central black hole, which often reaches billions of times the mass of our Sun. Even for the earliest quasars, such as J1342+0928, we can get up to a mass of 800 million solar masses just 690 million years after the Big Bang: when the Universe was just 5% of its current age.

    3
    This artist’s concept shows the most distant supermassive black hole ever discovered. It is part of a quasar from just 690 million years after the Big Bang. Robin Dienel/Carnegie Institution for Science.

    If you try to build a black hole in the conventional way, by having massive stars go supernova, form small black holes, and have them merge together, you run into problems. Star formation is a violent process, as when nuclear fusion ignites, the intense radiation burns off the remaining gas that would otherwise go into forming progressively more and more massive stars. From nearby star-forming regions to the most distant ones we’ve ever observed, this same process seems to be in place, preventing stars (and, hence, black holes) beyond a certain mass from ever forming.

    4
    An artist’s conception of what the Universe might look like as it forms stars for the first time. While stars might reach many hundreds or even a thousand solar masses, it’s very difficult to see how you could get a black hole of the mass the earliest quasars are known to possess. NASA/JPL-Caltech/R. Hurt (SSC).

    We have a standard scenario that’s very powerful and compelling: of supernova explosions, gravitational interactions, and then growth by mergers and accretion. But the early quasars we see are too massive too quickly to be explained by this. Our other known pathway to create black holes, from merging neutron stars, provides no further help. Instead, a third scenario of direct collapse may be responsible. This idea has been helped along by three pieces of evidence in the past year:

    1.The discovery of ultra-young quasars like J1342+0928, in possession of black holes many hundred of millions of solar masses.
    2.Theoretical advances that show how, if the direct collapse scenario is true, we could form early “seed” black holes a thousand times as massive as the ones formed by supernova.
    3.And the discovery of the first stars that become black holes via direct collapse, validating the process.

    5
    In addition to formation by supernovae and neutron star mergers, it should be possible for black holes to form via direct collapse. Simulations such as the one shown here demonstrate that, under the right condition, seed black holes of 100,000 to 1,000,000 solar masses could form in the very early stages of the Universe. Aaron Smith/TACC/UT-Austin.

    Normally, it’s the hottest, youngest, most massive, and newest stars in the Universe that will lead to a black hole. There are plenty of galaxies like this in the early stages of the Universe, but there are also plenty of proto-galaxies that are all gas, dust, and dark matter, with no stars in them yet. Out in the great cosmic abyss, we’ve even found an example of a pair of galaxies just like this: where one has furiously formed stars and the other one may not have formed any yet. The ultra-distant galaxy, known as CR7, has a massive population of young stars, and a nearby patch of light-emitting gas that may not have yet formed a single star in it.

    6
    Illustration of the distant galaxy CR7, which last year was discovered to house a pristine population of stars formed from the material direct from the Big Bang. One of these galaxies definitely houses stars; the other may not have formed any yet. M. Kornmesser / ESO.

    In a theoretical study published in March [Nature Astronomy] of this year, a fascinating mechanism for producing direct collapse black holes from a mechanism like this was introduced. A young, luminous galaxy could irradiate a nearby partner, which prevents the gas within it from fragmenting to form tiny clumps. Normally, it’s the tiny clumps that collapse into individual stars, but if you fail to form those clumps, you instead can just get a monolithic collapse of a huge amount of gas into a single bound structure. Gravitation then does its thing, and your net result could be a black hole over 100,000 times as massive as our Sun, perhaps even all the way up to 1,000,000 solar masses.

    6
    Distant, massive quasars show ultramassive black holes in their cores. It’s very difficult to form them without a large seed, but a direct collapse black hole could solve that puzzle quite elegantly. J. Wise/Georgia Institute of Technology and J. Regan/Dublin City University.

    There are many theoretical mechanisms that turn out to be intriguing, however, that aren’t borne out when it comes to real, physical environments. Is direct collapse possible? We can now definitively answer that question with a “yes,” as the first star that was massive enough to go supernova was seen to simply wink out of existence. No fireworks; no explosion; no increase in luminosity. Just a star that was there one moment, and replaces with a black hole the next. As spotted before-and-after with Hubble, there is no doubt that the direct collapse of matter to a black hole occurs in our Universe.

    7
    The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation. NASA/ESA/C. Kochanek (OSU).

    Put all three of these pieces of information together, and you arrive at the following picture for how these supermassive black holes form so early.

    A region of space collapses to form stars, while a nearby region of space has also undergone gravitational collapse but hasn’t formed stars yet.
    The region with stars emits an intense amount of radiation, where the photon pressure keeps the gas in the other cloud from fragmenting into potential stars.
    The cloud itself continues to collapse, doing so in a monolithic fashion. It expels energy (radiation) as it does so, but without any stars inside.
    When a critical threshold is crossed, that huge amount of mass, perhaps hundreds of thousands or even millions of times the mass of our Sun, directly collapses to form a black hole.
    From this massive, early seed, it’s easy to get supermassive black holes simply by the physics of gravitation, merger, accretion, and time.

    It might not only be possible, but with the new array of radio telescopes coming online, as well as the James Webb Space Telescope, we may be able to witness the process in action.

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    SKA Square Kilometer Array


    SKA/ASKAP radio telescope at the Murchison Radio-astronomy Observatory (MRO) in Mid West region of Western Australia


    SKA Murchison Widefield Array, Boolardy station in outback Western Australia, at the Murchison Radio-astronomy Observatory (MRO)

    The galaxy CR7 is likely one example of many similar objects likely to be out there. As Volker Bromm, the theorist behind the direct collapse mechanism first said [RAS], a nearby, luminous galaxy could cause a nearby cloud of gas to directly collapse. All you need to do is begin with a

    “primordial cloud of hydrogen and helium, suffused in a sea of ultraviolet radiation. You crunch this cloud in the gravitational field of a dark-matter halo. Normally, the cloud would be able to cool, and fragment to form stars. However, the ultraviolet photons keep the gas hot, thus suppressing any star formation. These are the desired, near-miraculous conditions: collapse without fragmentation! As the gas gets more and more compact, eventually you have the conditions for a massive black hole.”

    8
    The directly collapsing star we observed exhibited a brief brightening before having its luminosity drop to zero, an example of a failed supernova. For a large cloud of gas, the luminous emission of light is expected, but no stars are necessary to form a black hole this way.
    NASA/ESA/P. Jeffries (STScI)

    With a little luck, by time 2020 rolls around, this is one longstanding mystery that might finally be solved.

    See the full article here .

    Please help promote STEM in your local schools.

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    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 2:01 pm on November 7, 2017 Permalink | Reply
    Tags: , AGN's - Active galactic nuclei, , , , , , Feeding Black Holes Through Galactic Bars   

    From astrobites via AAS NOVA: ” Feeding Black Holes Through Galactic Bars” 


    AAS NOVA

    Astrobites bloc

    astrobites

    1
    Hubble view of NGC 1300, a barred spiral galaxy. [NASA, ESA, and The Hubble Heritage Team (STScI/AURA)].AAS NOVA

    Title: Galaxy-Scale Bars in Late-Type Sloan Digital Sky Survey Galaxies Do Not Influence the Average Accretion Rates of Supermassive Black Holes
    Authors: A.D. Goulding, E. Matthaey, J.E. Greene, et al.
    First Author’s Institution: Princeton University

    Status: Accepted to ApJ, open access

    When it comes to picking their host galaxies, active galactic nuclei (or AGN) are rather promiscuous. They reside in all types of galaxies: ellipticals, irregulars, and spirals. AGN of the same feather tend to flock together — the more luminous and radio-loud ones are found in elliptical galaxies while the lower luminosity ones are more often found in spiral galaxies. This is a manifestation of the black hole mass-host galaxy luminosity correlation, where spiral galaxies like our Milky Way tend to have less massive black holes than elliptical galaxies. Besides spiral arms, spiral galaxies sometimes also boast of having bars, if the right mood strikes. How are bars related to their AGN? Could they trigger the central black holes to light up as AGN?

    Galactic bars are thought to contribute to the dynamical evolution of their host galaxies. Numerical studies show that they can funnel in gas from the outskirts to the central regions of the galaxies, triggering star formation and possibly AGN activity. It is still unclear whether bars actually help trigger AGN, as previous studies have produced conflicting results and tend to suffer from small number statistics and biased AGN diagnostics. In today’s paper, the authors bring better tools to bear on the problem, by utilizing the large wealth of information from the SDSS Galaxy Zoo citizen science project and X-ray stacking analyses.

    2
    Fig. 1: Sample unbarred (blue borders), ambiguously barred (yellow borders), and barred (red borders) spiral galaxies from the Galaxy Zoo project, as determined by fbar, which is the fraction of votes by citizen scientists for the presence of bars. [Goulding et al. 2017]

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 10:05 am on August 23, 2017 Permalink | Reply
    Tags: 2017 HEAD: Day 2, a well-studied tidal disruption event ASASSN-14li, , AGN coronae — the incredibly luminous compact regions that lie directly above the accretion disks of supermassive black holes, AGN's - Active galactic nuclei, , , , , The Very High Energy Universe as Viewed with VERITAS and HAWC   

    From AAS NOVA: ” 2017 HEAD: Day 2″ 

    AASNOVA

    American Astronomical Society

    23 August 2017
    Susanna Kohler

    1
    The gamma-ray excess at the heart of M31. [NASA/DOE/Fermi LAT Collaboration and Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF]

    Session: AGN 1

    Ehud Bahar (Technion) opened the meeting’s first session on active galactic nuclei (AGN) by discussing eclipses of a different kind than the one we observed on Monday. Light from AGN is often obstructed on its path to us by warm, outflowing, intervening material that absorbs some of the AGN’s light. Bahar explained the difference between what he termed “absorbers” and “obscurers”: absorbers are slow and steady outflows from the AGN that change very little over long timescales. These provide us with the opportunity to probe their detailed physics. Obscurers, on the other hand, are fast-moving and transient outflows, briefly causing dramatic drops in the X-ray flux of the AGN.

    2
    Artist’s impression of the tidal disruption event ASASSN-14li, in which a supermassive black hole destroyed a star, launching outflows. [NASA GSFC]

    Two speakers in the session discussed the idea of particularly fast outflows from AGN: Michael Nowak (MIT Kavli Institute) presented data on ultrafast outflows moving at 5–20% of the speed of light from the AGN PG 1211+143 (that’s 15,000–60,000 km/s, as compared to more typical outflow speeds of 100–1,000 km/s), and Erin Kara (University of Maryland) discussed what we can learn from ultrafast outflows from tidal disruption events. Kara’s talk demonstrated how we can use our observations of a well-studied tidal disruption event, ASASSN-14li, to learn about how an accretion disk around a black hole can transition from a super-Eddington (especially high) accretion phase that launches winds to a sub-Eddington (lower) accretion phase in which the wind is shut off.

    Andrew Fabian (University of Cambridge) wrapped up the session by providing an overview of what we know about AGN coronae — the incredibly luminous, compact regions that lie directly above the accretion disks of supermassive black holes. Coronae are the source of the majority of the hard X-ray emission from AGN, and we have used observations of this emission to constrain the size of AGN coronae to a mere 10 gravitational radii. We’ve learned that coronae are extremely hot, at 30–300 keV, and are highly magnetized and dynamic, likely containing outflowing plasma.

    Session: The Very High Energy Universe as Viewed with VERITAS and HAWC

    HAWC High Altitude Cherenkov Experiment, located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters, at WikiMiniAtlas 18°59′41″N 97°18′30.6″W.

    CfA/VERITAS, a major ground-based gamma-ray observatory with an array of four 12m optical reflectors for gamma-ray astronomy in the GeV – TeV energy range. Located at FLWO in AZ, USA

    The session on very high energy observations opened with a talk by Brenda Dingus (LANL). Dingus introduced us to the High Altitude Water Cherenkov (HAWC) gamma-ray observatory, a new observatory located in Mexico that maps the northern sky in high-energy gamma rays. HAWC has a wide field of view, observing roughly 2/3 of the sky each day with long integration times. This means that the observatory is sensitive to the highest energy gamma rays. HAWC has recently released its very first catalog, 2HWC, and this is only the beginning — there is much more science expected from this observatory in the future!

    The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is another high-energy observatory, located in southern Arizona; Philip Kaaret (University of Iowa) provided us with an overview of this set of telescopes. VERITAS has a narrower field of view than HAWC, but its sensitivity and angular resolution are higher, allowing it to probe sources at a deeper level. It’s therefore often used for follow-up observations of known targets.

    So what sources are high-energy observatories like VERITAS and HAWC observing? They hunt for photons from astrophysical sources like supernova remnants, pulsar wind nebulae, active galactic nuclei, gamma-ray bursts, and dark matter annihilation. Oleg Kargaltsev (George Washington University), Sara Buson (NASA GSFC), and Matthew Baring (Rice University) each explained some of the insights we’ve obtained about these objects from observatories like HAWC, VERITAS, Fermi, and MAGIC in conjunction with observatories exploring other wavelengths.

    So what sources are high-energy observatories like VERITAS and HAWC observing? They hunt for photons from astrophysical sources like supernova remnants, pulsar wind nebulae, active galactic nuclei, gamma-ray bursts, and dark matter annihilation. Oleg Kargaltsev (George Washington University), Sara Buson (NASA GSFC), and Matthew Baring (Rice University) each explained some of the insights we’ve obtained about these objects from observatories like HAWC, VERITAS, Fermi, and MAGIC in conjunction with observatories exploring other wavelengths.

    NASA/Fermi Telescope

    MAGIC Cherenkov gamma ray telescope on the Canary island of La Palma, Spain

    Mid-Career Prize Talk: X-ray Winds from Black Holes

    Tuesday afternoon kicked off with the HEAD Mid-Career Prize Talk, given this year by Jon Miller (University of Michigan). Miller spoke in further depth about a topic introduced earlier in the day: winds emitted from black hole disks. He argued that these winds are worth studying because they provide information about how mass is accreted onto black holes, and therefore how the black holes grow and their spins evolve.

    The dense and ionized winds from black-hole disks can potentially carry away more mass than is accreted — and this appears to hold true across the mass scale, from X-ray binaries containing stellar-mass black holes to Seyfert galaxies containing supermassive black holes. Miller discussed the different mechanisms that may launch these winds, and how observations indicate that magnetic driving is important, although other forces may also be at work.

    ESA/Athena spacecraft

    Miller argued that many tests of disk physics are now within reach of data and simulations, such as measurements of disk magnetic fields. He also showed how extreme settings such as tidal disruption events can provide a unique and interesting regime in which to explore disks and winds, as the mass accretion rate in these events changes drastically on observable timescales.

    As a final point, Miller discussed how our understanding of black hole disk winds will change with upcoming observatories. Missions like Xarm, ARCUS, ATHENA, Lynx [no image available], etc. will be transformative; ATHENA, for instance, will be able to produce observations outstripping the sensitivity and resolution of any observations obtained so far with current instrumentation, in “less than the time it took you to have lunch today,” Miller explained.

    2
    Xarm satelite

    3
    NASA/ARCUS

    Session: ISM & Galaxies

    Xian Hou (Yunnan Observatories) opened the session on the interstellar medium (ISM) and galaxies by discussing our view of M31 (the Andromeda galaxy) with the Fermi Large Area Telescope.

    NASA/Fermi LAT

    Andromeda Galaxy Adam Evans

    M31 is the only other large spiral local galaxy — and it’s nearby, providing an excellent opportunity for resolved analysis of high-energy emission from a large, star-forming, spiral galaxy similar to the Milky Way. The >1 GeV emission tracked by Fermi LAT was found to be concentrated only in the inner region of the galaxy; it is not correlated with interstellar gas or star-formation sites. What could be this emission’s source, then? Hou suggests that possibilities include a population of millisecond pulsars in the galactic center, or annihilation/decay of dark matter.

    4
    NuSTAR observations of M31. The bright blue point in the inset is the intermediate-mass pulsar candidate. [NASA/JPL-Caltech]

    NASA NuSTAR X-ray telescope

    Later in the session, Ann Hornschemeier (NASA GSFC) provided a complementary discussion of observations of M31 — this time in the form of NuSTAR’s deep survey of of our nearest galactic neighbor. Hornschemeier reminded us that before NuSTAR, we were unable to spatially resolve hard X-ray sources (energies over 10 keV) in other galaxies. Now, with NuSTAR, we can resolve point sources — and their hard X-ray color can help us to identify whether they are black hole X-ray binaries, neutron-star X-ray binaries, pulsars, etc. A number of neutron stars were identified in globular clusters in M31, as well as a particularly high energy source that is likely an intermediate-mass X-ray pulsar.

    The work done by Francesca Fornasini (Harvard-Smithsonian CfA) and collaborators explores how low-luminosity AGN activity and star formation in its host galaxy are connected. Is there a correlation between these two types of activity? If there’s a positive correlation, we can infer that AGN feedback suppresses star formation; if there is a negative correlation, both types of activity may be fueled by a common mechanism. On the other hand, there may be no correlation at all! Because AGN are variable, and because the relation between AGN activity and star formation rate can vary with other host galaxy properties like stellar mass and redshift, we need a very large sample that covers the whole phase space to test for correlation. Fornasini and collaborators achieve this by building X-ray stacks from data from 123,000 galaxies in the Chandra COSMOS Legacy Survey. Their work is still underway, but thus far it has revealed no correlation between the black-hole accretion rate and the star formation rate of the host galaxies.

    See the full article here .

    Please help promote STEM in your local schools.

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    1

    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 4:11 pm on June 19, 2017 Permalink | Reply
    Tags: , AGN's - Active galactic nuclei, , , , Geometric dependence of AGN types, Hidden Black Holes Revealed?,   

    From AAS NOVA: ” Hidden Black Holes Revealed?” 

    AASNOVA

    American Astronomical Society

    19 June 2017
    Susanna Kohler

    1
    Artist’s illustration of the thick dust torus thought to surround supermassive black holes and their accretion disks. [ESA / V. Beckmann (NASA-GSFC)]

    Supermassive black holes are thought to grow in heavily obscured environments. A new study now suggests that many of the brightest supermassive black holes around us may be escaping our detection as they hide in these environments.

    2
    The geometric dependence of AGN types in the unified AGN model. Type 1 AGN are viewed from an angle where the central engine is visible. In Type 2 AGN, the dusty torus obscures the central engine from view. [Urry & Padovani, 1995]

    A Torus Puzzle

    The centers of galaxies with bright, actively accreting supermassive black holes are known as active galactic nuclei, or AGN. According to a commonly accepted model for AGN, these rapidly growing black holes and their accretion disks are surrounded by a thick torus of dust. From certain angles, the torus can block our direct view of the central engines, changing how the AGN appears to us. AGN for which we can see the central engine are known as Type 1 AGN, whereas those with an obscured central region are classified as Type 2.

    Oddly, the fraction of AGN classified as Type 2 decreases substantially with increasing luminosity; brighter AGN seem to be more likely to be unobscured. Why? One hypothesis is that the torus structure itself changes with changing AGN luminosity. In this model, the torus recedes as AGN become brighter, causing fewer of these AGN to be obscured from our view.

    But a team of scientists led by Silvia Mateos (Institute of Physics of Cantabria, Spain) suggests that we may instead be missing the bigger picture. What if the problem is just that many of the brightest obscured AGN are too well hidden?

    Geometry Matters

    3
    Type 2 AGN fraction vs. torus covering factor for AGN in the authors’ three luminosity bins. The black line shows the 1-to-1 relation describing the expected Type 2 AGN fraction; the black data points show the observed fraction. The red points show the best-fit model including the “missing” AGN, and the inset shows the covering-factor distribution for the missing sources. [Mateos et al. 2017]

    Mateos and collaborators built a sample of nearly 200 X-ray-observed AGN from the Bright Ultra-hard XMM-Newton Survey (BUXS). They then determined the intrinsic fraction of these AGN that were obscured (i.e., classified as Type 2) at a given luminosity, for redshifts between 0.05 ≤ z ≤ 1.

    ESA/XMM Newton

    The team next used clumpy torus models to estimate the distributions of AGN covering factors, the geometric factor that describes the fraction of the sky around the AGN central engine that’s obscured.

    The pointing directions for AGN should be randomly distributed, and geometry then dictates that the covering factor distributions combined over the total AGN population should match the intrinsic fraction of AGN classified as Type 2 AGN. Instead, the sample from BUXS reveals a “missing” population of high-covering-factor tori that we have yet to detect in X-rays.

    Missing Sources

    When they include the missing AGN, Mateos and collaborators find that the total fraction of Type 2 AGN is around 58%. They also show that more of these AGN are missing at higher luminosities. By including the missing ones, the total fraction of obscured AGN therefore has a much weaker dependence on luminosity than we thought — which suggests that the receding torus model isn’t necessary to explain observations.

    Mateos and collaborators’ results support the idea that the majority of very bright, rapidly accreting supermassive black holes at redshifts of z ≤ 1 live in nuclear environments that are extremely obscured. These black holes are so well embedded in their environments that they’ve escaped detection in X-ray surveys thus far.

    Citation

    S. Mateos et al 2017 ApJL 841 L18. doi:10.3847/2041-8213/aa7268

    Related Journal Articles
    See the full article for a list of further references with links.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    1

    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
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