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  • richardmitnick 12:56 pm on January 12, 2017 Permalink | Reply
    Tags: Active galactic nuclei, , IC 3639, Monster black holes, , , NGC 1448, , ,   

    From Space Science Laboratory at UC Berkeley: “NuSTAR – Black Holes Hide in our Cosmic Backyard” 

    UC Berkeley

    UC Berkeley

    SSL UC Berkeley

    Space Science Laboratory

    1
    No image caption. No image credit.

    NASA/NuSTAR

    NuSTAR

    January 12, 2017
    Christopher Scholz

    Monster black holes sometimes lurk behind gas and dust, hiding from the gaze of most telescopes. But they give themselves away when material they feed on emits high-energy X-rays that NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) mission can detect. That’s how NuSTAR recently identified two gas-enshrouded supermassive black holes, located at the centers of nearby galaxies.

    “These black holes are relatively close to the Milky Way, but they have remained hidden from us until now,” said Ady Annuar, a graduate student at Durham University in the United Kingdom, who presented the results at the American Astronomical Society meeting in Grapevine, Texas. “They’re like monsters hiding under your bed.”

    Both of these black holes are the central engines of what astronomers call “active galactic nuclei,” a class of extremely bright objects that includes quasars and blazars. Depending on how these galactic nuclei are oriented and what sort of material surrounds them, they appear very different when examined with telescopes.

    Active galactic nuclei are so bright because particles in the regions around the black hole get very hot and emit radiation across the full electromagnetic spectrum — from low-energy radio waves to high-energy X-rays. However, most active nuclei are believed to be surrounded by a doughnut-shaped region of thick gas and dust that obscures the central regions from certain lines of sight. Both of the active galactic nuclei that NuSTAR recently studied appear to be oriented such that astronomers view them edge-on. That means that instead of seeing the bright central regions, our telescopes primarily see the reflected X-rays from the doughnut-shaped obscuring material.

    “Just as we can’t see the sun on a cloudy day, we can’t directly see how bright these active galactic nuclei really are because of all of the gas and dust surrounding the central engine,” said Peter Boorman, a graduate student at the University of Southampton in the United Kingdom.

    Boorman led the study of an active galaxy called IC 3639, which is 170 million light years away.

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    IC 3639, a galaxy with an active galactic nucleus, is seen in this image combining data from the Hubble Space Telescope and the European Southern Observatory.

    This galaxy contains an example of a supermassive black hole hidden by gas and dust. Researchers analyzed NuSTAR data from this object and compared them with previous observations from NASA’s Chandra X-Ray Observatory and the Japanese-led Suzaku satellite.

    NASA/Chandra Telescope
    NASA/Chandra Telescope

    JAXA/Suzaku satellite
    JAXA/Suzaku satellite

    The findings from NuSTAR, which is more sensitive to higher energy X-rays than these observatories, confirm the nature of IC 3639 as an active galactic nucleus that is heavily obscured, and intrinsically much brighter than observed.

    Researchers analyzed NuSTAR data from this object and compared them with previous observations from NASA’s Chandra X-Ray Observatory and the Japan-led Suzaku satellite. NuSTAR also provided the first precise measurement of how much material is obscuring the central engine of IC 3639, allowing researchers to determine how luminous this hidden monster really is.

    More surprising is the spiral galaxy that Annuar focused on: NGC 1448.

    6
    NGC 1448 (also designated NGC 1457 and ESO 249-16) is a spiral galaxy located about 60 million light-years away in the constellation Horologium. It has a prominent disk of young and very bright stars surrounding its small, shining core. The galaxy is receding from us with 1168 kilometers per second.

    NGC 1448 has recently been a prolific factory of supernovae, the dramatic explosions that mark the death of stars: after a first one observed in this galaxy in 1983 (SN 1983S), two more have been discovered during the past decade.

    Visible as a red dot inside the disc, in the upper right part of the image, is the supernova observed in 2003 (Type II supernova SN 2003hn), whereas another one, detected in 2001 (Type Ia supernova SN 2001el), can be noticed as a tiny blue dot in the central part of the image, just below the galaxy’s core. If captured at the peak of the explosion, a supernova might be as bright as the whole galaxy that hosts it.

    A Type Ia supernova is a result from the violent explosion of a white dwarf star. This category of supernovae produces consistent peak luminosity. The stability of this luminosity allows these supernovae to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.

    A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum. Type II supernovae are mainly observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies.

    This image was obtained using the 8.2-metre telescopes of ESO’s Very Large Telescope. It combines exposures taken between July 2002 and the end of November 2003.

    ESO/VLT at Cerro Paranal, Chile, ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level
    ESO/VLT at Cerro Paranal, Chile, ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

    Credit: ESO

    The black hole in its center was only discovered in 2009, even though it is at the center of one of the nearest large galaxies to our Milky Way. By “near,” astronomers mean NGC 1448 is only 38 million light years away (one light year is about 6 trillion miles).

    Annuar’s study discovered that this galaxy also has a thick column of gas hiding the central black hole, which could be part of a doughnut-shaped region. X-ray emission from NGC 1448, as seen by NuSTAR and Chandra, suggests for the first time that, as with IC 3639, there must be a thick layer of gas and dust hiding the active black hole in this galaxy from our line of sight.

    Researchers also found that NGC 1448 has a large population of young (just 5 million year old) stars, suggesting that the galaxy produces new stars at the same time that its black hole feeds on gas and dust. Researchers used the European Southern Observatory New Technology Telescope to image NGC 1448 at optical wavelengths, and identified where exactly in the galaxy the black hole should be. A black hole’s location can be hard to pinpoint because the centers of galaxies are crowded with stars. Large optical and radio telescopes can help detect light from around black holes so that astronomers can find their location and piece together the story of their growth.

    “It is exciting to use the power of NuSTAR to get important, unique information on these beasts, even in our cosmic backyard where they can be studied in detail,” said Daniel Stern, NuSTAR project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, California.

    NuSTAR is a Small Explorer mission led by Caltech and managed by JPL for NASA’s Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR’s mission operations center is at UC Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission’s ground station and a mirror archive. JPL is managed by Caltech for NASA.

    See the full article here .

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    Founded in the wake of the gold rush by leaders of the newly established 31st state, the University of California’s flagship campus at Berkeley has become one of the preeminent universities in the world. Its early guiding lights, charged with providing education (both “practical” and “classical”) for the state’s people, gradually established a distinguished faculty (with 22 Nobel laureates to date), a stellar research library, and more than 350 academic programs.

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  • richardmitnick 3:34 pm on December 1, 2016 Permalink | Reply
    Tags: Active galactic nuclei, , , , X-Ray Beasts and Magnetic fields   

    From astrobites: “X-Ray Beasts and Magnetic fields” 

    Astrobites bloc

    Astrobites

    Nov 30, 2016
    Suk Sien Tie

    Title: Revisiting the structure and spectrum of the magnetic-reconnection-heated corona in luminous AGNs
    Authors: J.Y. Liu, E.L Qiao, and B.F. Liu
    First Author’s Institution: National Astronomical Observatories, Chinese Academy of Sciences

    Deep in the fathomless centers of galaxies, there lurk fantastic beasts of incredible energy and power. Legend has it that our ancestors have long since been aware of their presence (e.g. Carl Seyfert in 1943 and Maarten Schmidt in 1963), but separated by our great divide and limited by our technologies, little has been known about them. Appearing in various shapes and sizes, some beasts have been spotted to spurt out jets, while others appear more docile. How to find these fantastic beasts, you ask? Well, most give away their presence in the optical and UV, while others show up in the radio. These shape shifters are unified by one thing: the source of their power through the accretion of matter onto supermassive black holes. Astronomers call them active galactic nuclei, or AGNs (shhh, here is a secret guide on their different shape shifting abilities).

    Although active galactic nuclei are predominantly discovered in the optical/UV and radio, they actually emit all the way up to X-rays, which is true with most highly energetic astrophysical phenomena. Their spectral energy distributions (SED), an example shown in Figure 1, are characterized by different components that arise from different parts of their structures. First, we have a blackbody-like bump — the “big blue bump” — in the optical and UV, which is thought to originate from their accretion disks. The big blue bump is well-fit by the standard accretion disk model, which is geometrically thin and opaque to radiation (or optically thick). Then, we have some emission in soft X-ray ( 5 keV) well-described by a power-law. The origin of the hard X-ray emission is not completely solved, but astronomers believe that the up scattering, or inverse Compton scattering, of accretion disk photons by plasma of hot electrons in a corona surrounding the disk produces these hard X-ray emissions.

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    Fig. 1 – An example spectral energy distribution (SED) of an AGN.

    The formation of the hot corona and its heating mechanism are still unclear. Leading models posit that it is formed by the evaporation of material from the underlying cool disk, and then continuously heated by the disk magnetic field. More specifically, magnetic loops produced by the disk emerge and reconnect with other magnetic loops in the corona in a phenomenon known as magnetic reconnection, thereby releasing magnetic energy to maintain the corona at high temperatures. The authors of today’s paper investigate the effect of magnetic field on the structure and spectrum of the magnetic-reconnection-heated disk-corona model, particularly focusing on luminous AGNs.

    The authors encapsulate the effect of magnetic field in their model in a parameter they call the magnetic parameter β0 = (Pgas + Prad)/PB, where magnetic pressure is assumed to be proportional to the sum of the gas pressure and radiation pressure in the disk. Larger β0 corresponds to weaker magnetic field strength and vice versa. By tuning β0 and the accretion rate, the authors solve for the disk structure and derive the emergent SED. Figure 2 shows the simulated SEDs for different β0 and accretion rates. When β0 is small, strong magnetic fields are produced, which gives rise to similar-looking spectra regardless of accretion rates. As the magnetic field gets weaker (going down the panel), the model with higher accretion rate (dashed line) produces less and less hard X-rays. Eventually, the spectrum is dominated by the disk’s “big blue bump”. This agrees with observational trend that hard X-ray spectra of luminous AGNs become softer at higher accretion rates.

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    Fig. 2 – Simulated SEDs of AGN at various β0 (different panels) and accretion rates (different lines). [Figure 4 from paper]

    To compare their models with observations, the authors plotted the observable Lbol/Lx as a function of accretion rate for different magnetic field strengths, as shown in Figure 3. The red crosses are observational samples of luminous AGNs with measured black hole masses from a different work. The model with β0=200 more or less agrees with observations at ṁ 0.2, the model with β0=100 at slightly higher magnetic field agrees with the last data point, although it fails to at lower accretion rates. Although the model is far from perfect, it agrees with observational results at large. As with all things research-related, there remains more work to be done before we can piece together a complete picture of those fantastic beasts called AGNs.

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    Fig. 3 – Lbol/Lx as a function of accretion rate for various β0, overlaid with observational data (red crosses). The β0=100 and 200 lines agree with the data while the β0=10 and 50 lines do not. [Figure 5 from paper]

    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 4:12 pm on November 9, 2016 Permalink | Reply
    Tags: Active galactic nuclei, Markarian 1018: Starvation Diet for Black Hole Dims Brilliant Galaxy, ,   

    From Chandra: “Markarian 1018: Starvation Diet for Black Hole Dims Brilliant Galaxy” 

    NASA Chandra Banner

    NASA Chandra Telescope

    NASA Chandra

    November 9, 2016

    1
    Credit X-ray: NASA/CXC/Univ of Sydney/R.McElroy et al, Optical: ESO/CARS Survey
    Release Date November 9, 2016
    Observation Date 27 Nov 2010, 25 Feb 2016

    Markarian 1018 is an “active galaxy” that has brightened and dimmed over about 30 years.

    Such shifting between bright and dim phases has never been studied before in such detail.

    By combining data from Chandra, VLT and several other telescopes, scientists have narrowed in on the explanation.

    It appears the dimming arises from the black hole at the center being deprived of enough fuel to illuminate its surroundings.

    Astronomers may have solved the mystery of the peculiar volatile behavior of a supermassive black hole at the center of a galaxy. Combined data from NASA’s Chandra X-ray Observatory and other observatories suggest that the black hole is no longer being fed enough fuel to make its surroundings shine brightly.

    Many galaxies have an extremely bright core, or nucleus, powered by material falling toward a supermassive black hole. These so-called “active galactic nuclei” or AGN, are some of the brightest objects in the Universe.

    Astronomers classify AGN into two main types based on the properties of the light they emit. One type of AGN tends to be brighter than the other. The brightness is generally thought to depend on either or both of two factors: the AGN could be obscured by surrounding gas and dust, or it could be intrinsically dim because the rate of feeding of the supermassive black hole is low.

    Some AGN have been observed to change once between these two types over the course of only 10 years, a blink of an eye in astronomical terms. However, the AGN associated with the galaxy Markarian 1018 stands out by changing type twice, from a faint to a bright AGN in the 1980s and then changing back to a faint AGN within the last five years. A handful of AGN have been observed to make this full-cycle change, but never before has one been studied in such detail. During the second change in type the Markarian 1018 AGN became eight times fainter in X-rays between 2010 and 2016.

    After discovering the AGN’s fickle nature during a survey project using ESO’s Very Large Telescope (VLT), astronomers requested and received time to observe it with both NASA’s Chandra X-ray Observatory and Hubble Space Telescope.

    ESO/VLT at Cerro Paranal, Chile
    ESO/VLT at Cerro Paranal, Chile

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    The accompanying graphic shows the AGN in optical light from the VLT (left) with a Chandra image of the galaxy’s central region in X-rays showing the point source for the AGN (right).

    Data from ground-based telescopes including the VLT allowed the researchers to rule out a scenario in which the increase in the brightness of the AGN was caused by the black hole disrupting and consuming a single star. The VLT data also cast doubt on the possibility that changes in obscuration by intervening gas cause changes in the brightness of the AGN.

    However, the true mechanism responsible for the AGN’s surprising variation remained a mystery until Chandra and Hubble data was analyzed. Chandra observations in 2010 and 2016 conclusively showed that obscuration by intervening gas was not responsible for the decline in brightness. Instead, models of the optical and ultraviolet light detected by Hubble, NASA’s Galaxy Evolution Explorer (GALEX) and the Sloan Digital Sky Survey in the bright and faint states showed that the AGN had faded because the black hole was being starved of infalling material.

    NASA/Galex telescope
    NASA/Galex telescope

    SDSS Telescope at Apache Point, NM, USA
    SDSS Telescope at Apache Point, NM, USA

    One possible explanation for this starvation is that the inflow of fuel is being disrupted. This disruption could be caused by interactions with a second supermassive black hole in the system. A black hole binary is possible as the galaxy is the product of a collision and merger between two large galaxies, each of which likely contained a supermassive black hole in its center.

    The list observatories used in this finding also include NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) mission and Swift spacecraft.

    NASA/NuSTAR
    NASA/NuSTAR

    NASA/SWIFT Telescope
    NASA/SWIFT Telescope

    This starvation also explains the fading of the AGN in X-rays.

    Two papers, one with the first author of Bernd Husemann (previously at ESO and currently at the Max Planck Institute for Astronomy) and the other with Rebecca McElroy (University of Sydney), describing these results appeared in the September 2016 issue of Astronomy & Astrophysics journal.

    See the full article here .

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    NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

     
  • richardmitnick 8:13 am on July 20, 2016 Permalink | Reply
    Tags: Active galactic nuclei, , , , Galactic mergers   

    From astrobites: “How black hole and merger can kill a galaxy” 

    Astrobites bloc

    Astrobites

    Jul 19, 2016
    Suk Sien Tie

    Title: How to quench a galaxy
    First Author: Andrew Pontzen, Michael Tremmel, Nina Roth et al.
    First Author’s Institution: Department of Physics and Astronomy, University College London
    Paper Status: Submitted to MNRAS

    Galaxies are stellar neighborhoods of young and old stars. However, these galactic enclaves are only for exclusive members based on age. Some are spattered with young stellar populations and bursting with active star formation; these galaxies are typically blue in color, such as our own Milky Way.

    Milky Way with Ophiuchus Stream
    Milky Way with Ophiuchus Stream, Sesar 2015 via AAS NOVA

    Some are run by retired veteran stars; these galaxies are “red and dead”, as star formation has been shut off — or quenched, if you want to be fancy. What causes star formation to shut off in these red and dead galaxies has been a long-standing cosmic riddle. Since the building blocks of stars are cold molecular gas, some mechanisms are thought to drive these gases out of the galactic vicinity.

    How about winds from supernovae (SNe) or massive stars, collectively known as stellar feedback? Various research (see this bite, for instance) has shown that stellar feedback may help regulate star formation in low mass ( 10¹² Msun) galaxies that have more gravity to hold on their gases. Galaxy mergers can lend a helping hand, by stripping an infalling galaxy of its gas supply and inducing intense starbursts (thus consuming gas and causing stellar feedback). However, merger+stellar feedback quenches a galaxy much slower than observed, a hint that another feedback is in action.

    Active galactic nuclei, which are black holes (BH) at the centers of galaxies activated by accretion of matter, can drive rapid outflows such that their immediate environments are too hot or too devoid of gas to form stars. This is AGN feedback. The authors of this paper investigated how mergers and AGN feedback cooperate to quench star formation, by simulating a high-mass (10¹² Msun) redshift z=2 galaxy in three different merger scenarios: enhanced merger, suppressed merger, and the original “reference” merger. Enhanced merger is achieved by increasing the mass of the infalling object while suppressed merger is achieved through a series of small accretion events. The authors used a method to fix the local environment and arrive at the same final galaxy mass despite the different merger histories, thereby isolating the specific role of AGN and merger in the quenching process.

    Figure 1 shows the simulated galaxy at z=2.3 for the BH+SNe and SNe-only cases in the three different merger scenarios. For the SNe-only case in all three merger scenarios, the simulated galaxy appears blue with a central bar/bulge, while the galaxy appears more quiescent, red, and elliptical for the BH+SNe case. These galaxy portraits suggest that star formation has ceased in the reference and enhanced merger scenarios for the BH+SNe case while star formation is still actively underway for the SNe-only case regardless of merger scenario. This is more concretely shown in Figure 2, which tracks the galaxy’s specific star formation rate (=star formation rate/stellar mass) over time. The galaxy is said to be quenched when its specific star formation rate falls below the horizontal line. The top panel shows that the enhanced-merger scenario (red) quenches permanently, the reference merger (black) quenches temporarily, and the suppressed-merger (blue) never quenches for the BH+SNe case, alluding to the importance of mergers in the quenching process.

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    Fig. 1 – The simulated galaxy in IVU wavelengths at z=2.3. Each column refers to the three different merger scenario: suppressed, reference, and enhanced. The top panel is for SNe-only while the bottom panel is for BH+SNe. Notice the differences in appearance of the galaxy between the BH+SNe reference and enhanced merger scenarios and all other figures. [Adapted from Figure 1 of the paper]

    3
    FIg. 2 – Specific star formation rate (sSFR) as a function of redshift and time. The top panel is for BH+SNe while the middle panel is for SNe-only. The bottom panel is the sSFR ratio between the BH+SNe and SNe-only simulation. Black line refers to the reference merger, blue line the suppressed merger, and red line the enhanced merger scenario. The gray band is when the galaxy is at the main sequence stage. When the galaxy’s specific star formation rate drops by below the gray line labeled “UVJ-quenched” (i.e. 2×10⁻¹⁰ Msun per year), the galaxy is defined as quenched. [Figure 3 of the paper]

    Mergers are AGNs’ wing-men, so to speak. They initiate the quenching process by disrupting the AGN disk. With no disk to confine the AGN’s rapid outflows and shield the star-forming regions, AGN feedback is increased, pushing the galaxy into a long-term quiescent state. In the suppressed merger scenario, the gaseous disk surrounding the AGN limits the effect of AGN by directing the outflow in a funnel perpendicular to the disk, thereby leaving star-formation uninterrupted. The presence of AGN is also crucial in maintaining the galaxy’s quenched state. When the authors manually turned off the AGN in the enhanced merger BH+SNe case when the galaxy is quenched for the first time, star-formation quickly re-establishes, as shown in Figure 3.

    It appears that mergers and AGN feedback work together synergistically to quench a high-mass galaxy. While AGN feedback is essential, stellar feedback is negligible, as outflows from supernova-driven winds struggle to escape the galaxy as mass increases. Alas, it all comes down to a race against gravity…!

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    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.

     
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