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Post Info TOPIC: Active galactic nuclei (AGNs)
Blobrana


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RE: Active galactic nuclei (AGNs)
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Title: Galaxy Zoo: the effect of bar-driven fueling on the presence of an active galactic nucleus in disc galaxies 
Author: Melanie A. Galloway, Kyle W. Willett, Lucy F. Fortson, Carolin N. Cardamone, Kevin Schawinski, Edmond Cheung, Chris J. Lintott, Karen L. Masters, Thomas Melvin, Brooke D. Simmons 

We study the influence of the presence of a strong bar in disc galaxies which host an active galactic nucleus (AGN). Using data from the Sloan Digital Sky Survey and morphological classifications from the Galaxy Zoo 2 project, we create a volume-limited sample of 19,756 disc galaxies at 0.01<z<0.05 which have been visually examined for the presence of a bar. Within this sample, AGN host galaxies have a higher overall percentage of bars (51.8%) than inactive galaxies exhibiting central star formation (37.1%). This difference is primarily due to known effects; that the presence of both AGN and galactic bars is strongly correlated with both the stellar mass and integrated colour of the host galaxy. We control for this effect by examining the difference in AGN fraction between barred and unbarred galaxies in fixed bins of mass and colour. Once this effect is accounted for, there remains a small but statistically significant increase that represents 16% of the average barred AGN fraction. Using the L[OIII]/MBH ratio as a measure of AGN strength, we show that barred AGN do not exhibit stronger accretion than unbarred AGN at a fixed mass and colour. The data are consistent with a model in which bar-driven fuelling does contribute to the probability of an actively growing black hole, but in which other dynamical mechanisms must contribute to the direct AGN fuelling via smaller, non-axisymmetric perturbations.

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Blobrana


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Overfed Black Holes Shut Down Galactic Star-Making

The Herschel Space Observatory has shown galaxies with the most powerful, active black holes at their cores produce fewer stars than galaxies with less active black holes. The results are the first to demonstrate black holes suppressed galactic star formation when the universe was less than half its current age.
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Blobrana


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Active Galactic Nuclei
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INTEGRAL observations suggest unified model for Active Galactic Nuclei requires a rethink

Scrutinising a large sample of Active Galactic Nuclei with INTEGRAL, astronomers have found that, unexpectedly, sources affected by stronger absorption at lower energies show an excess emission in the hard X-ray band when compared to their less obscured counterparts. The excess is likely due to reflection of X-rays off the same dense clouds responsible for absorption. Unaccounted for in the unified paradigm of Active Galactic Nuclei, the finding calls for a rethinking of some of this model's facets. The reflected radiation could also represent the long-sought missing part of the Cosmic X-ray Background.
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Blobrana


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Active galactic nuclei (AGNs)
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Title: More evidence for extinction of activity in galaxies
Authors: Andrzej Marecki, Bartlomiej Swoboda (Torun Centre for Astronomy, N. Copernicus University, Torun)

This Research Note amends an article in which we showed that radio-loud quasars can become radio-quiet. Exploring the analogy between galactic nuclei and X-ray binaries (XRB), we pointed out there that this transition in quasars could be identified with a switch from low/hard to high/soft state in microquasars. Here, we present the evidence that traces of past occurrences of this kind of phenomena can be found in normal but once active galaxies. Based on the properties of a few such "post-active" galaxies that are representative for a much wider group, it has been argued that they have reached the evolutionary stages when their nuclei, which were radio-loud in the past, now, mimicking the behaviour of XRBs, remain in the intermediate state on their way towards quiescence or even have already entered the quiescent state. It follows that the full evolutionary track of XRBs can be mapped onto the evolution of galaxies. The above findings are in line with those reported recently for IC 2497, a galaxy that 70,000 years ago or less hosted a quasar but now appears as a normal one. This scenario stems from the presence of Hanny's Voorwerp, a nebulous object in its vicinity excited by that QSO in the epoch when IC 2497 was active. The post-active galaxies we deal with here are accompanied by extremely weak and diffuse relic radio lobes that were inflated during their former active period. These relics can be regarded as radio analogues of Hanny's Voorwerp.

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Blobrana


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Active Galaxies
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Swift Survey Finds 'Missing' Active Galaxies
Seen in X-rays, the entire sky is aglow. Even far away from bright sources, X-rays originating from beyond our galaxy provide a steady glow in every direction. Astronomers have long suspected that the chief contributors to this cosmic X-ray background were dust-swaddled black holes at the centers of active galaxies. The trouble was, too few of them were detected to do the job.
An international team of scientists using data from NASA's Swift satellite confirms the existence of a largely unseen population of black-hole-powered galaxies. Their X-ray emissions are so heavily absorbed that little more than a dozen are known. Yet astronomers say that despite the deeply dimmed X-rays, the sources may represent the tip of the iceberg, accounting for at least one-fifth of all active galaxies.



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Blobrana


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Posts: 131433
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RE: Active galactic nuclei (AGNs)
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Galactic mergers not the major feeding mechanism for mass monsters

A new study has obtained unexpected new insight into the feeding habits of the giant black holes, which are responsible for the emissions of some of the brightest objects in the universe: active galactic nuclei. Previous models had often assumed that mergers between galaxies are instrumental in driving matter into these black holes. But this systematic study of 1400 galaxies - the largest sample ever examined for the purpose - presents strong evidence that, at least for the past eight billion years, black holes have acquired their food more peacefully. (Astrophysical Journal, January 10, 2011)
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Blobrana


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Colliding galaxies don't shake up enough food to feed enormous black holes lurking in their centers. A new study suggests that less violent events, like gravitational disturbances within the galaxy, are probably black holes' primary source of fuel.
Almost every galaxy is centred on a supermassive black hole hundreds of millions of times more massive than the sun. Some of them, like the black hole at the center of the Milky Way, are relatively docile. But others draw a huge, hot disk of gas that glows white-hot just before it vanishes forever. These disks, called active galactic nuclei, can outshine the rest of the host galaxy.
Astronomers have suspected for decades that these ravenous black holes get their fuel from major mergers between two large galaxies. In all the chaos of a galactic pile-up, plenty of gas should get funnelled toward the center and gobbled up, astronomers reasoned. Simulations have found that black holes and galaxies grow together. Some observations even suggested that galaxies with active galactic nuclei were slightly distorted, a sign of a recent collision.
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Blobrana


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How Often do Giant Black Holes Become Hyperactive?

This two-panel graphic contains two composite images of galaxies used in a recent study of supermassive black holes. In each of the galaxies, data from NASA's Chandra X-ray Observatory are blue, and optical data from the Sloan Digital Sky survey are shown in red, yellow and white. The galaxy on the left, Abell 644, is in the center of a galaxy cluster that lies about 1.1 billion light years from Earth. On the right is an isolated, or "field," galaxy named SDSS J1021+1312, which is located about 900 million light years away. At the center of both of these galaxies is a growing supermassive black hole, called an active galactic nucleus (AGN) by astronomers, which is pulling in large quantities of gas.
A newly published study from Chandra tells scientists how often the biggest black holes in field galaxies like SDSS J1021+1312 have been active over the last few billion years. This has important implications for how environment affects black hole growth. The scientists found that only about one percent of field galaxies with masses similar to the Milky Way contain supermassive black holes in their most active phase. They also found that the most massive galaxies are the most likely to host these AGN, and that there is a gradual decline in the AGN fraction with cosmic time. Finally, the AGN fraction for field galaxies was found to be indistinguishable from that for galaxies in dense clusters, like Abell 644.
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Blobrana


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Massive Black holes "switch on" due to galaxy collision

The centre of most galaxies harbours a massive Black Hole. So does our Milky Way - the exotic object there however is pretty calm, unlike some supermassive gravity monsters in other galaxies. Scientists at the Max Planck Institute for Extraterrestrial Physics and other institutions around the world have now analysed 199 of these galaxies and discovered what makes the black holes at the galaxy centre become active: The black holes switched on some 700 million years ago after major galaxy merger events.
While in our own galaxy, the Milky Way, the central black hole was measured to have about 4 million solar masses, the 199 galaxies analysed now host black holes with a typical mass of about 300 million solar masses. These galaxies are much larger than our Milky Way and they are "active", which means that the inner region of the galaxy has a much higher luminosity than normal.
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Blobrana


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Active Galactic Nuclei
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Title: Active Galactic Nuclei and their role in Galaxy Formation and Evolution
Authors: Steve Kraemer (CUA), Rogier Windhorst (ASU), Kenneth G. Carpenter (NASA-GSFC), Mike Crenshaw (GSU), Martin Elvis (CfA), Margarita Karovska (CfA)

There are several key open questions as to the nature and origin of AGN including: 1) what initiates the active phase, 2) the duration of the active phase, and 3) the effect of the AGN on the host galaxy. Critical new insights to these can be achieved by probing the central regions of AGN with sub-mas angular resolution at UV/optical wavelengths. In particular, such observations would enable us to constrain the energetics of the AGN "feedback" mechanism, which is critical for understanding the role of AGN in galaxy formation and evolution. These observations can only be obtained by long-baseline interferometers or sparse aperture telescopes in space, since the aperture diameters required are in excess of 500 m - a regime in which monolithic or segmented designs are not and will not be feasible and because these observations require the detection of faint emission near the bright unresolved continuum source, which is impossible from the ground, even with adaptive optics. Two mission concepts which could provide these invaluable observations are NASA's Stellar Imager (SI; Carpenter et al.) interferometer and ESA's Luciola (Labeyrie 2008) sparse aperture hypertelescope.

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