Galaxy mergers not the major feeding mechanism for giant black holes
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. The new study will be published in the Astrophysical Journal on January 10.
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The emissions of active galactic nuclei (AGN) are driven by matter falling into the galaxy's supermassive central black hole. But it is an open question in the physics of active galaxies how matter traverses the final hundreds of light-years to reach the immediate neighborhood of the black hole and be swallowed.
Following a study by David Sanders and collaborators from the late 1980s, most astronomers thought they had the answer: Mergers between galaxies of similar sizes ("major mergers") would dramatically disturb the galaxies' gas, and make some of it fall towards the central black hole.
While this is a plausible scenario, only a systematic study can show whether or not this is indeed how giant black holes acquire their food. This is what Mauricio Cisternas and Knud Jahnke from the Max Planck Institute for Astronomy (MPIA) set out to do in 2008. Cisternas explains: "A study of this scope has become possible only recently, with the large surveys undertaken using the HUBBLE Space Telescope. Before, there was simply no way to examine sufficiently many active galaxies at large cosmic distances in sufficient detail."
Cisternas and his collaborators obtained data for 140 active galactic nuclei (AGN), identified as such by X-ray observations from the XMM-Newton space telescope as part of the multi-wavelength survey COSMOS. Light from the most distant of these AGN has been traveling for almost 8 billion years to reach us (redshift z=1): We see those AGN as they were 8 billion years ago, and the sample probes most of the black hole growth during the second half of cosmic history.
What makes this study special is the systematic way the astronomers selected a "control group" of ordinary galaxies without an active black hole – in other words, which do not have a black hole swallowing copious amounts of matter. For each of the AGNs in the study, nine non-active galaxies at roughly the same redshift, and thus roughly in the same stage of cosmic evolution, were selected from the same HUBBLE images, for a grand total of 1400 galaxies. This selection procedure allowed for a direct comparison between AGN and a matching population of ordinary, inactive galaxies.
The tell-tale sign that a galaxy has undergone a major merger over the past few hundred millions of years are distortions of its shape. For galaxies this distant, on images of the given resolution, a computerized, automatic identification of the degree of distortion cannot currently compete with visual inspection of the images by astronomers. Co-author Knud Jahnke (MPIA) says: "We were faced with the problem of bias. We knew that mergers were a plausible driver of AGN activity, so would we be more likely to classify AGN as distorted because of what we expected to find?"
In order to eliminate possible bias, the researchers set up a blind study – standard operating procedure in fields like medicine or psychology, but unusual in astronomy. Cisternas removed tell-tale signs of galactic activity from the images so there would be no way to directly distinguish between the images of active and inactive galaxies.
The images were then given to ten galaxy experts based at eight different institutions, who were asked to judge each galaxy as "distorted" or "not distorted". While their individual judgements showed significant variation, there was unanimity on the crucial question: None of the classifications showed a significant difference between AGN and inactive galaxies. There was no significant correlation between a galaxy's activity and its distortion, between its black hole being well-fed and its involvement in a major merger.
While mergers are a common phenomenon, and are thought to play a role at least for some AGN, the study shows that they provide neither a universal nor a dominant mechanism for feeding black holes. By the study's statistics, at least 75%, and possibly all of AGN activity over the last 8 billion years must have a different explanation. Possible ways of transporting matter towards a central black hole include instabilities of structures like a spiral galaxy's bar, the collisions of giant molecular clouds within the galaxy, or the fly-by of another galaxy that does not lead to a merger ("galactic harrassment").
Could there still be a causal connection between mergers and activity in the more distant past? That is the next question the group is gearing up to address. Suitable data is bound to come from two ongoing observational programs ("Multi-Cycle Treasury Programs") with the HUBBLE Space Telescope, as well as from observations by its successor, the James Webb Space Telescope, which is scheduled for launch after 2014.
The work described here will be published as Cisternas et al., "The bulk of black hole growth since z~1 occurs in a secular universe: no major merger-AGN connection" in December 10, 2010 issue of the Astrophysical Journal (volume 726, p. 57ff.).
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The team members are Mauricio Cisternas, Knud Jahnke, Katherine J. Inskip (all Max Planck Institute for Astronomy), Jeyhan Kartaltepe (NOAO), Anton M. Koekemoer (STScI), Thorsten Lisker (Heidelberg University), Aday R. Robaina (MPIA and University of Barcelona), Marco Scodeggio (IASF-INAF), Kartik Sheth (California Institute of Technology), Jonathan R. Trump (University of Arizona), Rene Andrae (MPIA), Takamitsu Miyaji (UNAM, Mexico, and University of California at San Diego), Elisabeta Lusso (INAF - Astronomical Observatory of BOLOGNA), Marcella Brusa (Max Planck Institute for Extraterrestrial Physics), Peter Capak (Caltech), Nico Cappelluti (MPE), Francesca Civano (Harvard Smithsonian Center for Astrophysics), Olivier Ilbert (Laboratoire d’Astrophysique de Marseille), Chris D. Impey (University of Arizona), Alexie Leauthaud (LBNL and University of California), Simon J. Lilly (ETH Zürich), Mara Salvato (Max Planck Institute for Plasma Physics), Nick Z. Scoville (Caltech), and Yoshi Taniguchi (Ehime University, Japan).
Why is transportation of matter towards a galactic black hole a problem?
It is a common misconception that black holes act like cosmic vacuum cleaners, sucking in any and all nearby matter. In reality, getting matter into a black hole is considerably more difficult. While matter that is heading straight for the black hole will assuredly fall in, matter that has some sideways motion, as well (in physics terms, matter with sufficient angular momentum) will orbit the black hole, instead. This is exactly analogous to standard planetary motion: Even though the Sun's gravity pulls the planets inwards, the combination of this strong pull with sideways motion results in the planets' elliptical orbits around the Sun.
Astronomers are familiar with a number of ways how this obstacle – sideways motion – can be overcome, and matter made to fall into a black hole. On smaller scales of a less than a few light-days friction within the accretion disk, a swirling disk of matter, serves to effectively transport matter into the galactic black hole's maw. On very large scales of a few thousands of light-years, unstable galactic-sized structures such as large-scale bars can efficiently drive matter towards the center of the galaxy. However, it is not clear how matter is transported on medium scales. This is the problem addressed by the present study: By what mechanism does matter bridge the gap?
Why were mergers thought to be a plausible solution?
The study by David Sanders and collaborators from the late 1980s was based on galaxies that are extremely bright in the infrared region of the spectrum ("ultra-luminous infrared galaxies", ULIRGs). It showed that nearly all of those galaxies sported an active galactic nucleus – the ultra-bright phenomenon associated with a galactic black hole fed by lots of infalling matter –, and, at the same time, showed the distortions in shape that is indicative of the galaxy having undergone a recent merger with another galaxy of similar size ("major merger"). Such mergers are a standard ingredient of current models of galaxy evolution, and they present a plausible solution to the matter transport problem: As two galaxies merge, tidal gravitational forces – similar to the forces behind the tides here on Earth – disturb the motion of gas in both galaxies, which could make at least some of the gas fall towards the central regions of the galaxy to be subsequently devoured by the supermassive black hole.
What makes this study different from previous studies of the same problem?
A number of studies published over the last 20 years sought to test the correlation between major and mergers and AGN activity, with mixed results: Some studies seemed to find a correlation, others not. These inconsistencies can be traced to different factors: Some studies looked at too small a number of objects, others used different criteria for selecting the active and the inactive galaxies, running the danger of comparing apples and oranges. The present study looks at a large sample of galaxies, and systematically selects both the active and inactive galaxies from the same parent data set (see next question). In this way, problems due to either small sample size or selection bias are avoided.
How was the sample selected?
Sample selection started with a total of 2000 X-ray emitting sources – candidates for active galactic nuclei – detected by the XMM-Newton space telescope. Follow-up observations used characteristic emission features – specific spectral lines where AGN are known to emit significant amounts of radiation – to identify about a thousand AGN among those candidates, as well as the distances to those AGN. Of those, 140 were within the distance range chosen for this survey, which is limited by the need to later be able to clearly distinguish the galaxy's shape. For each of these 140 AGN, nine galaxies at a comparable redshift were selected as a control group, resulting in a total of 1400 galaxies.
How was the blind study set up?
The optical image of an active galaxy shows a bright nucleus in the center. The shape and intensity of the image of the nucleus can be modelled very precisely, and Cisternas used this information to remove all signs of an active nucleus from the central portion of each galaxy. No such removal is perfect, and the procedure typically left some residuals (such as some dark pixels in the center of the image). Cisternas then artificially added similar residuals to the images of inactive galaxies to make sure active and inactive galaxies really could not be distinguished. In this way, the astronomers looking at the images did not know whether they were looking at active or inactive galaxies. The astronomers varied significantly in their judgement of which galaxies were distorted: Some applied stricter criteria and classified a mere 3% of the galaxies as distorted, others a whopping 30%. For the result of the study, this is not important; what is crucial is that all these different classifications showed no significant differences between distortions of active and of inactive galaxies.
What telescopes were used for this study?
The study was realized as part of the COSMOS collaboration, an international endeavour to observe the largest contiguous field ever imaged with the HUBBLE Space Telescope, and at wavelengths other than that of visible light with ESA's XMM-Newton and NASA's Chandra X-Ray space telescopes, NASA's infrared Spitzer Space Telescope, complemented by observations of various groundbased facilities.
The study is based on images taken by the NASA/ ESA HUBBLE Space Telescope: These are the key for undertaking a study of this type since, in order to tell whether or not a galaxy has undergone a merger, one needs to study its shape; before the large-scale HUBBLE surveys, there was simply not enough data to allow for a study of sufficiently many galaxies in sufficient detail. Candidates for AGNs were chosen using observations with ESA's XMM-Newton X-ray telescope, and their activity confirmed with ESO's Very Large Telescope (VLT) and the Carnegie Institution's Magellan Telescope.