Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


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 study will be published in the Astrophysical Journal on January 10.

If major mergers are an important factor in transporting matter towards galaxies\' central black holes, which makes the black hole shine out brightly as an Active Galactic Nucleus (AGN), then distortions - the tell-tale traces of such mergers - should be more frequent for active galaxies (left) than inactive ones (right). Credit: NASA/ESA and M. Cisternas (MPIA)

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.


Mauricio Cisternas (lead author)
Max Planck Institute for Astronomy
Phone: +49 6221 528-335
Dr. Knud Jahnke (second author)
Max Planck Institute for Astronomy
Phone +49 6221 528-398
Dr. Markus Pössel (public relations)
Max Planck Institute for Astronomy
Phone: (+49|0) 6221 – 528 261
Background information
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.). 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).

Dr. Markus Pössel | Max-Planck-Institut
Further information:

More articles from Physics and Astronomy:

nachricht From rocks in Colorado, evidence of a 'chaotic solar system'
23.02.2017 | University of Wisconsin-Madison

nachricht Prediction: More gas-giants will be found orbiting Sun-like stars
22.02.2017 | Carnegie Institution for Science

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Breakthrough with a chain of gold atoms

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

Im Focus: DNA repair: a new letter in the cell alphabet

Results reveal how discoveries may be hidden in scientific “blind spots”

Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...

Im Focus: Dresdner scientists print tomorrow’s world

The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.

The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...

Im Focus: Mimicking nature's cellular architectures via 3-D printing

Research offers new level of control over the structure of 3-D printed materials

Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...

Im Focus: Three Magnetic States for Each Hole

Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".

Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...

All Focus news of the innovation-report >>>



Event News

Booth and panel discussion – The Lindau Nobel Laureate Meetings at the AAAS 2017 Annual Meeting

13.02.2017 | Event News

Complex Loading versus Hidden Reserves

10.02.2017 | Event News

International Conference on Crystal Growth in Freiburg

09.02.2017 | Event News

Latest News

From rocks in Colorado, evidence of a 'chaotic solar system'

23.02.2017 | Physics and Astronomy

'Quartz' crystals at the Earth's core power its magnetic field

23.02.2017 | Earth Sciences

Antimicrobial substances identified in Komodo dragon blood

23.02.2017 | Life Sciences

More VideoLinks >>>