Mauricio Cisternas and Knud Jahnke on the Growth of Supermassive Black Holes
According to Essential Science Indicators, a subset of the Web of Knowledge from Thomson Reuters, the 2011 paper “The bulk of the black hole growth since z ~1 occurs in a secular universe: No major merger-AGN connection,” (M. Cisternas, et al., Astrophys. J., 726(2): No. 57, 10 January 2011) has been identified as a Fast Breaking Paper in Space Science. This designation is based on the paper’s increase in citations as tracked over two recent, successive bimonthly periods. The report, cited 28 times in the Web of Science as of May 31, 2012, displayed a higher bimonthly citation increase than any other paper of comparable age in its field.
Lead author Mauricio Cisternas is now based at the Instituto de Astrofîsica de Canarias La Laguna, Tenerife, Spain, while his main collaborator, Knud Jahnke, is at the Max Planck Institute for Astronomy, Heidelberg, Germany. On this paper, their team included 22 coauthors.
Below, Drs. Cisternas and Jahnke answer a few questions about their fast breaking report.
SW: Why do you think your paper is highly cited?
The discovery that supermassive black holes are located in most massive galaxies is less than 15 years old. Our paper provides a very direct answer to a fundamental question in galaxy evolution: what triggers the growth of these black holes? For the last 30 years it was thought that major collisions between galaxies were to blame. Such an event would disturb the gas content of the galaxies in such a way that it would be driven to its center to be subsequently swallowed by the supermassive black hole.
We believe our paper was well received because it provides a simple yet solid analysis showing that for the last 7.5 billion years this was not the case for most galaxies, and that other, less-extreme processes are prevalent at these stages. Our paper was also timely, since it used the best and largest of the recent high-quality datasets from the Hubble Space Telescope. This was a prerequisite for these kinds of studies, something that would not have been possible a few years ago.
SW: Does it describe a new discovery or new synthesis of knowledge?
“The discovery that supermassive black holes are located in most massive galaxies is less than 15 years old. Our paper provides a very direct answer to a fundamental question in galaxy evolution: what triggers the growth of these black holes?”
Both. First we show that major galaxy mergers are not more common for "active galaxies"—those that currently host a growing supermassive black hole—when compared to normal quiescent galaxies. We also show that the fraction of active galaxies that recently experienced a major merger is pretty low, at around 15%. These findings tell us that collisions between galaxies are not happening for active galaxies at a rate higher than what you would expect for normal galaxies, and hence, conversely, they cannot be blamed for "activating" the growth of these supermassive black holes.
But then, the question remains: if not major mergers, what triggers the feeding of these supermassive black holes? By putting together our results with other recent findings from the literature in the current broader picture of galaxy evolution, we argue that other, less-spectacular mechanisms can also provide the necessary fuel to activate these supermassive black holes.
Our findings show that clearly other mechanisms than galaxy collisions are responsible—other, less-violent mechanisms have to be at work.
SW: Would you summarize the significance of your paper in layman's terms?
Today we know that nearly every massive galaxy in the universe—including our own Milky Way—hosts at its center a black hole with a mass ranging between 1 million and 1,000 million times the mass of the Sun. How did these supermassive black holes become so massive? It is thought that these black holes grew most of their mass during periods of vigorous activity during which they consume large amounts of gas and their surroundings emit huge amounts of radiation. On one side, this means that we can easily detect and identify them. On the other, could it be that this energy, emitted in the form of radiation, is impacting the evolution of the surrounding galaxy?
In our paper we tackled the problem of how to supply the necessary "food" for a black hole to consume. Early observations of some of the most-bright active galaxies, known as quasars, revealed that their morphologies were distorted, suggesting that they had been recently involved in a major collision with another galaxy. This led to a scenario in which a major merger had to precede an episode of black hole activity. Nevertheless, there was not a consistent study actually comparing the frequency of collisions occurring on active galaxies against the "normal" galaxy population—that is, galaxies hosting a dormant supermassive black hole. This is relevant since currently there are a number of galaxies merging in the universe, albeit not every one of them hosts an active black hole.
The significance of our paper is that we show that active and inactive galaxies share the same rate of mergers. Additionally, we also show that this merging rate is actually pretty modest, at around 15%. This means that active black holes do not "prefer" to show up in colliding galaxies, and the majority of active galaxies have not been involved in a collision. Placed in the big picture, our study suggests that the universe is not evolving in such a violent way as previously thought, and other, less-extreme mechanisms are responsible for feeding a large number of these active galaxies.
"Example of active and inactive galaxies arranged according to how disturbed their morphologies are. Our study found that for each of these distortion classes, both samples showed equivalent fractions, indicating that active galaxies do not undergo collisions more often than inactive galaxies."
Credit: NASA, ESA, M. Cisternas
SW: How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes you've encountered along the way?
KJ was centrally involved in the COSMOS project which provided this dataset. Through an opening for PhD research in KJ's group at the Max Planck Institute for Astronomy in Heidelberg, Germany, MC started to work on this idea to study the connection between galaxy merging and active galaxies.
This project required visually analyzing in detail over a thousand galaxy images by independent scientists from different institutions around the world, and one of the main challenges was for a graduate student to coordinate and make senior researchers contribute their part of the work.
SW: Where do you see your research leading in the future?
Our work contributed to initiating a paradigm-shift regarding the understanding of the processes that regulate galaxy evolution during different periods in cosmic history. While a great deal of effort has been put forth, by both observational and theoretical astrophysicists, to study extreme processes such as major galaxy mergers, we hope that our work serves as a motivation to look into other, less-spectacular mechanisms that can also play a leading role in the evolution of galaxies.
Another aspect is to understand whether major galaxy merging could be the main mechanism in triggering black hole growth for particular populations of galaxies; whether it is for the most massive ones, for the most violently growing galactic nuclei, or for those that lived in a much earlier universe. These are still open questions we hope to help answer in the near future.
SW: Do you foresee any broader social implications or impact for your research?
Our research helped to improve the understanding of one of the many aspects of galaxy evolution. While in general it can be very tedious for us astrophysicists to introduce the general public to our methods and findings, in our case it came very natural: our analysis applied techniques that are frequently used in many other research fields, e.g., medicine and psychology. In order to identify merging galaxies in our samples of active and inactive galaxies, we decided to analyze the galaxies visually, since there is no computer algorithm than can compete with the human brain. One drawback is that such an analysis can be subjective, e.g., a person may have a preference to look more carefully and find more distortions on active galaxies and be less meticulous with the inactive galaxies. To remedy this issue we did a blind study: we mixed both samples, and the people analyzing the images did not know whether it was an normal galaxy or an active one, thereby removing any potential bias against a particular sample.
Dr. Mauricio Cisternas
Instituto de Astrofîsica de Canarias
La Laguna, Tenerife, Spain
Dr. Knud Jahnke
Max Planck Institute for Astronomy
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