By comparing 140 galaxies that had Active Galactic Nuclei with over 1200 galaxies in a "control group", the likelihood that mergers are the cause of AGN has been brought into doubt. NASA, ESA, M. Cisternas (Max-Planck Institute for Astronomy)

Galactic Mergers Fail to Feed Black Holes

Article Updated: 26 Apr , 2016

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The large black holes that reside at the center of galaxies can be hungry beasts. As dust and gas are forced into the vicinity around the black holes, it crowds up and jostles together, emitting lots of heat and light. But what forces that gas and dust the last few light years into the maw of these supermassive black holes?

It has been theorized that mergers between galaxies disturbs the gas and dust in a galaxy, and forces the matter into the immediate neighborhood of the black hole. That is, until a recent study of 140 galaxies hosting Active Galactic Nuclei (AGN) – another name for active black holes at the center of galaxies – provided strong evidence that many of the galaxies containing these AGN show no signs of past mergers.

The study was performed by an international team of astronomers. Mauricio Cisternas of the Max Planck Institute for Astronomy and his team used data from 140 galaxies that were imaged by the XMM-Newton X-ray observatory. The galaxies they sampled had a redshift between z= 0.3 – 1, which means that they are between about 4 and 8 billion light-years away (and thus, the light we see from them is about 4-8 billion years old).

They didn’t just look at the images of the galaxies in question, though; a bias towards classifying those galaxies that show active nuclei to be more distorted from mergers might creep in. Rather, they created a “control group” of galaxies, using images of inactive galaxies from the same redshift as the AGN host galaxies. They took the images from the Cosmic Evolution Survey (COSMOS), a survey of a large region of the sky in multiple wavelengths of light. Since these galaxies were from the same redshift as the ones they wanted to study, they show the same stage in galactic evolution. In all, they had 1264 galaxies in their comparison sample.

The way they designed the study involved a tenet of science that is not normally used in the field of astronomy: the blind study. Cisternas and his team had 9 comparison galaxies – which didn’t contain AGN – of the same redshift for each of their 140 galaxies that showed signs of having an active nucleus.

What they did next was remove any sign of the bright active nucleus in the image. This means that the galaxies in their sample of 140 galaxies with AGN would essentially appear to even a trained eye as a galaxy without the telltale signs of an AGN. They then submitted the control galaxies and the altered AGN images to ten different astronomers, and asked them to classify them all as “distorted”, “moderately distorted”, or “not distorted”.

Since their sample size was pretty manageable, and the distortion in many of the galaxies would be too subtle for a computer to recognize, the pattern-seeking human brain was their image analysis tool of choice. This may sound familiar – something similar is being done with enormous success with people who are amateur galaxy classifiers at Galaxy Zoo.

When a galaxy merges with another galaxy, the merger distorts its shape in ways that are identifiable – it will warp a normally smooth elliptical galaxy out of shape, and if the galaxy is a spiral the arms seem to be a bit “unwound”. If it were the case that galactic mergers are the most likely cause of AGN, then those galaxies with an active nucleus would be more probable to show distortion from this past merger.

The team went through this process of blinding the study to eliminate any bias that those looking at the images would have towards classifying AGN as more distorted. By both having a reasonably large sample size of galaxies and removing any bias when analyzing the images, they hoped to definitively show whether the correlation between AGN and mergers exists.

The result? Those galaxies with an Active Galactic Nucleus did not show any more distortion on the whole than those galaxies in the comparison sample. As the authors state in the paper, “Mergers and interactions involving AGN hosts are not dominant, and occur no more frequently than for inactive galaxies.”

This means that astronomers can’t point towards galactic mergers as the main reason for AGN. The study showed that at least 75% of AGN creation – at least between the last 4-8 billion years – must be from sources other than galactic mergers. Likely candidates for these sources include: “galactic harrassment”, those galaxies that don’t collide, but come close enough to gravitationally influence each other; the instability of the central bar in a galaxy; or the collision of giant molecular clouds within the galaxy.

Knowing that AGN aren’t caused in large part by galactic mergers will help astronomers to better understand the formation and evolution of galaxies. The active nuclei in galaxies that host them greatly influence galactic formation. This process is called ‘AGN feedback’, and the mechanisms and effects that result from the interplay between the energy streaming out of the AGN and the surrounding material in the center of a galaxy is still a hot topic of study in astronomy.

Mergers in the more distant past than 8 billion years might yet correlate with AGN – this study only rules out a certain population of these galaxies – and this is a question that the team plans to take on next, pending surveys by the Hubble Space Telescope and the James Webb Space Telescope. Their study will be published in the January 10 issue of the Astrophysical Journal, and a pre-print version is available on Arxiv.

Source: HST news release, Max Planck Institute for Astronomy, Arxiv paper

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Fozzie
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Fozzie
January 5, 2011 2:34 AM

Why all the complicated hypotheses? The simplest answer to me as to why an AGN is active could just be that there is more material near the black hole to get sucked in!

Steve Nerlich
Member
January 5, 2011 2:59 AM

Presumably AGN are active because the supermassive black hole within is sucking down an accretion disk of material, which is the source of the radiation. This should eventually run out of steam if there is no fresh supply of gas and dust to populate the accretion disk. However, the link between galactic mergers and AGN might be more correlation than causation?

Astrofiend
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Astrofiend
January 5, 2011 3:12 AM
Fozzie January 5, 2011 at 2:34 am The thing is, 1) there just can’t be enough material in the central regions to continuously power most AGN for any period of time without significant material inflow. 2) observations generally show that most galaxies are very gas-poor in their nuclear regions, and gas rich in the outer regions, and 3) gas in the outer regions of galaxies should be in stable orbits around the galactic centre in the absence of any perturbation. Putting all of this together means that disturbances to galactic systems are generally required in most models to feed material to the central regions of AGN to account for both their observed prevalence and luminosity distribution. The generally… Read more »
Torbjorn Larsson OM
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Torbjorn Larsson OM
January 5, 2011 1:43 PM

Thanks, a very useful quick walk through for this layman!

damian
Member
January 5, 2011 3:39 AM

Active Galactic Nuclei, I like that much better then Black Hole. Black holes just suck. smile

I still dont buy the Status Quo hypothesis of what these things actually are. Why should these Galactic Nuclei have to (devour or feed on) anything ? I’m a lay-person, so is there some flaw in the cosmic maths that needs this interpretation?

I’m curious, UT had an article (last year) pointing out that galactic accretion discs can form via interactions with the interstellar medium (dust) ‘without needing mergers’. This study could form the basis of validation of such a hypothesis. Where is the unity of ideas in scientific circles?

Astrofiend
Member
Astrofiend
January 5, 2011 4:25 AM

“observations generally show that most galaxies are very gas-poor in their nuclear regions, and gas rich in the outer regions, ”

> should have just left that as gas poor in the nuclear regions – not all galaxies are gas rich in the outer regions, particularly ellipticals of course…

Astrofiend
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Astrofiend
January 5, 2011 4:35 AM
“Active Galactic Nuclei, I like that much better then Black Hole. Black holes just suck.” Nice pun , however, the terms aren’t really interchangeable. Black holes power AGN; the term AGN referring more to the observational fact that some galaxies appear to emit radiation from their core regions in an amount and with characteristics that cannot be explained merely by the presence of stars or associated activity… I guess in a nutshell the AGN is the observable energetic activity powered by the BH engine. Anyway – AGN triggering is one of the most persistent problems in astronomy today. There is no reason why the current paradigm absolutely has to be correct, just that this is where a vast… Read more »
Lawrence B. Crowell
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Lawrence B. Crowell
January 5, 2011 4:49 AM

Galactic mergers do not necessarily mean their black holes merge or even enter into a tight orbit around each other. I suspect it might be common for galaxies to merge and for their large black holes to continue on an orbit which leaves the merged galaxy. It requires a large amount of drag or friction to force billions of solar masses to congeal. This is easy to understand with gas clouds, but less so with multi-million solar mass black holes. It also requires a near dead on impact trajectory for the two black holes to actually merge.

LC

wjwbudro
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wjwbudro
January 5, 2011 9:19 AM

I’m far from the grade level most of you are at but, given that we have a certain confidence in the inflation model and that we also believe that that laws of physics/conservation are universal and unchanging, at least in our bubble; smile, would it not be reasonable to presume that mass formation and interaction dynamics would differ dramatically at different stages of the expansion?
LC, wouldn’t the probability for BH mergers be higher, gravitationally, in the early confined and denser environment? This study is for the 4-8 BY old galaxies. What about the 12+ BY old ones? As Astrofiend said, hopefully well have more clarity in the next 5 years or so.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 5, 2011 2:28 PM
given that we have a certain confidence in the inflation model … would it not be reasonable to presume that mass formation and interaction dynamics would differ dramatically at different stages of the expansion? Close. What you refer to as “the inflation model” is actually the standard cosmology, where the inflation mechanism is but a part. It is also only one mechanism for universe expansion. Nancy’s recent post has a great figure of the observable universe in the standard model. Notice how “inflation” points only to the early exponential expansion where the universe really ‘blows up’? The later, much more sloven pace is AFAIU just the universe freewheeling, near enough, from the first stage. (Mostly because there’s no… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 5, 2011 2:37 PM

More explanatory: “our universe arose from a “Planck volume”” – our universe arose from a minute “Planck volume”. Those suckers are really small, which is why we have trouble (need tremendously high energies and/or long times) to see into such scales.

wjwbudro
Member
wjwbudro
January 5, 2011 7:47 PM
Thank you for taking your time to reply. Although I sounded quite naive I actually have a few years of trying to absorb the cosmology controversy. Also I have been a member of the U.T. forum since close to it’s beginning and it has been part of my inspiration to seek a deeper insight into the hidden mysteries. I even suffered through the heated “iron/electric sun” b.s. I’m now retired and can now spend more time getting more confused. I have seen that depiction in Nancy’s “Shedding new light…” article more than once, More Thoughts (and now math!) On What Came Before the Big Bang and I do understand that “inflation theory” is but a part of the… Read more »
Lawrence B. Crowell
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Lawrence B. Crowell
January 5, 2011 4:55 PM
Galactic coalescence occurs for galaxies in a cluster. These are gravitationally bound to each other and are in mutual orbits. The dynamics is no different from the motion of a binary star system; it is just bigger, more massive and the orbital period much longer. If the orbit takes the galaxies close to each other they then interact with each other. Most of a galaxy is dark matter which does not interact by means other than gravity — for all practical purposes. So the DM halos will continue merrily along their dynamic path. The luminous stuff is mostly gas, which does interact and friction slows that down so it sticks at a collision spot. Stars are somewhat slowed… Read more »
The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
January 5, 2011 11:15 AM
“Black holes power AGN; the term AGN referring more to the observational fact that some galaxies appear to emit radiation from their core regions in an amount and with characteristics that cannot be explained merely by the presence of stars or associated activity…” The activity in the centre of the AGN galaxies is caused probably by the accelerating ionised gases whizzing madly around their galactic centres, therefore emitting the radiation. Variability in the black hole in the centre is consuming stars and gas at a rapid rate, in which the matter “screams” before reaching the event horizon. If galaxies absorb little ones to make the ‘mother’ galaxy bigger, the material mostly avoids the core, but is torn apart… Read more »
The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
January 5, 2011 11:22 AM

Oops! I said “The area of a one million solar mass central black hole might be about a single parsec or so,”

By this I mean the gravitational influence dragging the material and accelerating the gas. I don’t mean the accession disk or the size +million solar mass event horizon. Sorry.

Lawrence B. Crowell
Member
Lawrence B. Crowell
January 5, 2011 5:13 PM
The coalescence takes a long time to happen. In the immediate aftermath of the collision the DM halo and black holes continue to orbit as if almost nothing happened. The luminous matter will however clump up fast and the complicated gravitational interplay will see that material chewed up like a piece of gum. This does rob the kinetic + gravitational energy of the DM halo and black holes (eg friction) so the DM halo will eventually settle into a single clump and the BHs will settle in as well. The resulting galaxy will then have two SMBHs that will over time have their orbital energy decrease, due to heating up of luminous matter, and they may after many… Read more »
wjwbudro
Member
wjwbudro
January 6, 2011 5:38 AM

Great explanation and I appreciate it. I’ll add orbital mechanics on my to do list. lol

Lawrence B. Crowell
Member
Lawrence B. Crowell
January 6, 2011 6:47 AM
The orbit of two galaxies is a classic two body problem in Newtonian mechanics. Where things diverge is due to the size of these galaxies, so that as they get close to each other tidal forces and other processes start to kick in. However, since DM only interacts by gravity (FAPP), and black holes (even huge ones) are little particles or bullets they orbit on as if virtually nothing is going on. The system does eventually settle down. The interactions between luminous matter, in particular gas, converts a lot of this orbital energy into heat and is a source of “friction.” So given some number of billions of years the two galaxies do eventually coalesce into a single… Read more »
The Eclectic Exterminator of Stupid Electricians
Member
The Eclectic Exterminator of Stupid Electricians
January 5, 2011 11:35 AM

AN IMPORTANT NOTE: Also read Nancy Atkinson’s later story on JANUARY 5, 2011; “Star Birth and Death in the Andromeda Galaxy”
Here the X-ray image supplied in the story shows the real violence of the colliding ionised gases in the core. The material (and stars) are being mixed up the variable gravitational field likely being destabilised by galaxies mergers.
If you look at the XMM Newton’s view in X-Ray. at the inner core of the central bulge the area is quite minuscule compared to the bulk of the galaxy!

wjwbudro
Member
wjwbudro
January 6, 2011 2:23 PM

I don’t mean to be a pest but this is intriguing and I thank you as well for your help. I don’t know the density but doesn’t this Alpha H filtered image show a substantial amount of far reaching gas surrounding the core as well as what looks like a collision remnant somewhat perpendicular to the galaxy plane? I did get permission to include the link. http://www.starscapeimaging.com/M31_Ha_1210.html

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