Maybe galaxy mergers don't cause nuclear activity!

Jan 06 2011 Published by under Astronomy & Physics

Knock me over with a feather. I really didn't expect this. I learned about this first over at Phil Plait's blog, but given the title of my blog I think I really ought to address it.

First, some background. My obsession with interacting galaxies goes back to my first year of grad school, when, in 1991, I started working with Tom Soifer. This wasn't too long after the publication of the IRAS Bright Galaxy Sample— if you follow the link, you'll see that Soifer is the first author on that article. A lot of follow-up work was done (and is still being done!) on the class of galaxies that was established in this sample. In particular, there are a class of galaxies that are particularly luminous at infrared wavelengths. There are things going on hidden behind dust that prevents them from always being particularly bright at optical wavelengths. Many of the galaxies, especially the brighter ones, in this sample show evidence of vigorous star formation. Additionally, many of these galaxies show evidence of being in the process of a massive collision, or being the recent result of a major merger.

The association between galaxy mergers and strong nuclear starbursts remains robust. A search at ADS on appropriate keywords can bring up a lot of recent papers, and perusing them shows that the observational evidence and theoretical reasoning that leads us to believe that major mergers of two big galaxies triggers a vigorous burst of star formation near the core of the merger.

Stars are formed from gas. A compact nuclear starburst associated with mergers means that gas is getting funnelled down to the centers of galaxies when they're merging. You'd also expect this theoretically, as gas clouds in the two galaxies run into each other and dissipate bulk orbital energy in the fluid interactions. What's more, we have known for more than a decade now that all, or at least most, big galaxies have a black hole at their core. Finally, we know that the phenomenon of Active Galactic Nuclei (AGN) is fuelled by accretion on to a black hole. If you put all of these things together, you would expect AGN activity to be correlated with galaxy mergers. To turn on an AGN, you have to feed the black hole at the center of a galaxy, which means getting all the gas down to the center. We know that mergers can send a lot of gas towards the center. Thus, you'd expect a correlation.

It is true that if you're talking about a "compact" nuclear starburst, it's not nearly as compact as the accretion disk around a black hole. The compact nuclear starburst will be of a size that's hundreds, or even a thousand or so, parsecs... whereas the size of the accretion disk is much, much less than a single parsec. So, just because you can get the gas down far enough to make a compact nuclear starburst, it may well be that you can't get the gas the rest of the way down to fuel an AGN as part of a merger, and that other processes are needed. It's so easy to say that now, with this new result (arXiv preprint of the paper) showing that AGN activity is not correlated with the morphological signatures of a major merger. If I am to be completely honest, before today I wouldn't have pointed out the scale difference between an AGN accretion disk and the (much larger) size of a compact nuclear starburst. Indeed, in popular talks I've given about interacting galaxies, I've repeated what the general consensus (or at least one popular model) was, that mergers can trigger AGN.

It will be interesting to see if this result holds. I do feel a small twinge of envy that I wasn't the one (or one of the ones) doing this statistical survey, as in a previous life it was something that I had on my list of research topics that would be interesting to try to confirm. But, clearly, it was a topic of interest to a lot of people, and it's great that we've got a result now... even if the result isn't the one I expected or hoped for. I look forward to the further unfolding of this story.

5 responses so far

  • Naved says:

    Rob, very interesting result - and right up your alley! Are you planning on continuing any of your research with your Quest students?

    Is the difference between a major and a minor interaction simply the masses of the galaxies? The abstract (I haven't read the paper) says that minor interactions could also trigger AGN, and it sounds like their "by-eye" classification method might not detect these minor interactions.

    Is there a way to calculate/guess the probability of a galaxy having a major interaction over its lifetime by using the space densities and velocities of galaxies in the Universe (taking into account the expansion over time)? Maybe such an estimate can be compared to the fraction of their galaxies that show signs of a major merger.

  • Bau Ur says:

    Is there more nuclear activity than can be accounted-for without contribution of matter to AGNs by galaxy mergers?

    Or was the amount of nuclear energy a calculation based on assumptions about AGNs -- rather than being an observation that calls for explanation?

  • rknop says:

    Bau -- the total luminosity of AGN tells you that the accretion rate on to the black hole has to be something like a few solar masses per year, if memory serves. (Typically; it varies a lot.) While that's a lot, compared to the amount of gas a galaxy has it's pretty small. So, there's no problem with having the fuel. But, you do have to get the gas down to the center somehow. Mergers was just one way to get the gas down to the center, and were appealing because we know they get gas *near* the center. But, if you have other ways to get gas to the center (galactic bars are another way that seems to work, at least sometimes), you don't need mergers. I'm not surprised that there are other triggering mechanisms; I think everybody would have expected that. What surprises me is that there's not even a correlation to suggest that mergers are really all that significant a contributor to the triggering.

    Naved -- yeah, mass ratio is more or less what is a major merger versus a minor interaction. Two big galaxies passing each other would be a minor interaction, but most of those are going to end up as a merger anyway as a result of kinematic friction.

    Re: continuing my research, I'm a bit out of it at the moment. Since I worked at Linden for two years, my research more or less stopped, except the parts of it on blazars I was continuing with Anders and Cameron (two undergrads after your time). One of the saddest things to happen was when a disk in my RAID array died and I could never read the array again. The whole point of having RAID is so that you can still get the data even if a disk dies, but, alas, it didn't work for me. Coupled with that was the loss of all the raw data on CDs that I had when my office at Vandy was moved twice. (They weren't paying me while I was advising to more honors theses, but they did let me have an office... they just moved it a couple of times. Somewhere in one of those moves the CDs disappeared. I looked for them, but they never turned up. They didn't turn up when I moved either, so I didn't lose them at home. I have no clue what happened to them.) I'm very sad about that, because it means that the VV114 data you worked on is gone, and all the blazar data is gone.

    If I still had that, I might try to push that to publication. In reality, though, I'm sort of three years removed from the last research I did. More recently, I've worked with folks in MICA on n-body simulation stuff and on using virtual worlds as a platform for scientific collaboration. Indeed, a year ago I was a co-PI of an NSF proposal to work on that stuff, which alas wasn't funded. Since I got to Quest, I've been teaching every block, so I haven't really sat down to think and figure out what the heck I'm going to do for research and scholarly stuff. At some point, I'm going to need to figure that out. In the mean time, I expect to dabble in OpenSim, and pay attention to what's going on with interacting galaxies.

  • Bau Ur says:

    Rob, I did not know about your loss of data before. It's horrifying. It makes me ill. My legs have gone numb and weak -- I am not sure I could even stand if I tried to do so. I'm terribly sorry that happened to you. And to you Naved, as it seems some of the data were also yours.

  • Naved says:

    Rob - so sorry to hear about the data loss. On top of everything else, that is a real bummer. I have occasional nightmares about losing 5 years worth of PhD data, even though I have 3 or 4 backups.

    Small piece of good news - I have the CTIO data from our 2004 October run (4 nights - this is when we got VV114, among others). It's about 400 MB total, and I have it on my hard drive. Maybe you can re-analyze and publish this with your students. Do you have blocks when you don't teach and can do research with undergrads? Anyway, let me know if you want this data and how I can get it to you.