Q: Who killed the Star Formation? A: The AGN did it.

One of the neat things about being a professional astronomer is sometimes knowing the authors of neat papers. Submitted to the arXiv on Friday was one such neat paper with an author list full of people I respect from their work and 1 collaborator. So let’s just say this is all neat and move on to the science.

So here is the paper. On a quick read, this is just another paper on what happens to gas when and star formation when an AGN gets involved. If you’ve listened to many episodes of Astronomy Cast, you might have heard me explain this before. AGN (short for Active Galactic Nuclei) are actively feeding black holes in the centers of galaxies. These giant monsters are capable of calmly devouring vast quantities of gas and dust with accretion rates (the rate they take stuff in) of anywhere from 1 to 100 Solar Masses a year. AGN can occur in all sorts of different galaxies, and once upon a time, our own Milky Way galaxy might have had one in its core. Spiral galaxies with lots of gas and dust can recover from their AGN phase to go on to billions of years of happy go lucky star formation. Elliptical galaxies aren’t so lucky.

If you look out at the sky with a reasonably large telescope and you explore the distribution of colors and shapes of galaxies, you’ll find that spirals are generally (but not always) blue, and elliptical galaxies are generally (but not always) red. Any of you who have played with Galaxy Zoo have probably seen this too, and cursed your fair share of boring red blobs. The only way to get a blue galaxy is to look at a system that contains lots of hot stars, and hot stars don’t live that long, so either you are looking at something with ongoing star formation or something that produced stars until very recently. This tells us that in general, spiral galaxies are actively forming stars, and ellipticals are not. As we look at these systems across other wavelengths, looking for gas and dust using Infrared and radio and all the colors of light in between, we also find that elliptical galaxies are poor in gas and dust and spirals are rich in those exact same things. This means ellipticals generally just don’t even have the stuff in them necessary to form stars.

The question is, how did they get that way?

The standard answer has always been that there is a burst of star formation (often triggered during the formation of the elliptical, or the merger of a spiral galaxy into an elliptical), and the gas and dust not involved in this sudden burst is driven into the super massive black hole in the center of the system (and when I say super, I mean something 10,000,000,000 solar masses or so in size!) This process causes the galaxy to first light up with star formation, then the AGN turns on, and in the end the AGN is all that is left signaling the recent event. The timescale for this process was originally thought to be determined largely by the rate of star formation, but star formation is a slow thing, with many billion years being required to eat away the gas and dust. But… with timescales that long we should see more blue(r) ellipticals. The longer something lives, the more likely we are to see it. (Think of being in a room with a variety of lights that randomly turn on and off that are on all sides of you. Those that turn on for the longest period of time are the ones that you are most likely to see, while those that flick on the smallest fraction of a second are very unlikely to be something you see well enough to learn anything about them.)

So here we were with a model and a universe that weren’t in perfect alignment.

But then came this paper, to tell us we had underestimated the effects of an AGN. In this paper, they look at a series of 24 galaxies (10 Star Forming, 10 AGN+Star Forming, and 4 AGN). What they found was about 200 Million years after the Star Forming turns on, the AGN kicks into full swing, and the Star Formation dies quickly – it dies in a fraction of the time predicted from models that use star formation to devour the dust. This indicates that the AGN is having more of an effect on the dust than previously thought.

They put forward in this paper that AGN feedback is responsible for destroying the gas that would have otherwise gone into star formation. This process was already thought to exist, but the magnitude of the role it plays hadn’t been assumed to be this big! It appears that the actively feeding AGN is able to heat and expel the gas very quickly. We knew this happened in GIANT galaxies, with GIANT AGN (think M87), the type of systems that often sport massive jets and other rather radical high energy phenomenas. What we didn’t know was that low luminosity AGN do the exact same thing. And that is just cool. (Or actually “That’s Hot”).

This paper gives us a cleaner insight into how effectively black holes can kill things. That is a very flippant remark, but to put it more scientifically – we now have new insights into how actively feeding black holes can disrupt the surrounding gas so effectively that they shut down star formation essentially instantly (on cosmic timescales at least). From alive to AGN to dead and red in a billion years flat, that may be the life of even the most mundane elliptical galaxy.

While this paper does use a lot of technical language, it explains itself well and is well referenced. If you feel like chewing through something that will hopefully change how we look at ellipticals, give it a read.


On a completely unrelated note: There is a remarkably ugly huricane on its way to Texas. One of the worst nights of my life involved a tropical storm parked over Houston that caused flooding and tornadoes as far north as Austin (the barn I used to keep horses at flooded and several hundred horses had to be evacuated in the middle of the night with water rising faster then a human could run). Just looking at the satillite imagery is making quesy with anxiety. My heart goes out to everyone on the Gulf coast. Stay safe everyone.


  1. Nicole September 12, 2008 at 12:10 pm #

    It’s noon and I still haven’t gotten to astro-ph yet. Rawr.

    Sounds like a great paper! I always knew that AGN ruled the universe.

  2. Jay Parkhill September 12, 2008 at 12:25 pm #

    Dear Pamela,

    If I understand this correctly, a 10B solar mass AGN can eat up to 100 solar masses worth of gas and dust per year- or does the paper say the rate is actually higher than that?

    If I can stretch and tie this in to the Large Hadron Collider being turned on this week, a friend (@jamesrosberg) and I had a silly conversation on Twitter about how quickly any micro-black holes created by the LHC could eat the Earth.

    My answer was a long time, but it’s a trick question because perception of time varies depending whether you are in the black hole or standing outside pretending you could actually see what is happening inside.

    It sounds like this paper has more to do with AGNs disrupting star formation than their feeding rate per se, but if it is possible to answer our hypothetical by analogy, we’d be grateful for your thoughts on this question:

    If we pretend that any micro black holes created by the LHC are permanent, dangerous and inclined to eat the Earth, what would their mass be and how long would it take to consume the mass of the Earth?

    Many thanks!

  3. Tanya Urrutia September 12, 2008 at 5:25 pm #

    Hey Pamela!

    I just discovered your Astronomy Cast and this website. Well done! I have a paper accepted on astro-ph about young, dust reddened quasars in which we find a very large fraction of LoBALs (low ionization broad absorption line) quasars. These BALs have very strong high velocity winds, which can be believed to be AGN (quasar) feedback in action. My objects are very luminous (otherwise we wouldn’t have found them), so they don’t really compare that well to the paper you posted. Anyway, if you are interested… http://arxiv.org/abs/0808.3668

    Also, good job on the podcast! Well researched and well done! Immediate addition to my iTunes. Keep up the good work.



  4. Beth September 12, 2008 at 8:06 pm #

    My brain isn’t up to understanding AGNs tonight, but I viscerally understand hurricanes on the Texas coast. I was living a block from the seawall on Corpus Christi Naval Air Station when Hurricane Celia roared ashore on August 3, 1970. Luckily we were in shelters when the roof tore off my house and caved in my bedroom wall.

    The budding scientist in me had been tracking that storm. It encouraged me to keep paying attention to science and especially the sky. That included both astronomy and meteorology. Never mind that I discovered I loved computer science even more. I still enjoy the other two.

    Storm Pulse has current as well as archive information on hurricanes.

    The people on the Texas (and Louisiana) coast are in my prayers.

  5. The Perky Skeptic September 19, 2008 at 11:46 am #

    This is so cool!!! 🙂

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