|From Ringberg Castle looking to the mountains. Image Credit: D. Kocevski|
|Chess and science in the Duke's office.Image Credit: D. Kocevski|
AGN, to this enamored scientist, are some of the most fantastic denizens of our astounding Universe. Our best picture of these beasts is one in which profoundly dense super-massive black holes, millions to billions of times as massive as our Sun, are growing as vast amounts of gas fall into them, in a process that is, as yet, poorly understood. Immense sources of energy, they pump radiation and powerful winds into their environments, and, through this, halt the formation of stars by blowing out or heating essentially all the gas in the galaxies they inhabit. It turns out, however, that it is genuinely very hard to pinpoint the moment in which AGN actually directly affect their 'host' galaxy, mostly because such 'feedback' happens over a very short time, a fleeting heartbeat of an instant in the long eons of cosmic time. Instead, galaxy modelers can tell observers like me what the long-term effects of feedback are and we can go out to our telescopes and vast datasets and try to search for the smoking gun, the imprint of AGN feedback on their host galaxies.
|Astronomers at the Ringberg AGN meeting, deep in the middle of a discussion session. |
Image Credit: D. Kocevski
At the meeting, some spectacular examples were shown of heating and winds from AGN, clearly supporting the notion that feedback from the black hole has its place in the panoply of astronomical processes. However, a number of new studies show a very weak relationship between star-formation and AGN activity -- not what is expected, either by models or if feedback is widely present. We had some interesting and fruitful discussions at the meeting about this apparent mismatch. Is feedback not as efficient at halting star-formation as we had thought? Do we understand well enough the timescales over which feedback acts on galaxies? Are we missing something in the models? A clear picture eluded us, but we moved on with a set of new experiments and concepts with which to tackle these problems.
Another interesting and relevant question that took up a lot of discussion is why AGN turn on in the first place: what triggers them? Bright AGN are rare - another poorly understood aspect of their nature. Their frequency, how common they are in galaxies, has a directly relation to how much they can affect the evolution of their galaxies, especially if their occurrence is tied to an important phase in a galaxy's life. For example, if a galaxy happens to enjoy a windfall of gas from intergalactic space which spurs the formation of stars, some of that gas may fall to the center and light up the black hole. The resultant AGN will then blow out the gas from the galaxy - the ultimate cosmic killjoy. In this case, the trigger is the process that dumps gas into the AGN's host galaxy and the processes that carry it to the center. Other popular triggers are galaxy mergers, vast collisions that roil up galaxies and spur huge bursts of stars and nuclear activity.
At the meeting, we learned about novel and interesting ways to fuel and trigger AGN in the early Universe. Back then, the Universe was a lot denser than it is now, and streams of gas would fall on to galaxies from intergalactic space. In a process similar to mergers, these flows of gas could shake up the galaxy and send a lot of gas into its center. Such a process is not expected to be important in the present-day Universe around us - a reminder of the arrow of time and the finite history of the Cosmos. In more local AGN, a clear sign of the role of galaxy mergers was shown from studies of pairs of AGNs in the Sloan Digital Sky Survey. In addition, a simple connection was shown to exist between AGN and the gas content of a galaxy, which suggested that, while mergers are a good way to trigger AGN, they are probably not the most important process in nearby galaxies. The meeting helped to underline the complex variety of ways that gas feeds the monstrous black holes in the centers of galaxies.
|Stories exchanged over beer in the fabulous Hexenzimmer.|
Image Credit: D. Kocevski