Wednesday, December 12, 2012

Astronomers in a Castle

From Ringberg Castle looking to the mountains. Image Credit: D. Kocevski
On a bluff in the foothills of the Bavarian Alps, overlooking the placid waters of the Tegernsee, sits a castle. Now, castles are not uncommon in Germany, but this particular one is special for astronomers and many other scientists around the world. In 1973, this castle, Schloss Ringberg, was donated to the Max Planck Society and ultimately converted into a location for scientific meetings and conferences. Here, as many as 60 scientists can get together in a picturesque location, away from the bustle of everyday academic life, and put their minds together to tackle interesting and relevant questions in their fields, in a setting that promises good food, comfortable quarters, gorgeous views, camaraderie and colorful insights into the romantic, and only a bit outlandish, inspiration of the castle's creators and architects, the Wittelsbach Duke Luitpold and the early twentieth century artist, Friedrich Attenhuber.

Chess and science in the Duke's office.Image Credit: D. Kocevski
Every room is a testament to Alpine culture and design: warm and dark wood paneling, green spectrum paint, runic filigree embellishments, enormous tiled stoves in each room, pastoral and woodland themed tapestries and murals on walls and balconies. It was to this rather unique setting that a group of astronomers and astrophysicists, including several CANDELS team members, converged on the 3rd of December, a cold and snowy Monday, for a conference to discuss the connection between Active Galactic Nuclei (AGN) and the galaxies in which they are found. This connection is of key interest to astronomers studying how galaxies form and change with time, as discussed in many older posts in this blog. The primary aim of the meeting was to bring together some of the best theoretical models of galaxy evolution and compare them with our fresh and recent understanding of AGN from various observational campaigns, many of which have gained from the superb coverage of CANDELS.

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
Over two and a half days of presentations and lengthy discussions, we pondered the physics of unfathomably incredible forces in the far reaches of our Universe as heavy snow piled up on the trees and coated the castle grounds. Over breaks for coffee and lunch, we huddled in smaller groups and debated over some of the finer points of theory or interpretation, skeptical inquiry that defines the method of science. Over big glasses of fresh Bavarian beer in the evenings, we traded our scientist hats for other metaphorical headgear, as we exchanged stories and banter, and got to know each other better on a personal basis. This is probably one of the lasting legacies of a meeting at Schloss Ringberg - collaborations, future plans, friendships. All because a Duke decided decades ago to build a picturesque castle that he couldn't quite afford on a hill in Germany.

No comments:

Post a Comment