Studying AGN

Introductory comments from Varoujan

I have been very interested in Active Galactic Nuclei (AGN) for some time. Here is a brief introduction to the key points.

Basically AGN are galaxies that emit an excess of light from their centers beyond the sum total of light that should be coming from their central stars. It was initially postulated that this was due to a great deal of star formation but over time it was realized that it would be impossible to have that much star formation in such a small volume. The accepted paradigm became where a large amount of gas was flowing into a supermassive black hole at the center of an AGN. As the gas flows in, it forms an accretion disk and the friction of the gas with itself as it spirals in causes the disk to heat up and emit a lot of light, especially in the UV. Then, any dust that surrounds the disk absorbs that light and re-emits it in the infrared.

What is strange about AGN is that there is as yet no definitive method of measuring the luminosity of the AGN. This is different than for stars, where we have known a way of measuring the luminosity of stars for a long time. The Hertzsprung-Russell diagram for stars sets up a nice relation between the temperature (color) and luminosity of a star. As long as you know the temperature you have a very good idea of what the luminosity of the star will be. I hope to be able to do the same for AGN by using two archival datasets. The GALEX satellite did an all-sky survey in UV wavelengths and so has imaged a large number of AGN. Spitzer, although not all-sky, also imaged a large number of AGN. I hope we can use these great new UV and IR data to come up with a diagram for AGN which hopefully will be like an H-R diagram, where by observing a few telling photometric properties we will know the luminosity of the AGN.

The key reason to study this of course is that once you can accurately determine the luminosity of an AGN, then, since they are the most continuously luminous objects in the universe, they can instantly become distance indicators to the far reaches of the universe!

This link has a bit more background on AGN in general. No need to understand all the details but I think it gives a nice history of these objects. http://nedwww.ipac.caltech.edu/level5/Cambridge/frames.html