The Incidence of AGN in Galaxy Groups, Part I: What is an AGN?

Seeing as I'm starting work on the poster for my presentation in two weeks' time, I figured writing about my SURF might be a good place to begin. This past summer I had the pleasure of being a visiting student at the Smithsonian Astrophysical Observatory in Cambridge, MA. For 10 weeks (and a few days) I worked with Dr. Andy Goulding and Dr. Christine Jones on a project regarding AGN in Boötes.

In light of Jackie's question from last week, let's talk about what exactly an AGN is. I could quote Wikipedia and say "An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum," but what's the fun in that? That sentence tells us everything and nothing about AGN. What causes AGN? And why do we even care about this mysterious AGN? Those are the real questions.

First of all, to clarify further from the Wikipedia definition, an AGN isn't just a region at the centre, it is the core of a galaxy which emits far greater radiation than the rest of the galaxy or galaxies that are not considered "active" (do not have AGN). These cores, or nuclei, occur due to accreting (growing) black holes at the centres of galaxies (Cappelluti et al., 2010). Now we have "what" and "how", but how about "why"? AGN are important because of the effect they have on galactic properties. They influence things such as stellar mass, star formation rate, and colour in their host galaxies. As a result, it is important to study AGN in order to better understand this influence (Coil et al., 2009; Hickox et al., 2009).

Next Time: In what ways can we study AGN?

References

Cappelluti, N., et al. 2010, ApJL, 716, L209

Coil, A. L., et al. 2009, ApJ, 701, 1484

Hickox, R. C., et al. 2009, ApJ, 696, 891

A brief introduction

Given the nature of this class, I suppose I should start with the most obvious answer: "I don't know". It's an answer to every question which has plagued me my entire life. At times it sounded more like "eye-unno" or something even less intelligible, but I have always come back to the response, "I don't know".

—What would you like for breakfast?

—I don't know.

—Ninth Doctor or Eleventh?

—I don't know.

—Why astrophysics?

—Astronomy, not astrophysics, and I don't know.

Notice a pattern? I honestly don't know why astrophysics. Why study this particular aspect of the universe? In my youth I went through the entire list: astronaut, palaeontologist, novelist, mechanical engineer, photographer, psychiatrist, FBI agent, then finally astronomer. Seems to have come full circle from the child who wanted to travel to the stars to the young adult who wants to study them. Never ignore a coincidence, so I can say with some certainty that my interest did not begin with John. I can call him that now that he's gone. Never quite managed it while he was here. Or at least, that's what I would like to convince myself. I'd like to tell myself the only thing I took from him is my insistence on calling myself an astronomer.

Whatever I got from him, our friendship brought me to the Harvard-Smithsonian Center for Astrophysics, where I would take in other astronomers' posters. And finally, I went back this past summer for a SURF and now I'll be making my own poster. And the topic of that poster? "I don't know." "I don't have the source statistics to solve this problem, but if I did, this is how I would try." I can't even pretend to understand all of what I studied. The Incidence of AGN in Galaxy Groups? Please. Give me a break. I worked on that project; devoted 3 months of my every waking moment to it. Yet I have to admit I can hardly explain to my mother what an active galactic nucleus is. But I can finally say, I have the answer to the question. Which question? Every question:

"I don't know, but I'm willing to find out"