The Problem with Black Holes

Image from radio observations
of the very brightradio galaxy 3C31.
This is an extreme example of the
type of source that could
interfere with a radio survey
for galaxy clusters.

Well, my Canadian adventure has come to an end, and it was quite the experience. Unfortunately there were more insects than there were large mammals to watch, but we did have a 10 foot beluga whale circle the canoe on the last day. That is now one of my favorite wildlife sightings, ranking 3rd behind a mountain lion just outside San Jose and a pack of wolves in Denali park.

After 17 days of paddling through the bush, we finally arrived in Kuujjuac with a couple hours of tourist time before our flight. After picking up a T-shirt, mug, and a pair of walrus ivory earrings that my girlfriend will probably never wear, I flew out for a 5 day layover in Montreal.

I took advantage of my layover to do some work with a friend who is now an assistant professor at McGill. We used to work in the same research group, and decided to take advantage of this stop in Montreal to run some computer simulations of the astronomy observations my old group is conducting in Owens Valley.

I have already discussed similar observations in one of my earlier posts. The observations in Owens Valley are also part of a search for galaxy clusters, using a radio wave telescope known as the Sunyaev-Zel'dovich Array (SZA).

To briefly summarize, the Sunyaev-Zel'dovich effect is basically a way of observing galaxy clusters in the radio regime of the electromagnetic spectrum. A large sample of such clusters would provide deep insights into the way matter and energy interact in the Universe.

One of the main difficulties in this type of observation is interference from black holes that are found at the center of galaxies. Some of these super-massive black holes power active galactic nuclei (AGN), which are among the most luminous sources in the universe. When an AGN, or other similar source shows significant radio emission, it is known as a radio galaxy.

It is these radio galaxies that make radio observations of clusters much more difficult. The clusters appear dark and the radio sources appear bright, and it requires a lot of effort to separate the two. In Montreal, we started running computer simulations to model this effort, and to help determine the best technique for extracting the cluster signal in the presence of radio galaxies. I imagine that we will have an answer by the end of the month.

I have to run now, I'm off for another 10 days of adventure, this time on the John Muir Trail. When I get back, I'll start attacking some of those fantastic questions you all dropped on me in my last post.

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.