We would like to know if active galactic nuclei change over cosmic time. Do black holes feed and grow in the same way over the history of the Universe?

We find active Super Massive Black Holes at the centers of massive galaxies. Our Galaxy also has its own black hole at its center measuring 2.6 million solar masses. Our black hole is not active today, but we presume it was active in the past.

Despite the enormous changes in the space density of back holes, the individual engines powering active galactic nuclei are remarkably stable.

Chandra has worked well for six years now. There is no reason why Chandra and Newton cannot continue to observe for another 10 or more years.

We wanted a census of all the black holes and we wanted to know what they are like. We also wanted to measure how black holes have grown over the history of the Universe.

Stress is the number of changes endured, divided by a short period of time. This includes a change of residence, death of family, change in job, etc.