VU Physics and Astronomy Colloquium

Friday, October 25. Refreshments in NSC Planetarium at 3:05 PM, followed by the colloquium in 221 at 3:30 PM.

Citizen Science, Giant Ionized Clouds, and the History of Galactic Nuclei

Dr. Bill Keel, University of Alabama, Department of Physics & Astronomy


The signature discovery of the Galaxy Zoo citizen-science project has been
Hanny's Voorwerp, a galaxy-sized gas cloud ionized by a quasar which
has faded so rapidly that we no longer see it when observing the
galaxy nucleus. Project participants have helped find a sample of
similar objects, giving us our first look at the history of active
galactic nuclei on timescales from 30,000-120,000 years. About 40%
of these clouds require much more energy input than the nuclear
source can provide, indicating that dramatic variability of active
nuclei is common on these timescales. This is faster than simple
models indicate for accretion disk changes; signs of gaseous outflow
may mean that the rate of accretion is changing less strongly than
its byproducts, switching to kinetic rather than radiative-energy
dominance. Current surveys show more similar cases in both high- and
low-power regimes; our snapshot of the population of accreting supermassive
black holes will be incomplete without including these faded objects.