KICP Seminars & Colloquia
KICP Seminars & Colloquia, Spring 2010
FRIDAY NOON SEMINARS
Light WIMPs, the Plot Thickens?
Juan Collar, The University of Chicago
The Environment of Major Mergers between Dark Matter Halos
Janice A Hester, California Institute of Technology
Major mergers are a popular theoretical mechanism for creating early type galaxies. Under the right conditions, simulated major mergers of gaseous spiral galaxies pass through a star burst phase, an active AGN phase, and finally form a red and dead elliptical remnant after feedback from the AGN heats and expels the gas from the galaxy. The remnants formed in these simulations are supported by random stellar motions, are centrally concentrated, have old red stellar populations, are surrounded by hot gaseous halos, and host dormant massive black holes. Anecdotal evidence supports this proposed mechanism; several classes of observed galaxy appear to correspond to the theoretical stages of the transformation of a major merger of two spiral galaxies into a red elliptical galaxy. Before we can claim to have revealed the source of the Hubble diagram, however, rigorous observational tests of the major merger driven evolutionary scenario must be developed and applied. Statistical tests, which can add the rigor of large numbers, cannot rely on detailed observations or analyses of individual galaxies. A novel probe is required in order to determine whether major mergers constitute a significant fraction of the populations of interest, such as E+A galaxies and luminous AGN. We studied major mergers between dark matter halos in the Millennium Simulation in order to determine whether environment can provide this probe. We found an excess of less luminous companion galaxies located at small physical distances from merger remnants which is a strong, potentially observable, signature of major merger populations. This diagnostic is robust against general assumptions about the role gas physics plays in determining whether a major merger follows the prescribed evolutionary path. We are working close to the limit of the simulation's ability to accurately simulate subhalos within the dense dark matter background of their hosts' halos. It is therefore imperative that we develop a physical understanding of the dynamics that determine the major merger rate and use this understanding to distinguish whether the measured environmental signatures are true diagnostics of major mergers or artifacts of the simulation. I will present a break down of subhalo dynamics and demonstrate their effects in the simulation. A subhalo's opportunities to transfer its orbital energy and angular momentum with respect to its host halo into the internal orbits of itself, its host, or its companions, play a key role in determining if and when the subhalo will merge with its host. Inelastic collisions between subhalos of the same host therefore greatly enhance the merger rate. This is the root cause of the measured excess of companions around merger remnants.