Emily Alicea-Munoz - Graduate Coop Student, NASA/GSFC, Code 663
NASA Goddard Space Flight Center
PSU Mailing Address:
Black Hole Mergers as Probes of Structure Formation
Thesis Advisor: M. Coleman Miller (UMD)
Co-Supervisors: Joan Centrella (NASA/GSFC), Pablo Laguna (PSU)
Intense structure formation and reionization dominate the first billion years of the universe. Thus it is important to quantify these processes through physical models in order to interpret the sparsely available data in this epoch. Essential clues about this critical era will come from the rate and properties of the mergers of massive black holes (MBHs). These are expected to be bright sources of gravitational radiation, detectable by space-based gravitational wave detectors such as LISA. Past efforts have been limited to calculating merger rates using different models in which many assumptions are made about the specific values of physical parameters of the mergers, resulting in merger rate estimates that span a very wide range (0.1 - 10^4 mergers/year). We will develop a semi-analytical, phenomenological model that includes plausible combinations of several physical parameters involved in the mergers, which we will turn around to determine how well LISA observations will be able to enhance our understanding of the universe during the critical z ~ 5 - 30 stucture formation era. We do this by generating synthetic LISA data from our model families, which we analyze using a Markov Chain Monte Carlo (MCMC) method. This allows us to constrain the physical parameters using merger rates, masses, and redshifts derived from LISA observations.
Our method encompasses three aspects: DM halo mergers; MBH-halo interactions; statistical analysis. In our first test case, a simple two-parameter model, we explored how halo mergers depend on the minimum mass of a halo to host a black hole (M_min) and the mass ratio threshold of parent halos.
On the left: merger rate as function of redshift for different values of M_min, with a fixed mass ratio threshold of 3:1. Small M_min leads to more mergers. Also notice that a small change in M_min leads to a large change in merger rates, especially at z>1. These results can be explained by the fact that the number density of high mass halos is smaller than that of low-mass halos at all redshifts.
On the right: merger rate as function of redshift for different mass ratio thresholds, for M_min = 10^10 M_sun. Mergers between halos of near-equal masses are fewer than mergers of halos with larger mass ratios. However, increasing the mass ratio threshold by a large amount does not significantly affect the total number of mergers. This can also be explained by the fewer number of high-mass halos that exist at all redshifts.