Binary Compact Object Coalescence Rates: The Role of Elliptical Galaxies

In this paper, we estimate binary compact object merger detection rates for LIGO, including the potentially significant contribution from binaries that are produced in elliptical galaxies near the epoch of peak star formation. Specifically, we convolve hundreds of model realizations of elliptical- and spiral-galaxy population syntheses with a model for elliptical- and spiral-galaxy star formation history as a function of redshift. Our results favor local merger rate densities of 4 × 10^-3 Mpc^-3 Myr^-1 for binary black holes (BHs), 3 × 10^-2 Mpc^-3 Myr^-1 for binary neutron stars (NSs), and 10^-2 Mpc^-3 Myr^-1 for BH-NS binaries. We find that mergers in elliptical galaxies are a significant fraction of our total estimate for BH-BH and BH-NS detection rates; NS-NS detection rates are likely dominated by the contribution from spiral galaxies. Limiting attention to elliptical-galaxy plus only those spiral-galaxy models that reproduce current observations of Galactic NS-NS, we find slightly higher rates for NS-NS and largely similar ranges for BH-NS and BH-BH binaries. Assuming a detection signal-to-noise ratio threshold of 8 for a single detector (in practice as part of a network, to reduce its noise), corresponding to radii D _bns of the effective volume inside of which a single LIGO detector could observe the inspiral of two 1.4 M _sun NSs of 14 Mpc and 197 Mpc, for initial and advanced LIGO, we find event rates of any merger type of 2.9 × 10^-2-0.46 and 25-400 yr^-1 (at 90% confidence level), respectively. We also find that the probability P _detect of detecting one or more mergers with this single detector can be approximated by (1) P _detect ~= 0.4 + 0.5 log(T/0.01 yr), assuming D _bns = 197 Mpc and it operates for T yr, for T between 2 days and 0.1 yr, or by (2) P _detect ~= 0.5 + 1.5 log(D _bns/32 Mpc), for 1 yr of operation and for D _bns between 20 and 70 Mpc.