The binary black hole merger rate from ultraluminous X-ray source progenitors

Abstract

Ultraluminous X-ray sources (ULXs) exceed the Eddington luminosity for a $\approx 10M_\odot$ black hole. The recent detection of black hole mergers by the gravitational wave detector ALIGO indicates that black holes with masses $> 10 M_\odot$ do indeed exist. Motivated by this, we explore a scenario where ULXs consist of black holes formed by the collapse of high-mass, low-metallicity stars, and that these ULXs become binary black holes (BBHs) that eventually merge. We use empirical relations between the number of ULXs and the star formation rate and host galaxy metallicity to estimate the ULX formation rate and the BBH merger rate at all redshifts. This assumes the ULX rate is directly proportional to the star formation rate for a given metallicity, and that the black hole accretion rate is distributed as a log-normal distribution. We include an enhancement in the ULX formation rate at earlier epochs due to lower mean metallicities. With simplified assumptions, our model is able to reproduce both the rate and mass distribution of BBH mergers in the nearby universe inferred from the detection of GW 150914, LVT 151012, GW 151226, and GW 170104 by ALIGO if the peak accretion rate of ULXs is a factor $\approx$1 --- 300 greater than the Eddington rate. Our predictions of the BBH merger rate, mass distribution, and redshift evolution can be tested by ALIGO in the near future, which in turn can be used to explore connections between the ULX formation and BBH merger rates over cosmic time.