Modelling the host galaxies of binary compact object mergers with
  observational scaling relations
  The merger rate density evolution of binary compact objects and the
properties of their host galaxies carry crucial information to understand the
sources of gravitational waves. Here, we present galaxyRate, a new code that
estimates the merger rate density of binary compact objects and the properties
of their host galaxies, based on observational scaling relations. We generate
our synthetic galaxies according to the galaxy stellar mass function of star
forming and passive galaxies. We estimate the metallicity according to both the
mass-metallicity relation (MZR) and the fundamental metallicity relation (FMR).
Also, we take into account galaxy-galaxy mergers and the evolution of the
galaxy properties from the formation to the merger of the binary compact
object. We find that the merger rate density evolution with redshift changes
dramatically depending on the choice of the star-forming galaxy main sequence,
especially in the case of binary black holes (BBHs) and black hole neutron star
systems (BHNSs). The slope of the merger rate density evolution of BBHs and
BHNSs is steeper if we assume the MZR with respect to the FMR, because the
latter predicts a shallower decrease of metallicity with redshift. In contrast,
binary neutron stars (BNSs) are only mildly affected by both the galaxy main
sequence and metallicity relation. Overall, BBHs and BHNSs tend to form in
low-mass metal-poor galaxies and merge in high-mass metal-rich galaxies, while
BNSs form and merge in massive galaxies. The metallicity distribution of the
host galaxies is significantly affected by the adopted metallicity relation. We
predict that passive galaxies host at least ~5-10 %, ~15-25 %, and ~ 15-35 % of
all BNS, BHNS and BBH mergers in the local Universe.
