The host galaxies of double compact objects across cosmic time

We explore the host galaxies of compact-object binaries (black hole–black hole binaries, BHBs; neutron star–black hole binaries, NSBHs; double–neutron stars; DNSs) across cosmic time, by means of population-synthesis simulations combined with the Illustris cosmological simulation. At high redshift (z 4), the host galaxies of BHBs, NSBHs, and DNSs are very similar and are predominantly low-mass galaxies (stellar mass M < 10¹¹ M). If z 4, most compact objects form and merge in the same galaxy, with a short delay time. At low redshift (z ≤ 2), the host galaxy populations of DNSs differ significantly from the host galaxies of both BHBs and NSBHs. DNSs merging at low redshift tend to form and merge in the same galaxy, with relatively short delay time. The stellar mass of DNS hosts peaks around ∼10¹⁰–10¹¹ M. In contrast, BHBs and NSBHs merging at low redshift tend to form in rather small galaxies at high redshift and then to merge in larger galaxies with long delay times. This difference between DNSs and black hole binaries is a consequence of their profoundly different metallicity dependence.