Host galaxies of merging compact objects: mass, star formation rate,
  metallicity and colours
  Characterizing the properties of the host galaxies of merging compact objects
provides essential clues to interpret current and future gravitational-wave
detections. Here, we investigate the stellar mass, star formation rate (SFR),
metallicity and colours of the host galaxies of merging compact objects in the
local Universe, by combining the results of MOBSE population-synthesis models
together with galaxy catalogs from the EAGLE simulation. We predict that the
stellar mass of the host galaxy is an excellent tracer of the merger rate per
galaxy ${\rm n}_{\rm GW}$ of double neutron stars (DNSs), double black holes
(DBHs) and black hole-neutron star binaries (BHNSs). We find a significant
correlation also between ${\rm n}_{\rm GW}$ and SFR. As a consequence, ${\rm
n}_{\rm GW}$ correlates also with the $r-$band luminosity and with the $g-r$
colour of the host galaxies. Interestingly, $\gtrsim{}60$ %, $\gtrsim{}64$ %
and $\gtrsim{}73$ % of all the DNSs, BHNSs and DBHs merging in the local
Universe lie in early-type galaxies, such as NGC 4993. We predict a local DNS
merger rate density of $\sim{}238~{\rm Gpc}^{-3}~{\rm yr}~^{-1}$ and a DNS
merger rate $\sim{}16-121$ Myr$^{-1}$ for Milky Way-like galaxies. Thus, our
results are consistent with both the DNS merger rate inferred from GW170817 and
the one inferred from Galactic DNSs.
