# The host galaxies of double compact objects across cosmic time

## TL;DR

This study combines population-synthesis and cosmological simulations to analyze the evolution and host galaxy properties of compact-object binaries across cosmic time, revealing distinct formation and merger patterns for different binary types.

## Contribution

It provides a comprehensive analysis of the host galaxy evolution of various compact-object binaries using combined simulation methods, highlighting differences in their formation and merger environments over cosmic history.

## Key findings

- At high redshift, host galaxies of all compact binaries are similar and mostly low-mass.
- Low-redshift DNSs tend to form and merge in the same galaxy with short delay times.
- Black hole binaries often form in small galaxies early and merge later in larger galaxies with long delay times.

## Abstract

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 ($z\gtrsim{}4$) the host galaxies of BHBs, NSBHs and DNSs are very similar and are predominantly low-mass galaxies (stellar mass $M<10^{11}$ M$_\odot$). If $z\gtrsim{}4$ most compact objects form and merge in the same galaxy, with a short delay time. At low redshift ($z\leq{}2$), 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 $\sim{}10^{10}-10^{11}$ M$_\odot$. 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.