The evolution of compact object mergers and their host galaxies across cosmic time

The next-generation network of gravitational wave observatories are expected to detect many more compact object mergers inside their host galaxies. In light of this future circumstances, I developed a theoretical tool that investigates the properties of formation and host galaxies of compact objects. The starting point of my new methodology is a population of star-forming galaxies across cosmic time obtained from observed scale relations. I varied the metallicity of galaxies, considering the fundamental metallicity relation (FMR) and the mass-metallicity relation (MZR). With my fast methodology, we are also able to compare different populations of compact objects. In this talk, we will see the impact of different population-synthesis simulations, where I varied the value of the alpha common envelope (α_CE = 1 , 3 and 5). I evaluated the merger rate density of compact objects for this set of models and I compared it with the 90% credibile intervals inferred by the LIGO-Virgo-KAGRA collaboration. We will see how different metallicity distributions strongly affect the merger rate density evolution of binary black holes (BBHs) with redshift. By looking at the properties of host galaxies, I learnt that low mass galaxies with stellar mass <10⁹ solar masses can host up to 50% of the total BBH mergers, with α_CE = 1 . For larger masses, the merger rate per galaxy correlates with stellar mass of galaxies for all compact objects types. Moreover, the typical masses of the host galaxies increase significantly as we approach the local Universe. An indicator of this trend is the rise in percentage of compact object mergers hosted in passive galaxies, which can be > 80% for α_CE = 5 .

I acknowledge financial support from the European Research Council for the ERC Consolidator Grant DEMOBLACK, under Contract No. 770017.