Binary compact object mergers, gravitational waves and their host galaxies
In 2015 the ground based interferometers of the LIGO collaboration detected a binary black hole merger for the first time, opening a new era for the Gravitational Wave (GW) Astrophysics. In the last years, the number of events detected has grown thanks to the improvements in sensitivity and new detectors joining to this endeavor (LIGO-Virgo-KAGRA collaboration). These detections provided information of the coalescence of binary black holes (BBH), binary neutron stars (BNS) and binary black hole- neutron star (BHNS) systems.
The properties of the binary compact objects mergers confirmed by the LIGO-Virgo-KAGRA collaboration have triggered new exciting questions in Astrophysics. Among others, some of these questions are: what are the formation pathays that the progenitors follow? are there physical mechanisms explaining the detection of black holes with masses of the order of ~100Msol or have we detected hierarchical mergers? have we detected primordial black holes? what are the conditions we need to detect a binary compact object merger and its electromagnetic counterpart?
My interest on the host galaxies of merging compact objects started a couple of years ago, after the first detection of a binary neutron star merger (BNS) named as GW170817 by the LIGO-Virgo collaboration and its electromagnetic counterpart. This event was (and still) very special, since it was possible to identify the host galaxy, NGC4993. The first host galaxy detected seemed to be a with poor star formation at present and identified as an early-type galaxy.
Understanding which are the most likely host galaxies of merging binary compact objects is important for several reasons: 1) it can help us to disentangle between the different formation channels; 2) it helps to improve the electromagnetic counterpart searches from GW events, and 3) they provide an alternative way to investigate the Hubble constant.
Using the simulated galaxy catalogues from the EAGLE simulation, I have explored the properties of the most likely host galaxies of merging binary compact objects cross cosmic time, between z=0, and z=6. My approach consists of combining the galaxy catalogs with the results from population synthesis models.
In my papers of 2019 and 2020, we show that the stellar mass of the host galaxy is the most fundamental tracer for the merger rate per galaxy (i.e., the number of events per unit time in a given galaxy). This trend is present for all the redshifts we studied.
Merger rate per galaxy (nGW) for binary black holes as a function of the stellar mass of the host galaxy at redshift z=0. Each point represents an individual galaxy from the EAGLE simulation. The colour code indicates the metallicity of the host galaxy.
We find a strong dependence of the merger rate per galaxy with the stellar mass. This dependence varies for each merging compact object. For further details, see: https://ui.adsabs.harvard.edu/abs/2019MNRAS.487.1675A/abstract
Star formation rate (SFR) as a function of the stellar mass of the host galaxies. The color code represents the merger rate per galaxy for binary neutron star mergers (here demarked as nDNS) at z~0.
These results show that the stellar mass of the host galaxy plays a fundamental role for the merger rate per galaxy, more than the SFR.
The black star represents the stellar mass and SFR property of NGC4993, the host galaxy of GW170817.
See: https://ui.adsabs.harvard.edu/abs/2019MNRAS.487.1675A/abstract