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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 Gravitational Wave (GW) Astrophysics. In recent years, the number of detected events has grown thanks to improvements in sensitivity and new detectors (LIGO-Virgo-KAGRA collaboration). These detections provided information on the coalescence of binary black holes (BBH), binary neutron stars (BNS), and binary black hole- neutron star (BHNS) systems.
The properties of binary compact-object mergers confirmed by the LIGO-Virgo-KAGRA collaboration have triggered new exciting questions in Astrophysics. Among other questions, one is: what formation pathways do the progenitors follow? Are there physical mechanisms that explain the detection of black holes with masses on the order of ~100 Msol, 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 in the host galaxies of merging compact objects began about a decade ago, following the first detection of a binary neutron star merger (BNS), GW170817, by the LIGO-Virgo collaboration and the detection of its electromagnetic counterpart. This event was (and still is) very special, since it was possible to identify the host galaxy, NGC4993. The first host galaxy detected seemed to have poor star formation at present and was 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.
My team and I have been exploring different aspects linking host galaxy properties with binary compact object (and their progenitors) across cosmic time.
Artale et al. (2019, 2020a, 2020b)
In these works, we use the EAGLE simulation with the population synthesis code MOBSE to study the merger rate per galaxy as a function of different galaxy properties. We also put constraints on the probability of finding a BCO merger according to the galaxy properties
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
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