Gravitational wave background and observed galaxy merger rate

The NANOGrav and others Pulsar Timing Arrays (PTAs) have released data confirming the presence of a Stochastic Gravitational Wave Background (SGWB) in the nHz frequency range. The current understanding of its origin is that it is generated by astrophysical sources, i.e. the mergers of Super Massive Black Holes (SMBHs) following their host’s merger, but there is still the possibility that this signal was generated in the early Universe from so-called cosmological sources. The objective of this work is focused on the astrophysical sources case, in particular it is an analysis of the SGWB using observed galactic merger rates to get information on the mechanisms of the SMBHs approach inside the newly formed galaxies. To achive this goal, a mock SGWB was built based on the observed galactic merger rates with a few parameters describing the unknown mechanisms of SMBH approach, searching for the values that maximised the likelihood of the models. The results seems to favour both prospects, meaning that the SGWB has both an astrophysical component and a cosmological component. The possibility of a singular strong source in one of the bin is also discussed, giving a criterion for the individuation of its host galaxy.