The co-evolution of supermassive black holes and galaxies in luminous AGN over a wide range of redshift

It is well known that supermassive black holes (SMBHs) and their host galaxies co-evolve. A manifestation of this co-evolution is the correlation that has been found between the SMBH mass, M_BH, and the galaxy bulge or stellar mass, M_*. The cosmic evolution of this relation, though, is still a matter of debate. In this work, we examine the M_BH − M_* relation, using 687 X-ray luminous (median log [L_{X,2−10 keV}(erg s⁻¹)] = 44.3), broad-line active galactic nuclei (AGN), at 0.2 < z < 4.0 (median z ≈ 1.4) that lie in the XMM-XXL field. Their M_BH and M_* range from 7.5 < log [M_BH (M_⊙)] < 9.5 and 10 < log [M_*(M_⊙)] < 12, respectively. Most of the AGN live in star-forming galaxies and their Eddington ratios range from 0.01 to 1, with a median value of 0.06. Our results show that M_BH and M_* are correlated (r = 0.47 ± 0.21, averaged over different redshift intervals). Our analysis also shows that the mean ratio of the M_BH and M_* does not evolve with redshift, at least up to z = 2 and has a value of log(M_BH/M_*)= − 2.44. The majority of the AGN (75%) are in a SMBH mass growth-dominant phase. In these systems, the M_BH − M_* correlation is weaker and their M_* tends to be lower (for the same M_BH) compared to systems that are in a galaxy mass growth phase. Our findings suggest that the growth of black hole mass occurs first, while the early stellar mass assembly may not be so efficient.