The role of AGN and obscuration in the position of the host galaxy relative to the main sequence

Abstract

We use X-ray active galactic nuclei (AGN) observed by the Chandra X-ray Observatory within the 9.3 deg² Boötes field of the NDWFS to study whether there is a correlation between X-ray luminosity (L_X) and star formation rate (SFR) of the host galaxy, at 0.5 < z < 2.0, with respect to the position of the galaxy to the main sequence (SFR_norm). About half of the sources in the X-ray sample have spectroscopic redshifts. We also construct a reference galaxy catalogue. For both datasets we use photometric data from the optical to the far-infrared compiled by the HELP project, and apply spectral energy distribution fitting, using the X-CIGALE code. We exclude quiescent sources from both the X-ray and the reference samples. We also account for the mass completeness of our dataset, in different redshifts bins. Our analysis highlights the importance of studying the SFR-L_X relation in a uniform manner, taking into account systematics and selection effects. Our results suggest, in less massive galaxies (log [M_*(M_⊙)] ∼ 11), that an AGN enhances the SFR of the host galaxy by ∼50% compared to non-AGN systems. A flat relation is observed for the most massive galaxies. The SFR_norm does not evolve with redshift. The results, although tentative, are consistent with a scenario where, in less massive systems, both AGN and star formation are fed by cold gas supplied by a merger event. In more massive galaxies the flat relation could be explained by a different supermasssive black hole fuelling mechanism that is decoupled from the star formation of the host galaxy (e.g., hot diffuse gas). Finally, we compare the host galaxy properties of X-ray absorbed and unabsorbed sources. Our results show no difference, which suggests that X-ray absorption is not linked with the properties of the galaxy.

Tables with the X-ray and host galaxy properties used in the analysis are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/653/A74