Active galactic nuclei vs. Host galaxy properties in the COSMOS field

Context. The coeval active galactic nuclei (AGN) and galaxy evolution, and the observed local relations between super massive black holes (SMBHs) and galaxy properties suggest some sort of connection or feedback between SMBH growth (i.e., AGN activity) and galaxy build-up (i.e., star formation history). Aims. We looked for correlations between average properties of X-ray detected AGN and their far-IR (FIR) detected, star forming host galaxies in order to find quantitative evidence for this connection, which has been highly debated in recent years. Methods. We exploited the rich multiwavelength data set (from X-ray to FIR) available in the COSMOS field for a large sample (692 sources) of AGN and their hosts in the redshift range 0.1 <z< 4. We use X-ray data to select AGN and determine their properties, such as X-ray intrinsic luminosity and nuclear obscuration, and broadband (from UV to FIR) SED fitting results to derive host galaxy properties, such as stellar mass (M∗) and star formation rate (SFR). Results. We find that the AGN 2-10 keV luminosity (LX) and the host 8-1000 μm star formation luminosity (LIRSF) are significantly correlated, even after removing the dependency of both quantities with redshift. However, the average host LIRSFhas a flat distribution in bins of AGN LX, while the average AGN LXincreases in bins of host LIRSFwith logarithmic slope of ~0.7 in the redshift range 0.4 <z< 1.2. We also discuss the comparison between the full distribution of these two quantities and the predictions from hydrodynamical simulations. No other significant correlations between AGN LXand host properties is found. On the other hand, we find that the average column density (NH) shows a clear positive correlation with the host M∗at all redshifts, but not with the SFR (or LIRSF). This translates into a negative correlation with specific SFR at all redshifts. The same is true if the obscured fraction is computed. Conclusions. Our results are in agreement with the idea, introduced in recent galaxy evolutionary models, that SMBH accretion and SFRs are correlated, but occur with different variability time scales. Finally, the presence of a positive correlation between NHand host M∗suggests that the column density that we observe in the X-rays is not entirely due to the circumnuclear obscuring torus, but may also include a significant contribution from the host galaxy.