Active galactic nuclei vs. host galaxy properties in the COSMOS field^⋆

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
e-mail: giorgio.lanzuisi2@unibo.it
² INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Department of Physics, University of Zagreb, Bijenička cesta 32, 10002 Zagreb, Croatia
⁴ Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, 10000 Zagreb, Croatia
⁵ Department of Physics & Astronomy, Clemson University, Clemson, SC 29634, USA
⁶ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁷ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁸ Department of Astronomy and Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, U. Park, PA 16802, USA
⁹ Institut d’Astrophysique de Paris, UPMC et CNRS, UMR 7095, 98bis Bd Arago, 75014 Paris, France

Received: 25 October 2016
Accepted: 22 February 2017

Abstract

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 (L_X) and the host 8−1000 μm star formation luminosity (L_IR^SF) are significantly correlated, even after removing the dependency of both quantities with redshift. However, the average host L_IR^SF has a flat distribution in bins of AGN L_X, while the average AGN L_X increases in bins of host L_IR^SF with 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 L_X and host properties is found. On the other hand, we find that the average column density (N_H) shows a clear positive correlation with the host M_∗ at all redshifts, but not with the SFR (or L_IR^SF). 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 N_H and 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.