The Mean Star Formation Rate of X-ray selected Active Galaxies and its Evolution from z=2.5: Results from PEP-Herschel

TL;DR

This study investigates the relationship between star formation rates and nuclear activity in X-ray selected AGNs up to redshift 2.5, revealing redshift-dependent trends and the evolving role of mergers in SMBH growth and galaxy evolution.

Contribution

It provides new insights into how the connection between star formation and AGN activity varies with luminosity and redshift, highlighting different fueling mechanisms over cosmic time.

Findings

At low AGN luminosities, SFR and accretion are uncorrelated across all redshifts.

High-luminosity AGNs show a correlation between SFR and accretion at low and moderate redshifts.

The role of mergers in SMBH and galaxy co-evolution diminishes at higher redshifts (z>1).

Abstract

(Abridged) We study relationships between the SFR and the nuclear properties of X-ray selected AGNs out to z=2.5, using far-IR data in three extragalactic deep fields as part of the PACS Evolutionary Probe (PEP) program. Guided by studies of intrinsic infra-red AGN SEDs, we show that the majority of the FIR emission in AGNs is produced by cold dust heated by star-formation. We uncover characteristic redshift-dependent trends between the mean FIR luminosity (L_fir) and accretion luminosity (L_agn) of AGNs. At low AGN luminosities, accretion and SFR are uncorrelated at all redshifts, consistent with a scenario where most low-luminosity AGNs are primarily fueled by secular processes in their host galaxies. At high AGN luminosities, a significant correlation is observed between L_fir and L_agn, but only among AGNs at low and moderate redshifts (z<1). We interpret this as a signature of the increasing importance of major-mergers in driving both the growth of super-massive black holes (SMBHs) and global star-formation in their hosts at high AGN luminosities. However, we also find that the enhancement of SFR in luminous AGNs weakens or disappears at high redshifts (z>1). This suggests that the role of mergers in SMBH-galaxy co-evolution is less important at these epochs. At all redshifts, we find essentially no relationship between L_fir and nuclear obscuration across five orders of magnitude in obscuring column density, suggesting that various different mechanisms are likely to be responsible for obscuring X-rays in active galaxies. We explain our results within a scenario in which two different modes of SMBH fueling operate among low- and high-luminosity AGNs. We postulate, guided by emerging knowledge about the properties of high redshift galaxies, that the dominant mode of accretion among high-luminosity AGNs evolves with redshift.