Star formation of X-ray AGN in COSMOS: The role of AGN activity and galaxy stellar mass

TL;DR

This study compares star formation rates of X-ray AGN and non-AGN galaxies in COSMOS, revealing that high-luminosity AGN often have elevated star formation, especially in certain stellar mass ranges.

Contribution

It provides a uniform comparison of star formation in AGN and non-AGN galaxies using spectral energy distribution fitting and a newly defined main sequence, minimizing systematic biases.

Findings

AGN at low to moderate luminosities have similar or lower SFR than non-AGN.

High-luminosity AGN show increased SFR in specific stellar mass ranges.

The study refines understanding of the relationship between AGN activity and star formation.

Abstract

We use $\sim 1,000$ X-ray sources in the COSMOS-Legacy survey and study the position of the AGN relative to the star forming main sequence (MS). We also construct a galaxy (non-AGN) reference sample that includes $\sim 90,000$ sources. We apply the same photometric selection criteria on both datasets and construct their spectral energy distributions (SEDs) using optical to far-infrared photometry compiled by the HELP project. We perform SED fitting, using the X-CIGALE algorithm and the same parametric grid for both datasets, to measure the star formation rate (SFR) and stellar mass of the sources. The mass completeness of the data is calculated at different redshift intervals and is applied on both samples. We define our own main sequence, based on the distributions of the specific SFR at different redshift ranges and exclude quiescent galaxies from our analysis. These allow us to compare the SFR of the two populations in a uniform manner, minimizing systematics and selection effects. Our results show that at low to moderate X-ray luminosities, AGN tend to have lower or, at most, equal star formation rates compared to non-AGN systems with similar stellar mass and redshift. At higher ($\rm L_{X,2-10keV} > 2-3\times 10^{44}\,erg\,s^{-1}$), we observe an increase of the SFR of AGN, for systems that have $\rm 10.5 < log\,[M_*(M_\odot)] < 11.5$.