Investigating the Star Formation Rates of AGN Hosts Relative to the Star-Forming Main Sequence

Abstract

A fundamental question in galaxy and black-hole evolution remains how galaxies and their supermassive black holes have evolved together over cosmic time. Specifically, it is still unclear how the position of X-ray active galactic nucleus (AGN) host galaxies with respect to the star-forming main sequence (MS) may change with the X-ray luminosity ($L_{\mathrm{X}}$) of the AGN or the stellar mass ($M_{\star }$) of the host galaxy. We use data from XMM-SERVS to probe this issue. XMM-SERVS is covered by the largest medium-depth X-ray survey (with superb supporting multiwavelength data) and thus contains the largest sample to date for study. To ensure consistency, we locally derive the MS from a large reference galaxy sample. In our analysis, we demonstrate that the turnover of the galaxy MS does not allow reliable conclusions to be drawn for high-mass AGNs, and we establish a robust safe regime where the results do not depend upon the choice of MS definition. Under this framework, our results indicate that less-massive AGN host-galaxies ($\log M_{\star }\sim 9.5-10.5$ $M_{\odot }$) generally possess enhanced SFRs compared to their normal-galaxy counterparts while the more-massive AGN host galaxies ($\log M_{\star }\sim 10.5-11.5$ $M_{\odot }$) lie on or below the star-forming MS. Further, we propose an empirical model for how the placement of an AGN with respect to the MS (SFR${}_{norm}$) evolves as a function of both $M_{\star }$ and $L_{\mathrm{X}}$.

15 pages, 10 figures, 1 table, accepted for publication in ApJ