Investigating the Star Formation Rates of Active Galactic Nucleus 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 X) of the AGN or the stellar mass (M ⋆) of the host galaxy. We use data from the XMM-Spitzer Extragalactic Representative Volume Survey (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 ⋆ ∼ 9.5 – 10.5 M ⊙ ) generally possess enhanced star formation rates compared to their normal-galaxy counterparts while the more massive AGN host galaxies ( log M ⋆ ∼ 10.5 – 11.5 M ⊙ ) 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 (SFRnorm) evolves as a function of both M ⋆ and L X.

Mapping the Growth of Supermassive Black Holes as a Function of Galaxy Stellar Mass and Redshift

“…Such a classification is more reliable at M å  10 10.5 M e , but it may become sensitive to the adopted method at higher M å and lower z (e.g., Donnari et al 2019). Cristello et al (2024) show that the star-forming and quiescent subpopulations cannot be safely defined for massive galaxies, and Feldmann (2017) also argued that the bimodal separation is not necessarily appropriate. The proposed redshift-dependent maximum M å values for reliable classifications in Cristello et al (2024) and they are explicitly plotted in Figure 10.…”

“…Cristello et al (2024) show that the star-forming and quiescent subpopulations cannot be safely defined for massive galaxies, and Feldmann (2017) also argued that the bimodal separation is not necessarily appropriate. The proposed redshift-dependent maximum M å values for reliable classifications in Cristello et al (2024) and they are explicitly plotted in Figure 10. We also plot the BHAR of the whole population in Figure 10, and it is similar to the star-forming BHAR below the M å threshold in Equation (31) and becomes more in the middle between the star-forming and quiescent BHAR with rising M å .…”

“…The black dashed curves denote the BHAR with all the galaxies included, i.e., those in Figure 6. The vertical black lines mark the maximum M å values where star-forming and quiescent galaxies can be reliably classified at the corresponding z (Cristello et al 2024). Star-forming galaxies have larger BHAR.…”