Gas Content Regulates the Lifecycle of Star Formation and Black Hole Accretion in Galaxies

TL;DR

This study investigates how gas content influences the lifecycle of star formation and black hole activity in galaxies, revealing that AGN feedback does not instantaneously deplete molecular gas in host galaxies.

Contribution

It provides observational evidence linking galaxy evolution stages with gas content and challenges models predicting immediate gas depletion by AGN feedback.

Findings

Gas content mediates galaxy star formation and AGN activity lifecycle.

Strong AGN activity does not instantaneously reduce molecular gas reservoirs.

Galaxy evolution involves phases from gas-rich star-forming to gas-poor quiescent states.

Abstract

Active galactic nucleus (AGN) feedback is expected to impact the amount of cold gas in galaxies by driving strong galactic winds, by preventing external gas inflows, or by changing the thermodynamical state of the gas. We use molecular gas mass estimates based on dust absorption (H$\alpha$/H$\beta$) to study gas content of large samples of type 2 AGN host galaxies in comparison with inactive galaxies. Using sparse principal component and clustering analysis, we analyze a suite of stellar and structural parameters of $\sim 27,100$ face-on, central galaxies at redshift $z = 0.02-0.15$ and with stellar mass $M_\star \approx 10^{10}-2\times 10^{11}\,M_\odot$. We identify four galaxy groups of similar mass and morphology (mass surface density, velocity dispersion, concentration, and S\'{e}rsic index) that can be evolutionarily linked through a lifecycle wherein gas content mediates their star formation rate (SFR) and level of AGN activity. Galaxies first consume their gas mostly through bursty star formation, then enter into a transition phase of intermediate gas richness in which star formation and AGNs coexist, before settling into retirement as gas-poor, quiescent systems with residual levels of AGN activity (LINERs). Strongly accreting black holes (Seyferts) live in gas-rich, star-forming hosts, but neither their gas reservoir nor their ability to form stars seem to be impacted \emph{instantaneously} (timescales $\lesssim 0.5$\,Gyr) by AGN feedback. Our results are inconsistent with AGN feedback models that predict that central, bulge-dominated, Seyfert-like AGNs in massive galaxies have significantly lower molecular gas fractions compared to inactive galaxies of similar mass, morphology, and SFR.