AGN Feedback and Star Formation of Quasar Host Galaxies: Insights from the Molecular Gas

TL;DR

This study uses ALMA CO(2-1) observations to explore the relationship between AGN activity, molecular gas, and star formation in nearby quasar host galaxies, revealing that AGN feedback does not suppress star formation in these systems.

Contribution

It provides the largest and most sensitive molecular gas survey of low-redshift quasars, showing that AGN activity is loosely coupled to cold gas and that feedback does not quench star formation.

Findings

AGN luminosity correlates with CO luminosity and black hole mass.

Host galaxies have molecular gas content similar to star-forming galaxies.

No evidence of molecular outflows or quenching of star formation.

Abstract

The molecular gas serves as a key probe of the complex interplay between black hole accretion and star formation in the host galaxies of active galactic nuclei (AGNs). We use CO(2-1) observations from a new ALMA survey, in conjunction with literature measurements, to investigate the molecular gas properties of a representative sample of 40 z<0.3 Palomar-Green quasars, the largest and most sensitive study of molecular gas emission to date for nearby quasars. We find that the AGN luminosity correlates with both the CO luminosity and black hole mass, suggesting that AGN activity is loosely coupled to the cold gas reservoir of the host. The observed strong correlation between host galaxy total infrared luminosity and AGN luminosity arises from their common dependence on the molecular gas. We argue that the total infrared luminosity, at least for low-redshift quasars, can be used to derive reliable star formation rates for the host galaxy. The host galaxies of low-redshift quasars have molecular gas content similar to that of star-forming galaxies of comparable stellar mass. Moreover, they share similar gas kinematics, as evidenced by their CO Tully-Fisher relation and the absence of detectable molecular outflows down to sensitive limits. There is no sign that AGN feedback quenches star formation for the quasars in our sample. On the contrary, the abundant gas supply forms stars prodigiously, at a rate that places most of them above the star-forming main sequence and with an efficiency that rivals that of starburst systems.