ALMA observations of cold molecular gas in AGN hosts at z ∼ 1.5 – evidence of AGN feedback?

Abstract

Similarly to the cosmic star formation history, the black hole accretion rate density of the Universe peaked at 1 < z < 3. This cosmic epoch is hence best suited for investigating the effects of radiative feedback from active galactic nucleus (AGN). Observational efforts are under way to quantify the impact of the AGN feedback, if any, on their host galaxies. Here, we present a study of the molecular gas content of AGN hosts at z ∼ 1.5 using CO[2−1] line emission observed with Atacama Large Millimeter/sub-mm Array (ALMA) for a sample of 10 AGNs. We compare this with a sample of galaxies without an AGN matched in redshift, stellar mass and star formation rate. We detect CO in three AGNs with LCO ∼ 6.3–25.1 × 109 L⊙, which translates to a molecular hydrogen gas mass of 2.5–10 × 1010 M⊙ assuming conventional conversion factor of αCO ∼ 3.6. Our results indicate a >99 per cent probability of lower depletion time-scales and lower molecular gas fractions in AGN hosts with respect to the non-AGN comparison sample. We discuss the implications of these observations on the impact that AGN feedback may have on star formation efficiency of z >1 galaxies.