LLAMA: Normal star formation efficiencies of molecular gas in the centres of luminous Seyfert galaxies

TL;DR

This study shows that the star formation efficiencies and molecular gas properties in the centers of luminous Seyfert galaxies are similar to inactive galaxies, indicating weak AGN influence on cold molecular gas in these regions.

Contribution

It provides the first controlled comparison of molecular gas and star formation efficiencies in active and inactive galaxy centers using new CO spectroscopy and multi-wavelength data.

Findings

Gas fractions and SFEs are similar in active and inactive galaxies when controlling for host properties.

Central molecular gas depletion times are comparable to those in normal spiral galaxy disks.

AGN nuclear radiation only weakly affects the cold molecular gas in galaxy centers.

Abstract

Using new APEX and JCMT spectroscopy of the CO 2-1 line, we undertake a controlled study of cold molecular gas in moderately luminous Active Galactic Nuclei (AGN) and inactive galaxies from the Luminous Local AGN with Matched Analogs (LLAMA) survey. We use spatially resolved infrared photometry of the LLAMA galaxies from 2MASS, WISE, IRAS & Herschel, corrected for nuclear emission using multi-component spectral energy distribution (SED) fits, to examine the dust-reprocessed star-formation rates (SFRs), molecular gas fractions and star formation efficiencies (SFEs) over their central 1 - 3 kpc. We find that the gas fractions and central SFEs of both active and inactive galaxies are similar when controlling for host stellar mass and morphology (Hubble type). The equivalent central molecular gas depletion times are consistent with the discs of normal spiral galaxies in the local Universe. Despite energetic arguments that the AGN in LLAMA should be capable of disrupting the observable cold molecular gas in their central environments, our results indicate that nuclear radiation only couples weakly with this phase. We find a mild preference for obscured AGN to contain higher amounts of central molecular gas, which suggests a connection between AGN obscuration and the gaseous environment of the nucleus. Systems with depressed SFEs are not found among the LLAMA AGN. We speculate that the processes that sustain the collapse of molecular gas into dense pre-stellar cores may also be a prerequisite for the inflow of material on to AGN accretion disks.