The Effect of AGN on the Global HI Content of Isolated Low-Mass Galaxies

TL;DR

This study examines how active galactic nuclei (AGN) influence the neutral hydrogen content in isolated low-mass galaxies, revealing that certain AGN-hosting galaxies are unexpectedly gas-depleted despite their isolation.

Contribution

It provides new evidence that AGN activity may lead to gas depletion in isolated low-mass galaxies, especially in a specific stellar mass range, suggesting internal processes like black hole feedback.

Findings

AGN activity does not affect HI content in galaxies with log(M*) > 9.5.

A subset of low-mass, gas-depleted galaxies are isolated and lack star formation.

Gas depletion may be caused by black hole feedback or starburst shocks, not environment.

Abstract

We investigate the global neutral hydrogen (HI) content of isolated galaxies selected from the SDSS spectroscopic survey with optical evidence of Active Galactic Nuclei (AGN). Our sample includes galaxies with unresolved HI observations from the ALFALFA 70% data release combined with deeper HI observations of low-mass galaxies with 7.0 < log(M*) < 9.5. We examine the HI masses of this sample using the distance from the star-forming sequence on the OIII\Hb and NII\Ha Baldwin Phillips Terlevich (BPT) diagram as a measurement of AGN activity. In agreement with previous studies, we find that, for galaxies with log(M*) > 9.5, AGN activity does not correlate with the global HI content. However, for galaxies with 9.2 < log(M*) < 9.5, we identify a set of objects at large distances from the BPT star-forming sequence and lower than expected HI masses. This gas-depleted sample is red in both g-r and NUV-r colors and compact without distinguishable signs of star formation. This is surprising because the vast majority of isolated galaxies in this stellar mass regime are both star-forming and gas-rich. These galaxies are greater than 1.5 Mpc from any massive galaxy, ruling out environmental processes as a source of the gas-depletion. We suggest that either black hole feedback or shocks from extremely bursty star formation cause the emission lines and have destroyed or otherwise consumed the cold gas.