A Plethora of AGN Among Lyman Alpha Galaxies at Low Redshift

TL;DR

This study reveals that a significant fraction of low-redshift Lyman alpha emitting galaxies host active galactic nuclei, which is notably higher than the AGN fraction observed at higher redshifts, impacting our understanding of galaxy evolution.

Contribution

The paper provides the first detailed analysis of AGN presence in low-redshift LAEs using multiple diagnostic methods, highlighting the importance of optical spectroscopy for AGN detection.

Findings

Low-redshift LAEs have an AGN fraction of approximately 43%.

The AGN fraction in low-redshift LAEs is significantly higher than at high redshift.

Optical spectroscopy is essential for identifying low-luminosity AGNs in LAEs.

Abstract

We investigate the fraction of z ~ 0.3 Lyman alpha emitting galaxies (LAEs) which host active galactic nucleus activity, which is typically from 1 - 5% at 2 < z < 6. Using optical spectroscopy of 23 LAEs at 0.2 < z < 0.45 selected with GALEX UV data, we probe for AGN with a variety of methods, including line widths, diagnostic line ratios, high-ionization emission, infrared activity and X-ray luminosity. We found that our sample of low-redshift LAEs has an AGN fraction of 43 ^{+18}{-26}%, significantly higher than at high redshift. While previous results have shown that low-redshift LAEs have a lower space density than their high-redshift analogs, these results show that star-forming LAEs at low-redshift are rarer still. Accounting for differences in available AGN classification methods, we conclude that rest-frame optical spectroscopy is necessary to identify low-luminosity AGNs in LAEs at high redshift, and that limits on the X-ray flux are necessary to determine the extent to which the AGN contaminates the Lyman alpha fluxes.