On the AGN Nature of Broad Balmer Emission in Four Low-Redshift
  Metal-Poor Galaxies
  We report on continued, $\sim$15 year-long, broad Balmer emission lines in
three metal-poor dwarf emission-line galaxies selected from Sloan Digital Sky
Survey spectroscopy. The persistent luminosity of the broad Balmer emission
indicates the galaxies are active galactic nuclei (AGNs) with virial black hole
masses of $\sim 10^{6.7}{-10^{7.0}\ M_{\odot}}$. The lack of observed hard
X-ray emission and the possibility that the Balmer emission could be due to a
long-lived stellar transient motivated additional follow-up spectroscopy. We
also identify a previously-unreported blueshifted narrow absorption line in the
broad H$\alpha$ feature in one of the AGNs, indicating an AGN-driven outflow
with hydrogen column densities of order $10^{17}$ cm$^{-2}$. We also extract
light curves from the Catalina Real-Time Transient Survey and the Zwicky
Transient Facility. We detect probable AGN-like variability in three galaxies,
further supporting the AGN scenario. This also suggests the AGNs are not
strongly obscured. This sample of galaxies are among the most metal-poor which
host an AGN ($Z=0.05 - 0.16\ Z_\odot$). We speculate they may be analogues to
seed black holes which formed in unevolved galaxies at high redshift. Given the
rarity of metal-poor AGNs and small sample size available, we investigate
prospects for their identification in future spectroscopic and photometric
surveys.
