On the AGN Nature of Broad Balmer Emission in Four Low-Redshift Metal-Poor Galaxies

Abstract

We report on continued, ∼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 10 6.4 −10 6.7 M ⊙ . 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 Hα 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, Pan-STARRS, and the Zwicky Transient Facility. We detect 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 ⊙ ). 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.

Multiple giant eruptions and X-ray emission in the recoiling AGN/LBV candidate SDSS1133

“…Considering the intrinsic rarity of the Balmer BAL AGNs, it is unreasonable to assume that the P-Cygni features of SDSS1133 (selected as a rare offset AGN) are due to the Balmer BAL nature. We should note that the discovery of SDSS1133 suggests that a fraction of Balmer BAL AGN candidates known to date (e.g., SDSS J1025 in Burke et al 2020a) can actually be SDSS1133-like LBVs.…”

“…About 10% of broad line AGNs are known to show blue-shifted broad absorption line (BAL) features in the UV wavelengths due to line absorption by high velocity disk winds (e.g., Hall et al 2002). However, only a few examples of hydrogen Balmer BAL AGNs are known to date, because the disk winds are generally too low-density and too highlyionized to keep neutral hydrogen atoms populated in the n = 2 shell (e.g., Hall 2007;Zhang et al 2015;Kokubo 2017;Burke et al 2020a). Considering the intrinsic rarity of the Balmer BAL AGNs, it is unreasonable to assume that the P-Cygni features of SDSS1133 (selected as a rare offset AGN) are due to the Balmer BAL nature.…”

“…Another similar object is recently reported by ZTF (AT2018kle at z = 0.013 with the peak absolute magnitude of Mg = −16.34; Perley et al 2020), which is classified as an eruption of a LBV. It is worth noticing that the spectral resemblance between SDSS1133 and broad line AGNs suggests that many SDSS1133-like objects are missed by the current surveys; for example, a fraction of nuclear/off-nuclear AGN-like objects can actually be LBVs (e.g., Izotov et al 2007;Izotov & Thuan 2008;Baldassare et al 2016;Simmonds et al 2016;Burke et al 2020a;Ward et al 2020). In this sense, the volumetric rate of the SDSS1133-like objects could be higher than the above estimate, and more comprehensive survey is needed to quantify the true occurrence rate of the SDSS1133-like objects.…”