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 ∼106.7 − 107.0 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 1017 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⊙). 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.

Decade-long time-monitoring of candidate Luminous Blue Variable Stars in the two very metal-deficient compact dwarf galaxies DDO 68 and PHL 293B

“…Allan et al (2020), from spectroscopic observations including the 2019 VLT X-Shooter data and radiation transfer modeling, report the absence of the broad emission component since 2011 and conclude that the LBV was in an eruptive state during 2001 -2011 that has ended. Burke et al (2021) also report the decrease of the I br /Inar ratio from 0.41 (SDSS, 2001) to ∼0.10, using Gemini observations taken in December 2019. Despite the fact that a AGN-like damped random walk model works well to fit the observed light curve, those authors concluded that a long-lived stellar transient of type SN IIn can better explain all the data for PHL 293B.…”

“…The flux variations, both in the narrow and broad and even very broad components (a sharp flux decrease in 2014 followed by a rise at the end of 2015, followed by another decrease in 2017) can be explained by inaccurate pointing when observing with a narrow slit of 0.9 arcsec. However the variations are likely to be real because the light curves derived by Burke et al (2021) from SDSS and DES (Dark Energy Survey) imaging between 1998 and 2018 (their figure 2) also show small-amplitude variability in the g and r bands during the same years. The synchronicity in the variability of fluxes in the narrow and broad components does not hold anymore starting 2018 (see also the fading of the broad component in Allan et al 2020;Burke et al 2021).…”

“…However the variations are likely to be real because the light curves derived by Burke et al (2021) from SDSS and DES (Dark Energy Survey) imaging between 1998 and 2018 (their figure 2) also show small-amplitude variability in the g and r bands during the same years. The synchronicity in the variability of fluxes in the narrow and broad components does not hold anymore starting 2018 (see also the fading of the broad component in Allan et al 2020;Burke et al 2021).…”

“…However, very strong broad emission (FWHM > 1000 km s −1 ) can also be present in the spectra of objects other than massive stars, such as Type IIn Supernovae (SNe) and Active Galactic Nuclei (AGNs). In these objects, the luminosities of the broad Hα component are larger and can reach values up to 10 40 -10 42 ergs s −1 (Izotov et al 2007;Sobral et al 2020;Kokubo 2021;Burke et al 2021).…”

Variable AGN in the GALEX Time Domain Survey

“…We estimated BH masses using the luminosity (L Hα ) and FWHM of the broad Hα component (Greene & Ho 2005;Reines et al 2013;Burke et al 2021). Specifically, we used the results from Burke et al (2021), where: log M BH M = 6.57 + 0.47 log L Hα 10 42 erg s −1 +2.06 log FWHM Hα 10 3 km s −1 .…”