Low-Mass Active Galactic Nuclei with Rapid X-Ray Variability

Abstract

We present a detailed study of the optical spectroscopic properties of 12 active galactic nuclei (AGNs) with candidate low-mass black holes (BHs) selected by Kamizasa et al. through rapid X-ray variability. The high-quality, echellette Magellan spectra reveal broad H$\hat{I}\pm$ emission in all the sources, allowing us to estimate robust viral BH masses and Eddington ratios for this unique sample. We confirm that the sample contains low-mass BHs accreting at high rates: the median $M_{\mathrm{BH}}=1.2\times 10^6{M}_{\odot }$ and median $L_{\mathrm{bol}}/L_{\mathrm{Edd}}=0.44$. The sample follows the $M_{\mathrm{BH}}-\ddot{I}{ƒ}_{\ast }$ relation, within the considerable scatter typical of pseudobulges, the probable hosts of these low-mass AGNs. Various lines of evidence suggest that ongoing star formation is prevalent in these systems. We propose a new strategy to estimate star formation rates in AGNs hosted by low-mass, low-metallicity galaxies, based on modification of an existing method using the strength of [O II] $\hat{I}»3727$, [O III] $\hat{I}»5007$, and X-rays.

To appear in ApJ