Multi-wavelength observations of SDSS J105621.45+313822.1, a broad-line, low-metallicity AGN

Abstract

In contrast to massive galaxies with Solar or super-Solar gas phase metallicities, very few Active Galactic Nuclei (AGN) are found in low-metallicity dwarf galaxies. Such a population could provide insight into the origins of supermassive black holes. Here we report near-infrared spectroscopic and X-ray observations of SDSS J105621.45+313822.1, a low-mass, low-metallicity galaxy with optical narrow line ratios consistent with star forming galaxies but a broad H$\hat{I}\pm$ line and mid-infrared colors consistent with an AGN. We detect the [Si VI] 1.96$\hat{I}¼$m coronal line and a broad Pa$\hat{I}\pm$ line with a FWHM of $850\pm 25$~km~s${}^{-1}$. Together with the optical broad lines and coronal lines seen in the SDSS spectrum, we confirm the presence of a highly accreting black hole with mass $(2.2\pm 1.3)\times 10^6$~M${}_{\odot }$, with a bolometric luminosity of $\approx 10^{44}$~erg~s${}^{-1}$ based on the coronal line luminosity, implying a highly accreting AGN. Chandra observations reveal a weak nuclear point source with $L_{\text{X,2-10 keV}}=(2.3\pm 1.2)\times 10^{41}$~erg~s${}^{-1}$, $\sim 2$ orders of magnitude lower than that predicted by the mid-infrared luminosity, suggesting that the AGN is highly obscured despite showing broad lines in the optical spectrum. The low X-ray luminosity and optical narrow line ratios of J1056+3138 highlight the limitations of commonly employed diagnostics in the hunt for AGNs in the low metallicity low mass regime.

13 pages, 6 figures, accepted to ApJ