Multiwavelength 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 (AGNs) are found in low-metallicity dwarf galaxies. Such a population could provide insight into the origins of supermassive black holes. Here we report near-IR 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 α line and mid-infrared colors consistent with an AGN. We detect the [Si vi ] 1.96 μ m coronal line and a broad Pa α line with an FWHM of 850 ± 25 km s −1 . Together with the optical broad lines and coronal lines seen in the Sloan Digital Sky Survey (SDSS) spectrum, we confirm the presence of a highly accreting black hole with mass (2.2 ± 1.3) × 10 6 M ⊙ , with a bolometric luminosity of ≈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 erg s −1 , ∼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.