Multi-wavelength observations of SDSS J105621.45+313822.1, a broad-line,
  low-metallicity AGN
  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$\alpha$ line and mid-infrared colors consistent
with an AGN. We detect the [Si VI] 1.96$\mu$m coronal line and a broad
Pa$\alpha$ 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
$\approx10^{44}$~erg~s$^{-1}$ based on the coronal line luminosity, implying a
highly accreting AGN. Chandra observations reveal a weak nuclear point source
with $L_{\textrm{X,2-10 keV}} = (2.3 \pm 1.2) \times 10^{41}$~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.
