Relics of Supermassive Black Hole Seeds: The Discovery of an Accreting
  Black Hole in an Optically Normal, Low Metallicity Dwarf Galaxy
  The detection and characterization of supermassive black holes (SMBHs) in
local low mass galaxies is crucial to our understanding of the origins of
SMBHs. This statement assumes that low mass galaxies have had a relatively
quiet cosmic history, so that their black holes have not undergone significant
growth and therefore can be treated as relics of the original SMBH seeds. While
recent studies have found optical signatures of active galactic nuclei (AGNs)
in a growing population of dwarf galaxies, these studies are biased against low
metallicity and relatively merger-free galaxies, thus missing precisely the
demographic in which to search for the relics of SMBH seeds. Here, we report
the detection of the [\ion{Si}{6}]1.963~$\mu$m coronal line (CL), a robust
indicator of an AGN in the galaxy SDSS~J160135.95+311353.7, a nearby
($z=0.031$) low metallicity galaxy with a stellar mass approximately an order
of magnitude lower than the LMC ($M_*\approx10^{8.56}$~M$_\odot$) and no
optical evidence for an AGN. The AGN bolometric luminosity implied by the CL
detection is $\approx10^{42}$~erg~s$^{-1}$, precisely what is predicted from
its near-infrared continuum emission based on well-studied AGNs. Our results
are consistent with a black hole of mass $\approx~10^5$~M$_\odot$, in line with
expectations based on its stellar mass. This is the first time a near-infrared
CL has been detected in a low mass, low metallicity galaxy with no optical
evidence for AGN activity, providing confirmation of the utility of infrared
CLs in finding AGNs in low mass galaxies when optical diagnostics fail. These
observations highlight a powerful avenue of investigation to hunt for low mass
black holes in the JWST era.
