Nuclear Activity in the Low Metallicity Dwarf Galaxy SDSS J0944-0038: A Glimpse into the Primordial Universe

Abstract

Local low metallicity dwarf galaxies are relics of the early universe and hold clues into the origins of supermassive black holes (SMBHs). In recent work, coronal lines have been used to unveil a population of candidate accreting black holes in dwarf galaxies with gas phase metallicities and stellar masses well below the host galaxies of any previously known AGNs. Using MUSE/VLT observations, we report the detection of [Fe X] $λ$6374 coronal line emission and a broad H$α$ line in the nucleus of SDSS J094401.87$-$003832.1, a nearby ($z=0.0049$) metal poor dwarf galaxy almost 500 times less massive than the LMC. The [Fe X] $λ$6374 emission is compact and centered on the brightest nuclear source, with a spatial extent of $\approx$100 pc. The [Fe X] luminosity is $\approx 10^{37}$ erg s$^{-1}$, within the range seen in previously identified AGNs in the dwarf galaxy population. This line has never been observed in gas ionized by hot stars. While it can be produced in supernova ejecta, the [Fe X] flux from SDSS J094401.87$-$003832.1 has persisted over the ~19 year time period between the SDSS and MUSE observations, ruling out supernovae as the origin for the emission. The black hole mass measured from the broad H$α$ FWHM and luminosity is $\approx 3150$ M$_\odot$, in line with its stellar mass if virial mass relations and black hole-galaxy scaling relations apply in this mass regime. These observations, together with previously reported multi-wavelength observations, can most plausibly be explained by the presence of an accreting intermediate mass black hole in a primordial galaxy analog. However, we cannot rule out the possibility that current stellar population models of metal poor stars significantly under-predict the stellar ionizing photon flux, and that metal poor stars can produce an extreme ionizing spectrum similar to that produced by AGNs.