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 are thought to hold clues into the origins of supermassive black holes. While recent studies are uncovering a growing population of active galactic nuclei (AGNs) in dwarf galaxies, the vast majority reside in galaxies with solar or supersolar metallicities and stellar masses comparable to that of the LMC. Using Multi-Unit Spectroscopic Explorer (MUSE) and Very Large Telescope 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. Unlike the emission from the lower-ionization nebular lines, the [Fe x ] λ 6374 emission is compact and centered on the brightest nuclear source, with a spatial extent of ≈100 pc, similar to that seen in well-known AGNs. The [Fe x ] luminosity is ≈10 ^37 erg s ^−1 , within the range seen in previously identified AGNs in the dwarf-galaxy population. The [Fe x ] emission has persisted over the roughly 19 yr time period between the SDSS and MUSE observations, ruling out supernovae as the origin for the emission. The FWHM of the broad component of the H α line is 446 ± 17 km s ^−1 and its luminosity is ≈1.5 × 10 ^38 erg s ^−1 , corresponding to a black hole mass of ≈ 3150 M _⊙ , 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 multiwavelength observations, can most plausibly be explained by the presence of an accreting intermediate-mass black hole in a primordial galaxy analog.

Keywords

• Dwarf irregular galaxies
• Supermassive black holes
• AGN host galaxies