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

Michael Reefe, Shobita Satyapal, Remington O. Sexton, Nathan J. Secrest, William Matzko, Emma Schwartzman, Kristina Nyland, Gabriela Canalizo, Barry Rothberg, Ryan W. Pfeifle, Jenna M. Cann, Archana Aravindin, Camilo Vasquez, Tracy Clarke

Submitted on 23 November 2022


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] λ6374 coronal line emission and a broad Hα line in the nucleus of SDSS J094401.87003832.1, a nearby (z=0.0049) metal poor dwarf galaxy at least fifty times less massive than the LMC. The [Fe X] λ6374 emission is compact and centered on the brightest nuclear source, with a spatial extent of 100 pc. The [Fe X] luminosity is 1037 erg s1, 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 J094401.87003832.1 has persisted over the ~19 year time period between the SDSS and MUSE observations, ruling out supernovae as the origin for the emission. The FWHM of the broad component of the Hα line is 446±17 km s1 and its luminosity is 1.5×1038 erg s1, lower than the broad line luminosities of previously identified low mass broad line AGNs. 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.


Comment: 12 pages, 5 figures, 1 table, submitted to ApJL. Comments welcome

Subject: Astrophysics - Astrophysics of Galaxies


Top panel: Hubble Space Telescope image of J0944$-$0038, constructed from UV (F336W, blue channel), H$\alpha$ (F657N, green channel), and visual/near-IR (F814W, red channel) archival data. Bottom panel: stellar mass distribution of the subset of dwarf galaxies in the CLASS sample compared to the samples of AGNs identified by \citet{2013ApJ...775..116R} and \citet{2021ApJ...922..155M}. The stellar mass of J0944$-$0038, and other coronal line emitters, is significantly lower than the mass regime probed by traditional optical narrow line ratios.