A Large Scale Survey of Galaxies with Coronal Line Emission Selected from the Sloan Digital Sky Survey

We conduct a wide-scale search for coronal line emission in the SDSS MPA/JHU Data Release 8 catalog containing over 900,000 optical spectra, looking for the complete set of 20 coronal lines visible within the SDSS optical regime (~3300-8000 Å). This includes often overlooked high ionization-potential (IP) lines such as [Ar X] λ5533, [S XII] λ7609, [Fe XI] λ7892, and [Fe XIV] λ5303. We find when accounting for measurement sensitivities and redshifted wavelength ranges that the most commonly found coronal lines are the [Ne V] λλ3346,3426 doublet, at 5.79% and 8.32% respectively; followed closely by [Fe X] λ6374, at 6.56%; and the [Fe VII] λλ5720,6087 doublet, at 5.49% and 5.20%. These are also the lines most often found together in the same galaxy, whereas the majority of other detections contain only a single line. We find a significant correlation with these high-IP lines being found more frequently within lower mass galaxies between 10⁷-10⁹ M_⊙ when compared to BPT-identified AGN, the more traditionally used metrics. This supports the theory that lower mass black holes found in lower mass galaxies produce a hotter accretion disk which enhances the higher ionization coronal line spectrum. Future surveys with JWST may reveal more low-mass AGNs, potentially also uncovering intermediate mass black holes.