A Hunt for Intermediate Mass Black Holes in Low Mass and Low Metallicity Galaxies in SDSS-IV/eBOSS

Understanding the demographics of Intermediate Mass Black Holes (IMBHs) is an important step in understanding how supermassive black holes form. IMBHs, which have masses between 10^2 - 10^5 solar masses, have historically been difficult to detect even in the local universe. To search for IMBHs, we analyzed optical spectra from the Sloan Digital Sky Survey’s (SDSS) Extended Baryon Oscillation Spectroscopic Survey (eBOSS), focusing on 1286 low-metallicity star-forming galaxies with mass lower than 3*10^8 solar masses at redshift z < 0.15. One signature of accreting IMBHs is powerful outflows, which could appear as broad emission line components in the H-alpha or [OIII] 5007 lines. To search for these features, we fit a Gaussian curve to both the broad and narrow emission components, and then use the Bayesian Information Criterion to determine if the broad component is statistically significant. We find 139 candidates with robust broad lines in either [OIII] 5007 or H-alpha and outflow velocities too high (> 300 km/s) to be due to star formation-powered outflows. Another possible contaminant in our sample could be individual supernovae; however, most of the galaxies do not appear to have a sufficiently broad component, nor do we see evidence for other low-ionization metal lines typically seen in supernova spectra. There appear to be a few candidates with extremely broad [OIII] components with no H-alpha emission that are excellent IMBH candidates, requiring additional follow-up.