SDSS J085431.18+173730.5: The First Compact Elliptical Galaxy Hosting an Active Nucleus

Abstract: We report the discovery of a rare compact early-type galaxy, SDSS J085431.18+173730.5 (hereafter cE_AGN). It has an half light radius of R${e}$ = 490 pc and a brightness of M${r}$ = $-$18.08 mag. Optical spectroscopy available from the Sloan Digital Sky Survey (SDSS) reveals the presence of prominent broad-line emissions with the H$\alpha$ broad component width of FWHM=2400 km/s. The black hole (BH) mass, as estimated from the luminosity and width of the broad H$\alpha$ emission, is 2.1$\times$10$^{6}$ M$_{\sun}$. With the help of surface photometry, we perform a detailed analysis of the structural properties. The observed light distribution is best modeled with a double S\'ersic function. Fixing the outer component as an exponential disk, we find that the inner component has a S\'ersic index of n = 1.4. Considering the inner component as bulge/spheroidal we find that cE_AGN remains consistent in both BH mass -- bulge mass relation and BH mass -- bulge S\'ersic index relation. Given these observational properties, we discuss its possible origin investigating the surrounding environment where it is located.