The Mini–Active Galactic Nucleus at the Center of the Elliptical Galaxy NGC 4552 withHubble Space Telescope

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Abstract
The complex phenomenology shown by the UV-bright, variable spike first detected with the Hubble Space Telescope (HST) at the center of the otherwise normal galaxy NGC 4552 is further investigated with both HST imaging (FOC) and spectroscopy (FOS). HST/FOC images taken in 1991, 1993, and 1996 in the near-UV have been analyzed in a homogeneous fashion, showing that the central spike has brightened by a factor ~4.5 between 1991 and 1993 and has decreased its luminosity by a factor ~2.0 between 1993 and 1996. FOS spectroscopy extending from the near-UV to the red side of the optical spectrum reveals a strong UV continuum over the spectrum of the underlying galaxy, along with several emission lines in both the UV and the optical ranges. In spite of the low luminosity of the UV continuum of the spike (~ 3 × 105 L☉), the spike is definitely placed among active galactic nuclei (AGNs) by current diagnostics based on the emission-line intensity ratios, being just on the borderline between Seyfert galaxies and LINERs. Line profiles are very broad, and both permitted and forbidden lines are best modeled with a combination of broad and narrow components, with FWHM of ~3000 km s-1 and ~700 km s-1, respectively. This evidence argues for the variable central spike being produced by a modest accretion event onto a central massive black hole (BH), with the accreted material having possibly being stripped from a star in a close flyby with the BH. The 1996 broad Hα luminosity of this mini-AGN is ~ 5.6 × 1037 ergs s-1, about a factor of 2 less than that of the nucleus of NGC 4395, heretofore considered to be the faintest known AGN. Combining all observational constraints, we estimate the mass of the BH at the center of NGC 4552 to be in the range between 3 × 108 and 2 × 109 L☉. The relevance for the demography of BHs in galaxies of the high (HST) resolution imaging and spectroscopy capable of revealing an extremely low level AGN activity in normal galaxies is briefly discussed.