The mini-active galactic nucleus at the center of the elliptical galaxy NGC 4552 with Hubble Space Telescope

The complex phenomenology shown by the UV-bright, variable spike first dete cted with the Hubble Space Telescope (HST) at the center of the otherwise n ormal galaxy NGC 4552 is further investigated with both HST imaging (FOC) a nd 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 centr al spike has brightened by a factor similar to 4.5 between 1991 and 1993 an d has decreased its luminosity by a factor similar to 2.0 between 1993 and 1996. FOS spectroscopy extending from the near-UV to the red side of the op tical spectrum reveals a strong UV continuum over the spectrum of the under lying galaxy, along with several emission lines in both the UV and the opti cal ranges. Tn spite of the low luminosity of the UV continuum of the spike (similar to 3 x 10(5) L.), the spike is definitely placed among active gal actic nuclei (AGNs) by current diagnostics based on the emission-line inten sity ratios, being just on the borderline between Seyfert galaxies and LINE Rs. Line profiles are very broad, and both permitted and forbidden lines ar e best modeled with a combination of broad and narrow components, with FWHM of similar to 3000 km s(-1) and similar to 700 km s(-1), respectively. Thi s 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 Ha luminosity of this mini-AGN is similar to 5.6 x 1 0(37) 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 obs ervational constraints, we estimate the mass of the BH at the center of NGC 4552 to be in the range between 3 x 10(8) and 2 x 10(9) 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 i n normal galaxies is briefly discussed.