Michele Cappellari

Abstract:

A self-consistent analysis of near-UV, HST/FOC images of the elliptical galaxy NGC 4552 is used to show that its central spike has brightened by a factor similar to 4.5 between 1991 and 1993, and has decreased its luminositv by a factor similar to 2.0 between 1993 and 1996. A strong UV continuum over the energy distribution of the underlying galaxy is concurrently revealed shortward of lambda similar to 3200 Angstrom by our FOS spectra extending from the near-UV to red wavelengths. Nuclear emission-line profiles of both permitted and forbidden lines are best modelled 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. Current diagnostics based on the emission line intensity ratios definitely places the spike among AGNs, just at the border between Seyferts and LINERs. 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 fly-by with the BH. In this regard, one has to look at NGC 4552 as the faintest known AGN.

Abstract:

We demonstrate the possibility of detecting mass concentrations (down to the level of 5 x 10(7) M.) in the center of galaxies with ground-based optical telescopes properly equipped, using Keplerian gaseous disks as probes. We predict that the high spatial resolution of HST equipped with STIS will allow to detect central mass concentrations down to the level of 5 x 10(6) M.. As a consequence, the detection of low mass concentrations in the range 10(6) - 10(7) M. will constitute a very appropriate use of HST.