  There is now ample evidence that most - perhaps all - galactic spheroids host
a supermassive BH at their center. This has been assessed using a variety of
observational techniques, from stellar and/or gas dynamics to megamasers. Yet
another promising technique is offered by the case of the Virgo elliptical NGC
4552, in which early HST/FOC observations revealed a central low-luminosity
flare. Subsequent HST/FOS observations with a 0.21 arcsec aperture have
revealed a rich emission-line spectrum with broad and narrow components with
FWHM of 3000 and 700 km/s, respectively. This variable, mini-AGN at the center
of NGC 4552 is most naturally the result of a sporadic accretion event on a
central BH. It has a H$\alpha$ luminosity of only $\sim 10,000 \lsun$, making
it the likely, intrinsically faintest AGN known today. Only thanks to the
superior resolution of HST such a faint object has been discovered and studied
in detail, but adaptive optics systems on large ground-based telescopes may
reveal in the future that a low level of accretion onto central massive BHs is
an ubiquitous phenomenon among galactic spheroids. FOC/FOS observations of a
central spike in NGC 2681 reveal several analogies with the case of NGC 4552,
while yet another example is offered by a recent exciting finding with STIS by
R.W. O'Connell in NGC 1399, the third galaxy in our original program.
garchingbh
