Supermassive Black Holes Can Hardly Be “Silent”

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 Hα luminosity of only ∼ 10, 000 L⊙, 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 groundbased 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. 1 Discovery of a Central Flare in NGC 4552 Sophisticated techniques have been used to infer the presence of supermassive black holes (BH) at the center of galactic spheroids: from detailed stellar dynamical modeling fitting 2D spectroscopy data cubes to megamaser observations, all aspects widely discussed at this meeting. Here I will present observational evidence concerning three randomly-selected galactic spheroids, showing that high spatial resolution imaging and/or spectroscopy can reveal sporadic mini-AGN activity likely related to the presence of a central BH. To investigate the UV upturn of ellipticals, in 1993 HST/FOC images in several UV bands were obtained for the central regions of the elliptical galaxies NGC 1399 and NGC 4552 and of the bulge of the S0/a galaxy NGC 2681. A point-like source – or spike – was evident at the center of both NGC 4552 and NGC 2681, with their photometric profile being indistinguishable from the PSF of the pre-COSTAR HST (Paper I [1]). Comparison with another FOC image of NGC 4552 taken in 1991 [2] showed that this spike had increased its luminosity in the U band (F342W) by a factor ∼ 7±1.5 between the two epochs, reaching ∼ 106L⊙ (Paper I). A second point-like source is also present in the 1991 image, ∼ 0.14 from the