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Abstract
Using the Hubble Space Telescope we observed at 7-pc resolution the nuclei of a luminosity-limited sample of fourteen elliptical and E/S0 galaxies in the Virgo Cluster. We find that, while none of the galaxies has an isothermal core, the nuclear and near-nuclear morphology confirms and strengthens the previously recognized dichotomy of elliptical galaxies into 'true' and 'disky' subtypes. The 'disky' ellipticals are the faintest members of the sample, and correspond to class E4 or later. In three of them we found evidence for a nuclear stellar disk of radius $\sim$100 pc. The more luminous 'true' ellipticals are classified as E4 or earlier and have absolute magnitude $M\sb{B}<{-}$19.2.<p> Seventy-five percent of our galaxies show dust in the nuclear region, the most remarkable example being a compact nuclear dust disk in NGC 4261, roughly perpendicular to the galaxy radio jet. For this particular galaxy, we obtained HST/WFPC2 V, R and I images and HST/FOS spectra. The images show that the disk is not in an equilibrium configuration: it is not coaxial with the major axis of the galaxy, and it is not centered on either the nucleus nor on the isophotal center of the galaxy. FOS spectra reveal that the ionized gas is confined in a disk in Keplerian motion around a central mass (1.2 $\pm$ 0.4) $\times\ 10\sp9$ M$\sb{\odot}$. The mass to light ratio is $(M/L)\sb{V}\approx 5200\ \rm M\sb{\odot}/L\sb{\odot}$ within the inner 14.5 pc, leading us to conclude that the majority of the central mass is concentrated in nuclear black hole.
In the final section of this thesis, we present near infrared images of the same sample of fourteen galaxies studied at the beginning. V-K, J-H and H-K colors are derived for the main body of the galaxies, the dust areas and the 100 pc size nuclear stellar disks detected in three of the galaxies, and are compared with the prediction of stellar population synthesis models.
