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S. Ravindranath, L. C. Ho (Carnegie Obs.), C. Y. Peng (Steward Obs.), A. V. Filippenko (UCB), W. L. W. Sargent (Caltech)
We present the results of surface photometry for a sample of 33 early-type (E, S0 and S0/a) galaxies observed with HST at 1.6 \mum. The high angular resolution images make it possible to probe structures close to the nucleus and reveal unresolved point sources or compact star clusters. Dust is found to occur mostly in galaxies which possess emission line nuclei and in some cases the presence of dust causes the isophotes to appear boxy. We performed a 2--dimensional modelling of the host galaxy light distribution using the empirical Nuker law, which proves to be an adequate representation of the surface brightness profiles for most early-type galaxies. The accurate modelling of the galaxy light allows us to estimate the magnitude of the unresolved central source without contamination from the underlying galaxy population. Our classification of galaxies into core type and power-law galaxies is consistent with the results from optical images, whereby cores occur in the most luminous galaxies and power-law profiles occur in galaxies with disky isophotes. Contrary to previous studies we find galaxies with inner slopes \gamma in the range 0.3--0.5. Also, some are best fit with Nuker law having \gamma = 0 implying that isothermal cores could be present in some of these early-type systems. Unresolved nuclear point sources are evident in about one-third of the sample, with magnitudes in the range 13.5 < mH < 17.3. The occurrence of point sources appears to be independent of whether the underlying galaxy has core-type or power-law brightness profile. The unresolved nuclei in core galaxies are all AGNs based on the spectral class. The H\alpha emission line luminosities correlate weakly with the magnitudes of the unresolved nuclei. The nuclear magnitudes do not correlate with the inner cusp slopes or with the velocity dispersion of the host galaxy, implying that the strength of activity is determined by the gas supply and accretion process rather than the mass of the central black hole.
The author(s) of this abstract have provided an email address for comments about the abstract: swara@ociw.edu