Galactic Bulges from HST NICMOS Observations : Nuclear Properties

Abstract: Hubble Space Telescope NICMOS images, we analyze the structural properties of bulges of S0-Sbc galaxies down to scales of ~10 pc. Isophotal profiles are matched with those from wider-field, ground-based K-band images enabling us to sample from the nucleus to the disk-dominated region of each galaxy, and thus to include proper bulge-disk decompositions in the analysis. The majority of galaxies (~90%) possess central light excesses above the Sersic model of the bulge, which can be modeled with central unresolved sources and/or inner exponentials. The unresolved sources are reminiscent of those found by others in ellipticals, bulges, and dwarf ellipticals. We find that their luminosities are correlated with bulge luminosity and scale as L^{0.5}. The extended nuclear components, with sizes of a few hundred pc, have specific isophotal and dynamical signatures, which suggest that they may be inner disks, rings, or bars; their colors are similar to those of the underlying bulge, arguing against a recent origin for their stellar populations. Surface brightness profiles rise inward to the resolution limit of the data, with a continuous distribution of logarithmic slopes from the low values typical of dwarf ellipticals (0.1 < gamma < 0.3) to the high values (gamma ~ 1) typical of intermediate luminosity ellipticals; the nuclear slope bi-modality reported by others is not present in our sample.