Authors: Marc Balcells, Alister W. Graham, Reynier Peletier
Abstract: Using Hubble Space Telescope NICMOS images, we analyze the structural properties of the 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 inner exponentials and/or central unresolved sources. All the extended nuclear components, with sizes of a few hundred pc, have disky isophotes, which suggest that they may be inner disks, rings, or bars; their colors are redder than those of the underlying bulge, arguing against a recent origin for their stellar populations. We observe unresolved nuclear sources (PSs), which we reason are star clusters, in 58% of our sample. PSs are reminiscent of those found by others in ellipticals, bulges, and dwarf ellipticals. Their photometric masses are related to their host bulge masses as MPS/MSun=107.75±0.15(MBulge/1010MSun)0.76±0.13. Put together with recent compilations of central black hole masses, we infer a non-linear dependency of the Central Massive Object mass on the host spheroid mass as MCMO/MSun=107.51±0.06(MBulge/1010MSun)0.84±0.06.
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 < γ < 0.3) to the high values (γ ≈ 1) typical of intermediate luminosity ellipticals; the nuclear slope bi-modality reported by others is not present in our sample.