Galactic Bulges fromHubble Space TelescopeNICMOS Observations: Central Galaxian Objects, and Nuclear Profile Slopes

Abstract

We have measured the central structural properties for a sample of S0-Sbc galaxies down to scales of ∼10 pc using Hubble Space Telescope NICMOS images. We find that the photometric masses of the central star clusters, which occur in 58% of our sample, are related to their host bulge masses such that MPS = 107.75±0.15 (MBul/1010M⊙)0.76±0.13. Put together with recent data on bulges hosting supermassive black holes, we infer a non-linear dependency of the 'Central Massive Object' mass on the host bulge mass such that MCMO/M⊙ = 107.51±0.06(MBul/1010 M⊙)0.84±0.06. We argue that the linear relation presented by Ferrarese et al. is biased at the low-mass end by the inclusion of the disc light from lenticular galaxies in their sample. Matching our NICMOS data with wider-field, ground-based K-band images enabled us to sample from the nucleus to the disk-dominated region of each galaxy, and thus to perform a proper bulge-disk decomposition. We found that the majority of our galaxies (∼90%) possess central light excesses which can be modeled with an inner exponential and/or an unresolved point source in the case of the nuclear star clusters. 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. Surface brightness profiles (of the total galaxy light, and the bulge component on its own) 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 ≤ ≤ 0.3) to the high values ( ∼ 1) typical of intermediate luminosity ellipticals; the nuclear slope bi-modality reported by others is not present in our sample. Subject headings: galaxies : spiral — galaxies : structure — galaxies : nuclei