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

We have measured the central structural properties for a sample of S0-Sbcgalaxies down to scales of ~10 pc using HST NICMOS images. We find that thephotometric masses of the central star clusters, which occur in 58% of oursample, are related to their host bulge masses such that MassPt =10^{7.75\pm0.15}(MassBul/10^{10}MassSun)^{0.76\pm 0.13}. Put together withrecent data on bulges hosting supermassive black holes, we infer a non-lineardependency of the `Central Massive Object' mass on the host bulge mass suchthat MassCMO = 10^{7.51\pm 0.06} (MassBul/10^{10}MassSun)^{0.84 \pm 0.06}. Weargue that the linear relation presented by Ferrarese et al. is biased at thelow-mass end by the inclusion of the disc light from lenticular galaxies intheir sample. Matching our NICMOS data with wider-field, ground-based K-bandimages enabled us to sample from the nucleus to the disk-dominated region ofeach galaxy, and thus to perform a proper bulge-disk decomposition. We foundthat the majority of our galaxies (~90%) possess central light excesses whichcan be modeled with an inner exponential and/or an unresolved point source inthe case of the nuclear star clusters. All the extended nuclear components,with sizes of a few hundred pc, have disky isophotes, which suggest that theymay be inner disks, rings, or bars; their colors are redder than those of theunderlying bulge, arguing against a recent origin for their stellarpopulations. Surface brightness profiles rise inward to the resolution limit ofthe data, with a continuous distribution of logarithmic slopes from the lowvalues typical of dwarf ellipticals (0.1 \leq gamma \leq 0.3) to the highvalues (gamma ~ 1) typical of intermediate luminosity ellipticals; the nuclearslope bi-modality reported by others is not present in our sample.Comment: Accepted by ApJ. Uses Apr 12, 2004 emulateapj.cls (included