Galactic Bulges from HST NICMOS Observations: Global Scaling Relations

We investigate bulge and disk scaling relations using a volume-corrected sample of early- to intermediate-type disk galaxies in which, importantly, the biasing flux from additional nuclear components on the inner bulge light-profile has been modeled and removed. Structural parameters are obtained from a seeing-convolved, bulge+disk+nuclear-component decomposition applied to NIR surface brightness profiles covering from ~10 pc to the outer disk. The profiles were obtained by combining HST NICMOS images with larger field-of-view ground-based images. Bulge and disk parameters, and bulge-to-disk ratios, are shown as a function of bulge luminosity, galaxy Hubble type, and bulge Sersic index n. Photometric parameters of bulges and disks are observed to correlate with bulge luminosity and with Sersic index n. In contrast, for the unbarred, early to intermediate types covered by the sample, Hubble type does not correlate with bulge and disk components, and their various ratios. In this sense, the early-to-intermediate spiral Hubble sequence is scale-free. However, galaxies themselves are not scale-free, the critical scale being the luminosity of the bulge. The central stellar density of the bulge is shown to increase strongly with bulge luminosity. Mathematical expressions are given for the stronger relations, which can be used to test and constrain galaxy formation models