Searching for Bulges at the End of the Hubble Sequence

Abstract

We investigate the stellar disk properties of a sample of 19 nearby spiral galaxies with low inclination and late Hubble type (Scd or later). We combine our high-resolution Hubble Space Telescope I-band observations with existing ground-based optical images to obtain surface brightness profiles that cover a high dynamic range of galactic radii. Most of these galaxies contain a nuclear star cluster, as discussed in a separate paper. The main goal of the present work is to constrain the properties of stellar bulges at these extremely late Hubble types. We find that the surface brightness profiles of the latest-type spiral galaxies are complex, with a wide range in shapes. We have sorted our sample in a sequence, starting with ``pure'' disk galaxies (approximately 30% of the sample). These galaxies have exponential stellar disks that extend inward to within a few tens of parsecs from the nucleus, where the light from the nuclear cluster starts to dominate. They appear to be truly bulgeless systems. Progressing along the sequence, the galaxies show increasingly prominent deviations from a simple exponential disk model on kiloparsec scales. Traditionally, such deviations have prompted ``bulge-disk'' decompositions. Indeed, the surface brightness profiles of these galaxies are generally well fitted by adding a second (exponential) bulge component. However, we find that most surface brightness profiles can be fitted equally well (or better) with a single Sérsic-type R1/n profile over the entire radial range of the galaxy without requiring a separate ``bulge'' component. We warn in a general sense against identification of bulges solely on the basis of single-band surface brightness profiles. Despite the narrow range of Hubble types in our sample, the surface brightness profiles are far from uniform. The differences between the various galaxies appear unrelated to their Hubble types, thus questioning the usefulness of the Hubble sequence for the subcategorization of the latest-type spiral galaxies. A number of galaxies show central excess emission on spatial scales of a few hundred parsecs that cannot be attributed to the nuclear cluster, the Sérsic-type description of the stellar disk, or what one would generally consider to be a bulge component. The origin of this light component remains unclear. Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal 8599.