Spiral Galaxies with HST/NICMOS. I. Nuclear Morphologies, Color Maps and Distinct Nuclei

We present surface photometry for the central regions of a sample of 33 early-type (E, S0, and S0/a) galaxies observed at 1.6 microns (H band) using the Hubble Space Telescope (HST). We employ a new technique of two-dimensional fitting to extract quantitative parameters for the bulge light distribution and nuclear point sources, taking into consideration the effects of the point-spread function. Parameterizing the bulge profile with a ``Nuker'' law, we confirm that the centra...

We investigate the structural properties of nuclear star clusters in late-type spiral galaxies. More specifically, we fit analytical models to HST images of 39 nuclear clusters in order to determine their effective radii after correction for the instrumental point spread function. We use the results of this analysis to compare the luminosities and sizes of nuclear star clusters to those of other ellipsoidal stellar systems, in particular the Milky Way globular clusters. Our n...

We summarize some recent results from our observational campaign to study the central regions of spiral galaxies of late Hubble type (Scd - Sm). These disk-dominated, bulgeless galaxies have an apparently uneventful merger history. The evolution of their nuclei thus holds important constraints on the mechanisms that are responsible for bulge formation and nuclear activity in spiral galaxies. We discuss the structural properties, stellar populations, and dynamical masses of th...

We have obtained HST_NICMOS images for 72 nearby spiral galaxy bulges in the H-band. These data show that galaxies with regular bulges have steeper nuclear cusps than galaxies with irregular bulges. We also show that galaxies with regular bulges fall on the same part of the (<\gamma>, magnitude)-diagram as elliptical galaxies. This implies that early-type spiral galaxy bulges have a formation process in common with ellipticals.

We present sub-arcsecond resolution ground-based near-infrared images of the central regions of a sample of twelve barred galaxies with circumnuclear star formation activity, which is organized in ring-like regions typically one kiloparsec in diameter. We also present Hubble Space Telescope near-infrared images of ten of our sample galaxies, and compare them with our ground-based data. Although our sample galaxies were selected for the presence of circumnuclear star formation...

(Abridged) The ACS Virgo Cluster Survey is an HST program to obtain high-resolution, g and z-band images for 100 early-type members of the Virgo Cluster, spanning a range of ~460 in blue luminosity. Based on this large, homogeneous dataset, we present a sharp upward revision in the frequency of nucleation in early-type galaxies brighter than M_B ~ -15 (66 < f_n < 82%), and find no evidence for nucleated dwarfs to be more concentrated to the center of Virgo than their non-nucl...

From H alpha images of a carefully selected sample of 57 relatively large, Northern spiral galaxies with low inclination, we study the distribution of the H alpha emission in the circumnuclear and nuclear regions. At a resolution of around 100 parsec, we find that the nuclear H alpha emission in the sample galaxies is often peaked, and significantly more often so among AGN host galaxies. The circumnuclear H alpha emission, within a radius of two kpc, is often patchy in late-t...

We present results of Hubble Space Telescope NICMOS H-band imaging of 73 of most luminous (i.e., log[L_IR/L_0]>11.4) Infrared Galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey (GOALS). This dataset combines multi-wavelength imaging and spectroscopic data from space (Spitzer, HST, GALEX, and Chandra) and ground-based telescopes. In this paper we use the high-resolution near-infrared data to recover nuclear structure that is obscured by dust at optical wavelengths...

We present a catalogue of photometric and structural properties of 228 nuclear star clusters (NSCs) in nearby late-type disk galaxies. These new measurements are derived from a homogeneous analysis of all suitable WFPC2 images in the $HST$ archive. The luminosity and size of each NSC is derived from an iterative PSF-fitting technique, which adapts the fitting area to the effective radius ($r_{\rm eff}$) of the NSC, and uses a WFPC2-specific PSF model tailored to the position ...

Recent observations with the Hubble Space Telescope (HST) have revealed that a large fraction of late-type (Sc and later) spiral galaxies harbor a bright, compact stellar cluster in their dynamical centers. Statistics of the mass, age, and star formation history of these clusters as a function of their host galaxy's Hubble type can be used to constrain models of secular galaxy evolution. Since late-type spirals by definition do not possess a prominent bulge, their nuclear clu...