Spiral galaxies with HST/NICMOS. I. Nuclear morphologies, color maps, and distinct nuclei

This is the first of two papers where we present the analysis of an HST/NICMOS2 near-infrared (NIR) snapshot survey in the F160W (H) filter for a sample of 78 spiral galaxies selected from the UGC and ESOLV catalogs. For 69 of these objects we provide nuclear color information derived by combining the H data either with additional NICMOS F110W (J) images or with V WFPC2/HST data. Here we present the NIR images and the optical-NIR color maps. We focus our attention on the properties of the photometrically distinct "nuclei" which are found embedded in most of the galaxies and provide measurements of their half-light radii and magnitudes in the H (and when available in the J) band. We find that (1) in the NIR the nuclei embedded in the bright early- to intermediate-type galaxies span a much larger range in brightness than the nuclei which are typically found embedded in bulgeless late-type disks: the nuclei embedded in the early- to intermediate-type galaxies reach, on the bright end, values up to H_AB ∼ -17.7 mag; (2) nuclei are found in both nonbarred and barred hosts, in large-scale (≳ 1 kpc) as well as in nuclear (up to a few 100 pc) bars; (3) there is a significant increase in half-light radius with increasing luminosity of the nucleus in the early/intermediate types (a decade in radius for ≈8 mag brightening), a correlation which was found in the V band and which is also seen in the NIR data; (4) the nuclei of early/intermediate-type spirals cover a large range of optical-NIR colors, from V-H ≈ -0.5 to 3. Some nuclei are bluer and others redder than the surrounding galaxy, indicating the presence of activity or reddening by dust in many of these systems; (5) some early/intermediate nuclei are elongated and/or slightly offset from the isophotal center of the host galaxy. On average, however, these nuclei appear as centered, star-cluster-like structures similar to those which are found in the late-type disks.