Morphological analysis and classification are powerful tools for assessing the current evolutionary state and earlier history of galaxies. To date, however, these techniques have been based overwhelmingly on observations in the relatively narrow optical band, 4000-7000 Å. Because the appearance of galaxies is strongly dependent on wavelength, extension of morphological studies into both the infrared and ultraviolet wavelength regimes promises new insights into the nature of galaxies as well as independent tests of the elaborate conceptual framework that has been developed on the basis of optical band imagery.
The optical band light of galaxies is a mix of warm and cool stellar components, in which older populations have significant influence (as discussed in Appendix A). In optical images of spiral galaxies, both the cool bulge and hot spiral arm components can be bright. The presence of the interstellar medium is revealed through low to moderate ionization emission lines (e.g., H I, [N II], [O II], and [O III]) and extinction effects from dust, especially at wavelengths below 5000 Å. Near-infrared light emphasizes cool stellar components, mainly the red giant branch or cool supergiants, while simultaneously reducing the effects of extinction from dust. Spiral galaxies tend to be more concentrated and smooth in images at 8200 Å as compared to the B band, and spiral structure in many cases is less prominent (Elmegreen 1981).
The vacuum ultraviolet (< 3200 Å) is more sensitive to hot stellar components (young massive stars and advanced evolutionary phases of low-mass stars) and to dust than are the optical and near-IR. As first indicated by emission line maps (e.g., Hodge 1966; Hodge & Kennicutt 1983), which trace the locations of gaseous regions containing ionizing far-UV continuum, the appearance of galaxies in the UV can be dramatically different from that in the optical and IR. There is growing interest in the UV properties of nearby galaxies because of their importance for interpreting the rapidly enlarging sample of high-redshift galaxies detected in deep ground-based and space surveys.
In this and a companion paper (Kuchinski et al. 2000), we present the first comparative atlas of bright nearby galaxies at UV and optical wavelengths. The UV images were obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 and Astro-2 Spacelab missions in 1990 and 1995, respectively. The Astro-1 data set discussed in this paper includes both far-UV (~ 1500 Å) and mid-UV (~ 2500 Å) images; data for a total of 43 galaxies are presented. The atlas consists of multiband, wide field comparisons of galaxies at ~ 1"-3" resolution together with corresponding surface photometry. The comparison sets of ground-based optical band CCD images were obtained with pixel scales and fields of view which approximately match those of the UIT imagery. Kuchinski et al. (2000) discuss far-UV data from the Astro-2 mission and optical comparisons for another 35 galaxies, emphasizing face-on spirals.
This paper is organized as follows. In the next section we discuss the scientific potential and technical development of ultraviolet imaging of galaxies and the motivations for our work. Sections 3 and 4 describe the UIT and ground-based observations, reduction, and analysis of the data, and the presentation of images and photometry in the atlas. Section 5 provides notes on individual objects. Section 6 discusses some of the implications of our data. In Appendix A we discuss the concept of star formation history weighting functions. A complete table of UV/optical aperture photometry for the primary targets is presented in Appendix B.