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Session 82 - Spirals & Irregulars.
Display session, Friday, January 09
Exhibit Hall,
The Ultraviolet Imaging Telescope (UIT) has returned the first significant sample of far-UV imagery of galaxies which combines a large field-of-view and spatial resolution comparable to ground-based observatories. The UIT data permit determination of both global FUV properties with improved photometric precision, and detailed investigation of galaxian morphology at intermediate (spiral arms, circumnuclear rings), and small (star-forming complexes) spatial scales. This spectral range is highly valuable for tracing the spatial distribution of recent star formation, and characterizing global star formation rates. An understanding of the FUV properties of local systems is also crucial for interpretation of systems at high redshift, where the observed light is the redshifted UV continuum.
During the Astro Spacelab missions, UIT obtained FUV (\lambda\lambda\ 1500 Åimagery of \sim\,60 galaxies with D_25 > 1^\prime, spanning the Hubble sequence from E0 to Im. We highlight several aspects of the global properties of the entire UIT galaxy dataset, including surface brightness distributions, comparative FUV/optical/NIR morphology, and the usefulness of the spatially-sampled FUV/H\alpha ratio as a stellar population diagnostic. We also provide a first description of the ultraviolet Hubble Sequence.
A variety of radial light profiles are found, including two systems, NGC 4214 and NGC 3310, which exhibit R^1/4 profiles produced by their Population I stellar components. Sb-Sc spirals often show a nuclear peak, followed by a region of approximately constant FUV surface brightness, and then an exponential decline at larger radii. Overall, elliptical systems display the most similar FUV/optical morphology, while early-type spirals show the most divergent. The UV and optical/NIR data will be combined in a series of Atlases which will become available in 1998.
