THE ULTRAVIOLET COLOR GRADIENT IN THE LATE-TYPE SPIRAL GALAXY-M33

The ultraviolet surface brightness and color distributions for the late-type spiral galaxy M33 are derived from images at 1520 and 2490 angstrom that were obtained by the Ultraviolet Imaging Telescope (UIT) on the Astro Spacelab mission. Although the surface brightness shows a general decline with radius, the dominant spiral arms cause significant deviations from an exponential fit. Colors in elliptical annuli become 0.2-0.3 mag bluer with increasing radius. Our measures of individual H II regions and sections of spiral arms follow this same trend. Interarm regions are redder than the arms. A metallicity gradient affecting only the colors of stars is inadequate in accounting for the observed color gradient. A plausible explanation invokes a combination of an LMC-type reddening curve and a radial gradient in the internal reddening in M33.