Comparing Galaxy Morphology at Ultraviolet and Optical Wavelengths

Abstract

We have undertaken an imaging survey of 34 nearby galaxies in far-ultraviolet (FUV, ̃ 1500 Å) and optical (UBVRI) passbands to characterize galaxy morphology as a function of wavelength. This sample, which includes a range of classical Hubble types from elliptical to irregular, with emphasis on spirals at low inclination angle, provides a valuable database for comparison with images of high-z galaxies whose FUV light is redshifted into the optical and near-infrared bands. Ultraviolet data are from the Ultraviolet Imaging Telescope (UIT) Astro-2 mission. We present images and surface brightness profiles for each galaxy, and we discuss the wavelength dependence of morphology for different Hubble types in the context of understanding high-z objects. In general, the dominance of young stars in the FUV produces the patchy appearance of a morphological type later than that inferred from optical images. Prominent rings and circumnuclear star formation regions are clearly evident in FUV images of spirals, while bulges, bars, and old, red stellar disks are faint to invisible at these short wavelengths. However, the magnitude of the change in apparent morphology ranges from dramatic in early-type spirals with prominent optical bulges to slight in late-type spirals and irregulars, in which young stars dominate both the UV and optical emission. Starburst galaxies with centrally concentrated, symmetric bursts display an apparent "E/S0" structure in the FUV, while starbursts associated with rings or mergers produce a peculiar morphology. We briefly discuss the inadequacy of the optically defined Hubble sequence in describing FUV galaxy images and estimating morphological k-corrections, and we suggest some directions for future research with this data set.