Quantitative morphology of galaxies observed in the ultraviolet

We present a quantitative study of the far-ultraviolet (FUV) and optical morphology in 32 nearby galaxies and estimate the "morphological k-correction" expected if these objects were observed unevolved at high redshift. Using the common indices of central concentration (C) and rotational asymmetry (A) to quantify morphology, we consider independently two phenomena that give rise to this k-correction. Bandshifting, the decrease in the rest-frame wavelength of light observed through optical filters, is explored by measuring these indices in several passbands for each galaxy, and it is found to be the primary driver of changes in C and A. In general, the optical trend found for decreasing C and increasing A when going to shorter wavelengths extends to the FUV. However, the patchy nature of recent star formation in late-type galaxies, which is accentuated in the FUV, results in poor quantitative correspondence between morphologies determined in the optical and FUV. We then artificially redshift our FUV images into the Hubble Deep Field (HDF) filters to simulate various cosmological distance effects, such as surface brightness dimming and loss of spatial resolution. Hubble types of many galaxies in our sample are not readily identifiable at redshifts beyond z similar to 1, and the galaxies themselves are difficult to detect beyond z similar to 3. Because only features of the highest surface brightness remain visible at cosmological distances, the change in C and A between simulated high-z galaxies and their unredshifted counterparts depends on whether their irregular features are primarily bright or faint. Our simulations suggest that k-corrections alone are indeed capable of producing the peculiar morphologies observed at high redshift; for example, several spiral galaxies have C and A indices typical of irregular or peculiar HDF objects viewed at z greater than or equal to 2. We briefly discuss some elements of a scheme to classify rest-frame UV images, mergers, protogalaxies, and other objects for which classical Hubble types do not adequately encompass the existing morphology.