The Internal Ultraviolet–to–optical Color Dispersion: Quantifying the Morphological K–correction

We present a quantitative measure of the internal color dispersion within galaxies, which quantifies differences in galaxy morphology as a function of observed wavelength. We apply this statistic to a sample of local galaxies with archival images at 1500 Å and 2500 Å from the Ultraviolet Imaging Telescope, and ground–based B–band observations in order to investigate how the internal dispersion between these colors relates to global galaxy properties (e.g., luminosity, color, morphological type). In general, the dispersion in the internal galaxy colors correlates with transformations in the galaxy morphology as a function of wavelength, i.e., our internal color dispersion statistic quantifies the morphological K–correction. Mid–type spiral galaxies exhibit the highest dispersion in their ultraviolet–to–optical internal colors, which stems from differences in the stellar content that constitute the bulge, disk, and spiral–arm components. Irregulars and late–type spirals show moderate internal color dispersion, although with lower values relative to the mid–type spirals. This implies that young stars generally dominate the ultraviolet–to–optical galaxy colors, modulo variations in the dust, gas, and stellar distributions. Ellipticals, lenticulars, and early–type spirals generally have low or negligible internal color dispersion, which indicates that the stars contributing to the ultraviolet–to–optical emission have a very homogeneous distribution. We discuss the application of the internal color dispersion to high–redshift galaxies in deep, Hubble Space Telescope images. By simulating the appearance of the local galaxy sample at cosmological distances, many of the galaxies have luminosities that are sufficiently bright at rest– frame optical wavelengths to be detected within the limits of the currently deepest near–infrared surveys even with no evolution. Under assumptions that the luminosity and color evolution of the local galaxies conform with the measured values of high–redshift objects, we show that galaxies’ intrinsic internal color dispersion remains measurable out to z ∼ 3. Subject headings: galaxies: fundamental parameters — galaxies: high-redshift — galaxies: photometry — galaxies: structure — methods: data analysis — ultraviolet: galaxies