Analysis of the intrinsic visible V-mid-infrared L colors of galaxies at redshifts z < 2

Ultraviolet and optical light from stars is reddened and attenuated by interstellar dust, where different sightlines across a galaxy suffer varying amounts of extinction. Tamura et al. (2009) developed an approximate method to correct for dust extinction, dubbed the “βV method,” by comparing the observed to an empirical estimate of the intrinsic flux ratio of visible and ∼3.5 μm emission. Moving beyond that empirical approach, through extensive modeling, I calibrated the βV -method for various filters spanning the visible through near infrared wavelength range, for a wide variety of simple stellar populations (SSP) and composite stellar populations (CSP). Combining Starburst99 and BC03 models, I built spectral energy distributions of SSP and CSP for various realistic star formation histories, while taking metallicity evolution into account. I convolved various 0.44–1.65 μm filter throughput curves with each model spectral energy distribution (SED) to obtain intrinsic flux ratios, βλ,0. To validate the modeling, I analyzed spatially resolved maps for the observed V- and g-band to 3.6 μm flux ratios and the inferred dust-extinction values AV for a sample of 257 nearby galaxies. Flux ratio maps are constructed using point-spread function-matched mosaics of Sloan Digitial Sky Survey g- and r-band images and Spitzer/InfraRed Array Camera 3.6μm mosaics, with all of the pixels contaminated by foreground stars or background objects masked out. Dust-extinction maps for each galaxy were created by applying the βV -method. The typical 1σ scatter in βV around the average, both within a galaxy and in each morphological type bin, is ∼20%. Combined, these result in a ∼0.4 mag scatter in AV. βV becomes insensitive to small-scale variations in stellar populations once resolution elements subtend an area larger than 10 times that of a typical giant molecular cloud. I find noticeably redder V−3.6 μm colors in the center of star-forming galaxies and galaxies with a weak AGN. The derived intrinsic V −3.6 μm colors for each Hubble type are generally consistent with the modeling. Finally, I discuss the applicability of the βV dust-correction method to more distant galaxies, for which large samples of well-matched Hubble Space Telescope rest-frame visible and James Webb Space Telescope rest-frame ∼3.5μm images will become available in the near future.