The Optical and Near-Infrared Colors of Galaxies

Mathematics – Logic

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Near Infrared Radiation, Color, Galactic Radiation, Classifications, Spectrum Analysis, Optical Measurement, Astronomical Photometry, Stellar Composition, Luminosity

Scientific paper

We use optical and near-infrared photometry of a well defined sample of field galaxies with spectroscopic redshifts to characterize the rest-frame colors to z less than or equal to 0.3. We construct a simple spectral synthesis model which serves to determine the information available in our five bands (UBJRFINK) to constrain the stellar composition of galaxies, as well as to classify spectrally our sample. A simple model consisting of two stellar spectral types reproduces the observed broad-band colors if the types are allowed to vary. Five primary galaxy spectral types result from this model (bk, bm, am, fm, and gm, where lower-case letters refer to stellar types B, K, etc.), each distinct in stellar type and mixture. Two types, am and fm, have similar rest-frame colors, although am-type galaxies are on average 2.5 times more luminous. We describe how our spectral synthesis model provides an accurate interpolative means for determining rest-frame colors. kappa-corrections calculated in this way are consistent with more sophisticated models and observed spectral energy distributions of local galaxies. kappa-corrections for the K band (tabulated) are shown to be very uniform for all galaxy types to z = 0.3, as predicted by models. Luminosities in all bands are also tabulated for individual galaxies. The rest-frame galaxy color distributions are comparable in range to local samples; no new spectral types exist at the intermediate redshifts and magnitude limits of our sample. In U-V and V-K, the trend of the rest-frame colors with galaxy spectral type well matches the mean distribution of local galaxies binned by morphological type. The scatter in the color distribution is due in part to color-luminosity effects observed, for example, in both V-K and U-V for all galaxy types over a combined range of 10 magnitudes. The range of colors at a fixed absolute magnitude is comparable to the change in mean color over the observed absolute magnitude range. In contrast to studies of galaxies segregated by Hubble type, we find no evidence for a strong galaxy spectral type dependence on the slope of the color-luminosity correlation.

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