The Optical and Near-Infrared Colors of Galaxies

Previous abstract Next abstract

Session 17 -- Normal Galaxies
Oral presentation, Monday, 10:30-12:00, Wheeler Room

[17.01] The Optical and Near-Infrared Colors of Galaxies

Matthew A. Bershady (Lick Observatory, UCSC)

We have used optical and near-infrared photometry of a well defined sample of field galaxies with spectroscopic redshifts to study and characterize the trends and dispersions of rest-frame colors. The galaxy sample has a characteristic B magnitude of 20 and extends to redshifts of 0.35.

We have constructed a simple spectral synthesis model which serves to determine both the information content in our four broad-band colors for constraining the stellar composition of galaxies, as well as to spectrally classify our sample. We find that a simple model consisting of two stellar spectral types can reproduce well the observed broad-band colors, but only if the types are allowed to vary. The distribution of galaxy spectral types resulting from this model is distinct both in type and stellar composition. In particular, the hottest galaxy spectral types have a very narrow range of stellar fractions, whereas the cooler types show a broader range of stellar fractions that are qualitatively consistent with more sophisticated stellar population synthesis models of similar stellar systems. The redshift distribution of galaxy k-corrections calculated via the simple model are also consistent with more sophisticated models as well as observed spectral energy distributions of galaxies. Our broad-band data can be spectrally interpolated reliably with the models to calculate accurate k-corrections to high redshift, particularly for redder bands.

We have used the k-corrections from the simple model to construct the rest-frame distribution in the U-V, V-K two-color plane. The trend of the rest-frame colors with galaxy spectral type well matches the mean distribution of local galaxies binned by morphological type. Particular attention is paid, however, to the scatter in these colors. Color-luminosity effects are observed 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. We find no evidence for a strong galaxy spectral type dependence of these effects. This implies that the correlation of color to luminosity occurs at a fundamental level.

Monday program listing