Colors of Ellipticals from GALEX to Spitzer

TL;DR

This study uses multi-wavelength photometry from GALEX to Spitzer to analyze the stellar populations of local elliptical galaxies, revealing complex metallicity distributions and the metallicity-driven UV upturn phenomenon.

Contribution

It demonstrates that elliptical galaxy colors are best explained by multi-metallicity models with no young stars, and highlights discrepancies between color-based and line index-based metallicity estimates.

Findings

Ellipticals are not single-age, single-metallicity populations.

Colors are consistent with multi-metallicity models using only metallicity variations.

UV upturn is driven by metallicity, as shown by BHB star models.

Abstract

Multi-color photometry is presented for a large sample of local ellipticals selected by morphology and isolation. The sample uses data from GALEX, SDSS, 2MASS and Spitzer to cover the filters NUV, ugri, JHK and 3.6mum. Various two-color diagrams, using the half-light aperture defined in the 2MASS J filter, are very coherent from color to color, meaning that galaxies defined to be red in one color are always red in other colors. Comparison to globular cluster colors demonstrates that ellipticals are *not* composed of a single age, single metallicity (e.g., [Fe/H]) stellar population, but require a multi-metallicity model using a chemical enrichment scenario. Such a model is sufficient to explain two-color diagrams and the color-magnitude relations for all colors using only metallicity as a variable on a solely 12 Gyrs stellar population with no evidence of stars younger than 10 Gyrs. The [Fe/H] values that match galaxy colors range from -0.5 to +0.4, much higher (and older) than population characteristics deduced from Lick/IDS line-strength system studies, indicating an inconsistency between galaxy colors and line indices values for reasons unknown. The NUV colors have unusual behavior signaling the rise and fall of the UV upturn with elliptical luminosity. Models with BHB tracks can reproduce this behavior indicating the UV upturn is strictly a metallicity effect.