Colors of Ellipticals From Galex to Spitzer

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 N U V , ugri, JHK and 3.6µm. 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 N U V 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.

Colors of Dwarf Ellipticals from GALEX to WISE

“…The colors of the new faint and dwarf ellipticals in our sample extend the relations found in Schombert (2016) by six more magnitudes in luminosity. The near colors are defined by the near-UV and optical filters (GALEX N U V , SDSS ugri).…”

“…The analysis of the colors of dwarf ellipticals follows the procedure outlined in Schombert (2016) for bright ellipticals; the empirical two-color relationships for all the optical and near-IR filters, comparison to multi-metallicity population models and the color-magnitude relation. For clarity, we divide the discussion of the two-color relations into those defined by filters close in wavelength (near colors) and those whose filters are widely separated in wavelength (far colors).…”

“…Again, as discussed in Schombert (2016), the bluer colors (u − g vs g − r) indicate that an internal metallicity distribution is required to match the global colors (so-called multi-metallicity models, Schombert & Rakos 2009). Where the bluer u − g colors are influenced by a small metal-poor population (also seen in the N U V colors), presumably a population of between 5 to 10% of the total stellar mass that is composed of the first generation of stars with near globular cluster metallicities.…”

“…Differing from the photometry of normal ellipticals in Schombert (2016), we have added archive data from the WISE mission to the whole dataset, mostly to offset the lack of 2MASS photometry for the dE's in the sample due to the surveys low limiting magnitude. The WISE MIDEX mission (Wright et al 2010) was a cryogenicallycooled 40 cm telescope equipped with a camera containing four mid-infrared focal plane array detectors that simultaneously imaged the same 47x47 arcmin field-ofview.…”

“…This study is an extension of the color analysis of bright ellipticals (M g < −20) from Schombert (2016) that focuses on presenting a large sample of morphologically classified dwarf ellipticals (class dE) and bridging the gap between normal and dwarf ellipticals (the luminosity range between −18 and −20). As demonstrated in Schombert (2017), this gap is populated by the rare class of faint ellipticals with power-law surface brightness profiles, in the same structural family as normal ellipticals.…”