GALEX UV color relations for nearby early-type galaxies

Abstract

We use GALEX/optical photometry to construct color-color relationships for early-type galaxies sorted by morphological type. We have matched objects in the GALEX GR1 public release and the first IR1.1 internal release, with the RC3 early-type galaxies having a morphological type -5.5 ≤ T < − 1.5, with mean error on T < 1.5 and mean error on (B − V)_T < 0.05. After visual inspection of each match, we are left with 130 galaxies with reliable GALEX pipeline photometry in the far-UV and near-UV bands. This sample is divided into ellipticals (-5.5 ≤ T < − 3.5) and lenticulars (-3.5 ≤ T < − 1.5). After correction for Galactic extinction, the color-color diagrams FUV − NUV versus (B − V)_Tc are plotted for the two subsamples. We find a tight anticorrelation between the FUV − NUV and (B − V)Tc colors for ellipticals, with the UV color getting bluer when the (B − V)Tc gets redder. This relationship very likely is an extension of the color-metallicity relationship in the GALEX NUV band. We suspect that the main source of the correlation is metal line blanketing in the NUV band. The FUV − NUV versus B − V correlation has larger scatter for lenticular galaxies; we speculate that this reflects the presence of low-level star formation. If the latter objects (i.e., those that are blue both in FUV − NUV and in B − V) are interpreted as harboring recent star formation activity, this would be the case for a few percent (~4%) of ellipticals and ~15% of lenticulars; this would mean about 10% of early-type galaxies have residual star formation in our full sample of 130 early-type galaxies. We also plot FUV − NUV versus the Mg2 index and central velocity dispersion. We find a tight anticorrelation between FUV − NUV and the Mg2 index; we suspect that this reflects blanketing in the NUV band being correlated with overall metallicity. We find a marginal anticorrelation of FUV − V_T with Mg_2 for elliptical galaxies.