GALEX UV color relations for nearby early-type galaxies

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 (similar to 4%) of ellipticals and similar to 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 Mg-2 index and central velocity dispersion. We find a tight anticorrelation between FUV-NUV and the Mg-2 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.