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 (∼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 Mg₂ for elliptical galaxies.