THE <i>GALEX</i> /S ⁴ G UV–IR COLOR–COLOR DIAGRAM: CATCHING SPIRAL GALAXIES AWAY FROM THE BLUE SEQUENCE

Abstract

We obtained GALEX FUV, NUV, and Spitzer/IRAC 3.6 m photometry for >2000 galaxies, available for 90% of the S 4 G sample. We find a very tightGALEX blue sequence (GBS) in the (FUV-NUV) versus (NUV-[3.6]) color-color diagram, which is populated by irregular and spiral galaxies, and is mainly driven by changes in the formation timescale () and a degeneracy between and dust reddening. The tightness of the GBS provides an unprecedented way of identifying star-forming galaxies and objects that are just evolving to (or from) what we call theGALEX green valley (GGV). At the red end of the GBS, at (NUV-[3.6]) > 5, we find a widerGALEX red sequence (GRS) mostly populated by E/S0 galaxies that has a perpendicular slope to that of the GBS and of the optical red sequence. We find no such dichotomy in terms of stellar mass (measured by M [3.6] ) since both massive