Using GALEX , UVOT and optical photometry , we explore the prevalence and strength of the UV-upturn in the spectra of quiescent early type galaxies in several nearby clusters .
Even for galaxies with completely passive optical colours , there is a large spread in vacuum UV colour consistent with almost all having some UV upturn component .
Combining GALEX and UVOT data below 3000Å , we generate for the first time comparatively detailed UV SEDs for Coma cluster galaxies .
Fitting the UV upturn component with a blackbody , twenty six of these show a range of characteristic temperatures ( 10000–21000K ) for the UV upturn population .
Assuming a single temperature to explain GALEX-optical colours could underestimate the fraction of galaxies with UV upturns and mis-classify some as systems with residual star formation .
The UV upturn phenomenon is not an exclusive feature found only in giant galaxies ; we identify galaxies with similar ( or even bluer ) FUV - V colours to the giants with upturns over a range of fainter luminosities .
The temperature and strength of the UV upturn are correlated with galaxy mass .
Under the plausible hypothesis that the sources of the UV upturn are blue horizontal branch stars , the most likely mechanism for this is the presence of a substantial ( between 4 % and 20 % ) Helium rich ( Y > 0.3 ) population of stars in these galaxies , potentially formed at z \sim 4 and certainly at z > 2 ; this plausibly sets a lower limit of \sim { 0.3 - 0.8 } \times 10 ^ { 10 } M _ { \odot } to the in situ stellar mass of \sim L ^ { * } galaxies at this redshift .