UV SEDs of early-type cluster galaxies: A new look at the UV upturn
  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 3000\AA,
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 $\sim {\rm 0.3 -
0.8} \times 10^{10}$ $M_{\odot}$ to the {\it in situ} stellar mass of $\sim
L^*$ galaxies at this redshift.
