  We exploit stellar population models of absorption line indices in the
ultraviolet (from 2000 - 3200 Angstrom) to study the spectra of massive
galaxies. Our central aim is to investigate the occurrence at high-redshift of
the UV upturn, i.e. the increased UV emission due to old stars observed in
massive galaxies and spiral bulges in the local Universe. We use a large
(~275,000) sample of z~0.6 massive (log M*/Msun > 11.5) galaxies using both
individual spectra and stacks and employ a suite of models including a UV
contribution from old populations, spanning various effective temperatures,
fuel consumptions and metallicities. We find that a subset of our indices, Mg
I, Fe I, and BL3096, are able to differentiate between old and young UV ages.
We find evidence for old stars contributing to the UV in massive galaxies,
rather than star formation. The data favour models with low/medium upturn
temperatures (10,000 - 25,000K) consistent with local galaxies, depending on
the assumed metallicity, and with a larger fuel (f ~ 0.065 Msun). Models with
one typical temperature are favoured over models with a temperature range,
which would be typical of an extended horizontal branch. Old UV-bright
populations are found in the whole galaxy sample (~92%), with a mass fraction
peaking around 20-30%. Upturn galaxies are massive and have redder colours, in
agreement with findings in the local Universe. We find that the upturn
phenomenon appears at z~1 and its frequency increases towards lower redshift,
as expected by stellar evolution of low mass stars. Our findings will help
constrain stellar evolution in the exotic UV upturn phase.

Astrophysics of Galaxies