UV-extended E-MILES stellar population models: young components in
  massive early-type galaxies
  We present UV-extended E-MILES stellar population synthesis models covering
the spectral range 1680-50000A at moderately high resolution. We employ the
NGSL space-based stellar library to compute spectra of single-age,
single-metallicity stellar populations in the wavelength range from 1680 to
3540A. These models represent a significant improvement in resolution and
age/metallicity coverage over previous studies based on earlier space-based
libraries. These model spectra were joined with those we computed in the
visible using MILES, and other empirical libraries for redder wavelengths. The
models span the metallicity range -1.79<[M/H]<+0.26 and ages above 30 Myr, for
a suite of IMF types with varying slopes. We focus on the behaviour of colours,
spectra and line-strength indices in the UV range as a function of relevant
stellar population parameters. Whereas some indices strengthen with increasing
age and metallicity, as most metallicity indicators in the visible, other
indices peak around 3 Gyr for metal-rich stellar populations, such as Mg at
2800A. Our models provide reasonably good fits to the integrated colours and
most line-strengths of the stellar clusters of the Milky-Way and LMC. Our
full-spectrum fits in the UV range for a representative set of ETGs of varying
mass yield age and metallicity estimates in very good agreement with those
obtained in the optical range. The comparison of UV colours and line-strengths
of massive ETGs with our models reveals the presence of young stellar
components, with ages in the range 0.1-0.5 Gyr and mass fractions 0.1-0.5%, on
the top of an old stellar population.
