The SAURON project-XXI. The spatially-resolved UV-line strength
  relations of early-type galaxies
  The unexpected rising flux of early-type galaxies at decreasing ultraviolet
(UV) wavelengths is a long-standing mystery. One important observational
constraint is the correlation between UV-optical colours and Mg2 line strengths
found by Burstein et al. (1988). The simplest interpretation of this phenomenon
is that the UV strength is related to the Mg line strength. Under this
assumption, we expect galaxies with larger Mg gradients to have larger UV
colour gradients. By combining UV imaging from GALEX, optical imaging from MDM
and SAURON integral-field spectroscopy, we investigate the spatially-resolved
relationships between UV colours and stellar population properties of 34
early-type galaxies from the SAURON survey sample. We find that galaxies with
old stellar populations show tight correlations between the FUV colours (FUV-V
and FUV-NUV) and the Mgb index, H{\beta} index and metallicity [Z/H]. The
equivalent correlations for the Fe5015 index, {\alpha}-enhancement
[{\alpha}/Fe] and age are present but weaker. We have also derived logarithmic
internal radial colour, measured line strength and derived stellar population
gradients for each galaxy and again found a strong dependence of the FUV-V and
FUV-NUV colour gradients on both the Mg b line strength and the metallicity
gradients for galaxies with old stellar populations. In particular, global
gradients of Mg b and [Z/H] with respect to the UV colour across galaxies are
consistent with their local gradients within galaxies, suggesting that the
global correlations also hold locally. From a simple model based on multi-band
colour fits of UV upturn and UV-weak galaxies, we have identified a plausible
range of parameters that reproduces the observed radial colour profiles. In
these models, the centers of elliptical galaxies, where the UV flux is strong,
are enhanced in metals by roughly 60% compared to UV-weak regions.
