The inside-out formation of nuclear discs and the absence of old central
  spheroids in barred galaxies of the TIMER survey
  The centres of disc galaxies host a variety of structures built via both
internal and external processes. In this study, we constrain the formation and
evolution of these central structures, in particular nuclear rings and nuclear
discs, by deriving maps of mean stellar ages, metallicities and [$\alpha$/Fe]
abundances. We use observations obtained with the MUSE integral-field
spectrograph for the TIMER sample of 21 massive barred galaxies. Our results
indicate that nuclear discs and nuclear rings are part of the same physical
component, with nuclear rings constituting the outer edge of nuclear discs. All
nuclear discs in the sample are clearly distinguished based on their stellar
population properties. As expected in the picture of bar-driven secular
evolution, nuclear discs are younger, more metal-rich, and show lower
[$\alpha$/Fe] enhancements, as compared to their immediate surroundings.
Moreover, nuclear discs exhibit well-defined radial gradients, with ages and
metallicities decreasing, and [$\alpha$/Fe] abundances increasing with radius
out to the nuclear ring. Often, these gradients show no breaks from the edge of
the nuclear disc until the centre, suggesting that these structures extend to
the very centres of the galaxies. We argue that continuous (stellar) nuclear
discs may form from a series of bar-built (initially gas-rich) nuclear rings
that grow in radius, as the bar evolves. In this picture, nuclear rings are
simply the (often) star-forming outer edge of nuclear discs. Finally, by
combining our results with those from a accompanying kinematic study, we do not
find evidence for the presence of large, dispersion-dominated components in the
centres of these galaxies. This could be a result of quiet merger histories,
despite the large galaxy masses, or perhaps high angular momentum and strong
feedback processes preventing the formation of these kinematically hot
components.
