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The centres of disc galaxies host a variety of structures built via bothinternal and external processes. In this study, we constrain the formation andevolution of these central structures, in particular nuclear rings and nucleardiscs, by deriving maps of mean stellar ages, metallicities and [$\alpha$/Fe]abundances. We use observations obtained with the MUSE integral-fieldspectrograph for the TIMER sample of 21 massive barred galaxies. Our resultsindicate that nuclear discs and nuclear rings are part of the same physicalcomponent, with nuclear rings constituting the outer edge of nuclear discs. Allnuclear discs in the sample are clearly distinguished based on their stellarpopulation properties. As expected in the picture of bar-driven secularevolution, 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 andmetallicities decreasing, and [$\alpha$/Fe] abundances increasing with radiusout to the nuclear ring. Often, these gradients show no breaks from the edge ofthe nuclear disc until the centre, suggesting that these structures extend tothe very centres of the galaxies. We argue that continuous (stellar) nucleardiscs may form from a series of bar-built (initially gas-rich) nuclear ringsthat grow in radius, as the bar evolves. In this picture, nuclear rings aresimply the (often) star-forming outer edge of nuclear discs. Finally, bycombining our results with those from a accompanying kinematic study, we do notfind evidence for the presence of large, dispersion-dominated components in thecentres of these galaxies. This could be a result of quiet merger histories,despite the large galaxy masses, or perhaps high angular momentum and strongfeedback processes preventing the formation of these kinematically hotcomponents.

The inside-out formation of nuclear discs and the absence of old central spheroids in barred galaxies of the TIMER survey