The central regions of disc galaxies hold clues to the processes thatdominate their formation and evolution. The TIMER project has obtained highsignal-to-noise and spatial resolution integral-field spectroscopy data of theinner few kpc of 21 nearby massive barred galaxies, allowing studies of thestellar kinematics with unprecedented spatial resolution. We confirmtheoretical predictions of the effects of bars on stellar kinematics, andidentify box/peanuts through kinematic signatures in mildly and moderatelyinclined galaxies, finding a lower limit to the fraction of massive barredgalaxies with box/peanuts at ~62%. Further, we provide kinematic evidence ofthe connection between barlenses, box/peanuts and bars. We establish thepresence of nuclear discs in 19 galaxies and show that their kinematics arecharacterised by near-circular orbits with low pressure support, and areconsistent with the bar-driven secular evolution picture for their formation.In fact, we show that these nuclear discs have, in the region where theydominate, larger rotational support than the underlying main galaxy disc. Wedefine a kinematic radius for the nuclear discs and show that it relates to barradius, ellipticity and strength, and bar-to-total ratio. Comparing our resultswith photometric studies, we find that state-of-the-art galaxy imagedecompositions are able to discern nuclear discs from classical bulges, if theimages employed have enough physical spatial resolution. In fact, we show thatnuclear discs are typically identified in such image decompositions asphotometric bulges with (near-)exponential profiles. However, we find that thepresence of composite bulges (galaxies hosting both a classical bulge and anuclear disc) can often be unnoticed in studies based on photometry alone, andsuggest a more stringent threshold to the Sersic index to identify galaxieswith pure classical bulges.

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.