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

Abstract

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 [α/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 exhibit lower [α/Fe] enhancements, as compared to their immediate surroundings. Moreover, nuclear discs exhibit well-defined radial gradients, with ages and metallicities decreasing, and [α/Fe] abundances increasing with radius out to the nuclear ring. Often, these gradients show no breaks from the edge of the nuclear disc up through the centre, suggesting that these structures extend to the very centres of galaxies. We argue that continuous (stellar) nuclear discs may form from a series of bar-built (initially gas-rich) nuclear rings that expand in their 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 taken 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, due to high angular momentum and strong feedback processes preventing the formation of these kinematically hot components.

Kinematic signatures of nuclear discs and bar-driven secular evolution in nearby galaxies of the MUSE TIMER project

“…Gadotti 2009, and references therein). In fact, the largest value of n found is below 3, which indicates that there is no galaxy in the TIMER sample with a dominant classical bulge, a conclusion that is corroborated in our accompanying paper (Bittner et al 2020) through an analysis of stellar populations properties. This may result from the selection of the TIMER sample, which favours galaxies hosting nuclear components with a disc-like morphology.…”

“…Figure 8 also suggests that more evolved bars tend to build larger nuclear discs, which may help putting constraints on the ages of bars, although more theoretical work on the formation and evolution of nuclear discs is necessary. Finally, in our accompanying paper (Bittner et al 2020), we present stellar population properties of the TIMER nuclear discs, and discuss how these properties too are consistent with the picture in which such nuclear discs are built via bar-driven processes.…”

“…However, Figs. 2 and 3 show that these nuclear structures are extended, and in our accompanying paper (Bittner et al 2020) we present evidence from the stellar population properties that these nuclear structures appear to extend all the way to the galaxy centre. We thus have very strong evidence that the TIMER galaxies host nuclear discs with kinematic properties consistent with a bar-driven origin.…”

“…In this paper, we take advantage of the integral-field spectroscopy data from the Time Inference with MUSE in Extragalactic Rings (TIMER) project to study the kinematic properties of barred galaxies and nuclear stellar structures with unprecedented spatial resolution. We also take advantage of the vast ancillary data for the TIMER sample to (i), provide evidence that bar-driven processes appear to be the main mechanism responsible for the formation of nuclear discs and related structures; (ii), demonstrate more rigorously the connection between nuclear discs, detected via their kinematic properties, and central exponential components, found via photometric decompositions, and (iii), provide further evidence that nuclear rings are the outer rims of nuclear discs (see also our accompanying paper, Bittner et al 2020).…”

“…The results discussed above suggest that the main processes providing the gaseous content for the formation of both nuclear rings and nuclear discs are bar-driven, and the connection between nuclear rings and nuclear discs suggests that a single set of joint mechanisms is in charge of transforming the gas component into these stellar structures. In our accompanying paper (Bittner et al 2020) we study this connection in more detail through the analysis of the stellar population properties of nuclear discs and nuclear rings.…”