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

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: adrian.bittner@eso.org
² Ludwig-Maximilians-Universität, Professor-Huber-Platz 2, 80539 München, Germany
³ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁴ Instituto de Física de Partículas y del Cosmos, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁵ Institute of Cosmology and Gravitation, University of Portsmouth, Burnaby Road, Portsmouth PO1 3FX, UK
⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching bei München, Germany
⁷ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, R. do Matão 1226, 05508-090 São Paulo, Brazil
⁸ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
¹⁰ Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Korea
¹¹ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹² Departamento de Física Teórica y del Cosmos, Universidad de Granada, Facultad de Ciencias, 18071 Granada, Spain
¹³ Instituto Universitario Carlos I de Física Teórica y Computacional, Universidad de Granada, 18071 Granada, Spain
¹⁴ Observatorio Astronómico Nacional, C/Alfonso XII 3, Madrid 28014, Spain
¹⁵ Caltech-IPAC, MC 314-6, 1200 E California Blvd, Pasadena, CA 91125, USA
¹⁶ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Wien, Austria

Received: 19 May 2020
Accepted: 2 September 2020

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.