Galaxies within galaxies in the TIMER survey: stellar populations of inner bars are scaled replicas of main bars

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 Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
⁵ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁶ Instituto de Física de Partículas y del Cosmos, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁷ Institute of Cosmology and Gravitation, University of Portsmouth, Burnaby Road, Portsmouth PO1 3FX, UK
⁸ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, R. do Matão 1226, 05508-090 São Paulo, Brazil
⁹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching bei München, Germany
¹⁰ Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Korea
¹¹ Departamento de Física Teórica y del Cosmos, Universidad de Granada, Facultad de Ciencias, 18071 Granada, Spain
¹² Instituto Universitario Carlos I de Física Teórica y Computacional, Universidad de Granada, 18071 Granada, Spain
¹³ Observatorio Astronómico Nacional, C/Alfonso XII 3, Madrid 28014, Spain
¹⁴ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Wien, Austria

Received: 23 September 2020
Accepted: 8 December 2020

Abstract

Inner bars are frequent structures in the local Universe and thought to substantially influence the nuclear regions of disc galaxies. In this study we explore the structure and dynamics of inner bars by deriving maps and radial profiles of their mean stellar population content and comparing them to previous findings in the context of main bars. To this end, we exploit observations obtained with the integral-field spectrograph MUSE of three double-barred galaxies in the TIMER sample. The results indicate that inner bars can be clearly distinguished based on their stellar population properties alone. More precisely, inner bars show significantly elevated metallicities and depleted [α/Fe] abundances. Although they exhibit slightly younger stellar ages compared to the nuclear disc, the typical age differences are small, except at their outer ends. These ends of the inner bars are clearly younger compared to their inner parts, an effect known from main bars as orbital age separation. In particular, the youngest stars (i.e. those with the lowest radial velocity dispersion) seem to occupy the most elongated orbits along the (inner) bar major axis. We speculate that these distinct ends of bars could be connected to the morphological feature of ansae. Radial profiles of metallicity and [α/Fe] enhancements are flat along the inner bar major axis, but show significantly steeper slopes along the minor axis. This radial mixing in the inner bar is also known from main bars and indicates that inner bars significantly affect the radial distribution of stars. In summary, based on maps and radial profiles of the mean stellar population content and in line with previous TIMER results, inner bars appear to be scaled down versions of the main bars seen in galaxies. This suggests the picture of a ‘galaxy within a galaxy’, with inner bars in nuclear discs being dynamically equivalent to main bars in main galaxy discs.