Galaxies within galaxies in the TIMER survey: stellar populations of
  inner bars are scaled replicas of main bars
  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 distinguished based on their stellar population properties alone.
More precisely, inner bars show elevated metallicities and depleted
[$\alpha$/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
[$\alpha$/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.
