The kinematics of young and old stellar populations in nuclear rings of MUSE TIMER galaxies

Rosado-Belza, D.; Falcón-Barroso, J.; Knapen, J. H.; Bittner, A.; Gadotti, D. A.; Neumann, J.; de Lorenzo-Cáceres, A.; Méndez-Abreu, J.; Querejeta, M.; Martín-Navarro, I.; Sánchez-Blázquez, P.; Coelho, P. R. T.; Martig, M.; van de Ven, G.; Kim, T.
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Astronomy and Astrophysics

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Context. Studying the stellar kinematics of galaxies is a key tool in the reconstruction of their evolution. However, the current measurements of the stellar kinematics are complicated by several factors, including dust extinction and the presence of multiple stellar populations.
Aims: We use integral field spectroscopic data of four galaxies from the Time Inference with MUSE in Extragalactic Rings (TIMER) survey to explore and compare the kinematics measured in different spectral regions that are sensitive to distinct stellar populations.
Methods: We derive the line-of-sight velocity and velocity dispersion of both a young (≲2 Gyr) and an old stellar population from the spectral regions around the Hβ line and the Ca II Triplet. In addition, we determine colour excess, mean age, and metallicity.
Results: We report a correlation of the colour excess with the difference in the kinematic parameters of the Hβ line and the Ca II Triplet range, which are dominated by young and old stellar populations, respectively. Young stellar populations, located primarily in nuclear rings, have higher velocity dispersions than old ones. These differences in the rings are typically ∼10 km s-1 in velocity dispersion but can have a mean value as high as ∼24 km s-1 in the most extreme case. Trends with age exist in the nuclear rings but are less significant than those with dust extinction. We report different degrees of correlation for these trends among the galaxies in the sample, which are related to the size of the Voronoi bins in their rings. No clear trends for the line-of-sight velocity differences are observed. The absence of these trends can be explained as a consequence of the Hβ line masking process during the kinematic extraction, as confirmed by dedicated simulations.
Conclusions: Our study demonstrates that kinematic differences caused by different stellar populations can be identified in the central regions of nearby galaxies, even from intermediate resolution spectroscopy. This opens the door to future detailed chemo-kinematic studies of galaxies, but also serves as a warning against deriving kinematics from full-spectrum fitting across very wide wavelength ranges when intense star formation is taking place.
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