Internal kinematics and stellar populations for the largest sample to date of galaxies with counter-rotating stellar disks

SAURON and ATLAS3D spectroscopic surveys provided a new classification of Early-Type Galaxies, based on their stellar kinematic maps. In this work we focus on the class of galaxies with two counter-rotating stellar disks, so far called ‘’2sigma-galaxies’’ because of the two peaks in the velocity dispersion map. These galaxies show different observed properties, and to date we do not have a clear picture of how they form; this is mostly because few counter-rotators have been found to date. The first goal of this work was to use data from the integral-field survey MaNGA, which observed ~10,000 galaxies, to build a large sample of such galaxies. Since in this survey the kinematic maps do not always show both the counter-rotation and the two peaks in sigma, we checked for at least one of these features to select counter-rotators candidates, and we designate them as ‘CRD’. To identify CRDs, we first selected a set of 603 galaxies from MaNGA, and, by visually inspecting the kinematic maps of the Pipeline, applying specific classification criteria, we obtained a final sample of 53 CRDs, the largest to date for such galaxies. The second goal was to produce the stellar velocity and dispersion maps for the sample; by fitting two components, we found spectroscopically confirmation of the two disks in about 1/3 of the sample. We then compared the velocity fields of the ionized gas and of the stars, and mostly found cases with gas corotating one of the two stellar disks, but also many cases of misalignment, suggesting recent gas accretion. Finally, we extracted the stellar population maps, looking for radial trends: almost all galaxies have higher metallicity in the central regions, and some also show radial trends in age. Evidences of multimodality in age and metallicity have been found, by performing regularized fits, in 13 galaxies, suggesting the presence of different stellar populations. Overall, our results support different formation paths for the class of CRDs.