The Carina Project. X. On the kinematics of old and intermediate-age stellar populations

TL;DR

This study provides detailed radial velocity measurements of stellar populations in the Carina dwarf galaxy, revealing its rotational properties and supporting its history as a disky dwarf galaxy shaped by tidal interactions with the Milky Way.

Contribution

It offers new RV data for faint stars in Carina and compares observed kinematics with N-body simulations, demonstrating the galaxy's transformation from a disky to a spheroidal shape.

Findings

Intermediate-age stars show a clear rotational pattern.

Old stars exhibit larger velocity dispersion without rotation.

Observed kinematics agree with simulations of a tidally disturbed disky dwarf galaxy.

Abstract

We present new radial velocity (RV) measurements of old (horizontal branch) and intermediate-age (red clump) stellar tracers in the Carina dwarf spheroidal. They are based on more than 2,200 low-resolution spectra collected with VIMOS at VLT. The targets are faint (20<V<21.5 mag), but the accuracy at the faintest limit is <9 kms-1. These data were complemented with RV measurements either based on spectra collected with FORS2 and FLAMES/GIRAFFE at VLT or available in the literature. We ended up with a sample of 2748 stars and among them 1389 are candidate Carina stars. We found that the intermediate-age stellar component shows a well defined rotational pattern around the minor axis. The western and the eastern side of the galaxy differ by +5 and -4 km s-1 when compared with the main RV peak. The old stellar component is characterized by a larger RV dispersion and does not show evidence of RV pattern. We compared the observed RV distribution with N-body simulations for a former disky dwarf galaxy orbiting a giant MilkyWay-like galaxy (Lokas et al. 2015). We rotated the simulated galaxy by 60 degrees with respect to the major axis, we kept the observer on orbital plane of the dwarf and extracted a sample of stars similar to the observed one. Observed and predicted Vrot/{\sigma} ratios across the central regions are in remarkable agreement. This evidence indicates that Carina was a disky dwarf galaxy that experienced several strong tidal interactions with the Milky Way. Owing to these interactions, Carina transformed from a disky to a prolate spheroid and the rotational velocity transformed into random motions.