Evidence for the formation of the young counter-rotating stellar disk from gas acquired by IC 719

TL;DR

This study uses detailed spectroscopic analysis to investigate the formation of counter-rotating stellar disks in galaxy IC 719, providing evidence supporting gas accretion as the primary formation mechanism.

Contribution

It offers a detailed 2D spectroscopic analysis of IC 719's counter-rotating disks, highlighting the role of gas accretion over mergers in their formation.

Findings

The main stellar disk is older, hotter, and thicker than the secondary disk.

The secondary stellar disk shares kinematics and spatial distribution with ionized gas.

Evidence favors gas accretion over binary mergers as the origin of counter-rotation.

Abstract

The formation scenario of extended counter-rotating stellar disks in galaxies is still debated. In this paper, we study the S0 galaxy IC 719 known to host two large-scale counter-rotating stellar disks in order to investigate their formation mechanism. We exploit the large field of view and wavelength coverage of the Multi Unit Spectroscopic Explorer (MUSE) spectrograph to derive two-dimensional (2D) maps of the various properties of the counter-rotating stellar disks, such as age, metallicity, kinematics, spatial distribution, the kinematical and chemical properties of the ionized gas, and the dust map. Due to the large wavelength range, and in particular to the presence of the Calcium Triplet 8498, 8542, 8662\AA (CaT hereafter), the spectroscopic analysis allows us to separate the two stellar components in great detail. This permits precise measurement of both the velocity and velocity dispersion of the two components as well as their spatial distribution. We derived a 2D map of the age and metallicity of the two stellar components as well as the star formation rate and gas-phase metallicity from the ionized gas emission maps. The main stellar disk of the galaxy is kinematically hotter, older, thicker and with larger scale-length than the secondary disk. There is no doubt that the latter is strongly linked to the ionized gas component: they have the same kinematics and similar vertical and radial spatial distribution. This result is in favor of a gas accretion scenario over a binary merger scenario to explain the origin of counter-rotation in IC 719. One source of gas that may have contributed to the accretion process is the cloud that surrounds IC 719.