Detectability of large-scale counter-rotating stellar disks in galaxies with integral-field spectroscopy

TL;DR

This study investigates the detectability of large-scale counter-rotating stellar disks in galaxies using mock integral-field spectroscopic data, focusing on the kinematic signatures and detection limits influenced by galaxy properties and observational setups.

Contribution

The paper provides a detailed analysis of the kinematic signatures of counter-rotating disks and establishes detection thresholds using mock MUSE data, enhancing understanding of their observational identification.

Findings

The symmetric double peak in velocity dispersion maps is the strongest signature of counter-rotation.

The size and shape of the 2σ peak depend on velocity separation and light contribution.

The $h_3$ map can diagnose counter-rotation when the 2σ peak is weak or undetectable.

Abstract

In recent years integral-field spectroscopic surveys have revealed that the presence of kinematically decoupled stellar components is not a rare phenomenon in nearby galaxies. However, complete statistics are still lacking because they depend on the detection limit of these objects. We investigate the kinematic signatures of two large-scale counter-rotating stellar disks in mock integral-field spectroscopic data to address their detection limits as a function of the galaxy properties and instrumental setup. We built a set of mock data of two large-scale counter-rotating stellar disks as if they were observed with the Multi-Unit Spectroscopic Explorer (MUSE). We accounted for different photometric, kinematic, and stellar population properties of the two counter-rotating components as a function of galaxy inclination. We extracted the stellar kinematics in the wavelength region of the calcium triplet absorption lines by adopting a Gauss-Hermite (GH) parameterization of the line-of-sight velocity distribution (LOSVD). We confirm that the strongest signature of the presence of two counter-rotating stellar disks is the symmetric double peak in the velocity dispersion map, already known as the $2\sigma$ feature. The size, shape, and slope of the 2$\sigma$ peak strongly depend on the velocity separation and relative light contribution of the two counter-rotating stellar disks. When the $2\sigma$ peak is difficult to detect due to the low signal-to-noise ratio of the data, the large-scale structure in the $h_3$ map can be used as a diagnostic for strong and weak counter-rotation. The counter-rotating kinematic signatures become fainter at lower viewing angles as an effect of the smaller projected velocity separation between the two counter-rotating components. We confirm that the observed frequency of $2\sigma$ galaxies represents only a lower limit of the stellar counter-rotation phenomenon.