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

¹ Dipartimento di Fisica e Astronomia “G. Galilei”, Università degli Studi Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
e-mail: michela.rubino@phd.unipd.it
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Instituto de Astronomìa y Ciencias Planetarias, Universidad de Atacama, Copayapu 485, Copiapó, Chile
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁵ INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini, 64100 Teramo, Italy
⁶ ADONI – Laboratorio Nazionale di Ottica Adattiva, Italy
⁷ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK

Received: 2 March 2021
Accepted: 25 June 2021

Abstract

Context. 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.

Aims. 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.

Methods. 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).

Results. 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 2σ feature. The size, shape, and slope of the 2σ peak strongly depend on the velocity separation and relative light contribution of the two counter-rotating stellar disks. When the 2σ peak is difficult to detect due to the low signal-to-noise ratio of the data, the large-scale structure in the h₃ 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 2σ galaxies represents only a lower limit of the stellar counter-rotation phenomenon.

Conclusions. The parameterization with a single GH function does not provide a good description of the LOSVD in the presence of strong counter-rotation. However, using GH parametric solutions is a practical way to reveal the large-scale counter-rotating stellar disks and could be used to detect faint counter-rotating components to improve the statistics of stellar counter-rotation.