# Kinematic and stellar population properties of the counter-rotating components in the S0 galaxy NGC 1366

## TL;DR

This study disentangles and analyzes the properties of counter-rotating stellar components in galaxy NGC 1366, revealing their ages, metallicities, and formation stage, which sheds light on galaxy evolution processes.

## Contribution

It provides the first spectroscopic separation and detailed stellar population analysis of counter-rotating components in NGC 1366, highlighting their different properties and formation stage.

## Key findings

- Counter-rotating component is younger and less α/Fe enhanced.
- Ionized gas has a disordered distribution and disturbed kinematics.
- Galaxy is in an intermediate stage of acquiring counter-rotating components.

## Abstract

Context. Many disk galaxies host two extended stellar components that rotate in opposite directions. The analysis of the stellar populations of the counter-rotating components provides constraints on the environmental and internal processes that drive their formation. Aims. The S0 NGC 1366 in the Fornax cluster is known to host a stellar component that is kinematically decoupled from the main body of the galaxy. Here we successfully separated the two counter-rotating stellar components to independently measure the kinematics and properties of their stellar populations. Methods. We performed a spectroscopic decomposition of the spectrum obtained along the galaxy major axis and separated the relative contribution of the two counter-rotating stellar components and of the ionized-gas component. We measured the line-strength indices of the two counter-rotating stellar components and modeled each of them with single stellar population models that account for the {\alpha}/Fe overabundance. Results. We found that the counter-rotating stellar component is younger, has nearly the same metallicity, and is less {\alpha}/Fe enhanced than the corotating component. Unlike most of the counter-rotating galaxies, the ionized gas detected in NGC 1366 is neither associated with the counter-rotating stellar component nor with the main galaxy body. On the contrary, it has a disordered distribution and a disturbed kinematics with multiple velocity components observed along the minor axis of the galaxy. Conclusions. The different properties of the counter-rotating stellar components and the kinematic peculiarities of the ionized gas suggest that NGC 1366 is at an intermediate stage of the acquisition process, building the counter-rotating components with some gas clouds still falling onto the galaxy.