# The kinematics of local thick discs do not support an accretion origin

## TL;DR

This study uses kinematic data from nearby edge-on galaxies to assess whether satellite accretion is the primary formation mechanism for thick discs, finding little evidence of retrograde material that would support accretion.

## Contribution

The paper provides observational evidence against satellite accretion as the main formation process for thick discs by analyzing their kinematics and retrograde fractions.

## Key findings

- Most thick discs show no strong retrograde signatures.
- The observed retrograde fraction is significantly lower than predicted by accretion models.
- Satellite accretion is unlikely to be the dominant formation mechanism for thick discs.

## Abstract

Thick discs are nearly ubiquitous components of the discs of present-day galaxies. It has been proposed that a fraction of their stars has been accreted. Here, we aim to find whether accretion of satellites is the main thick disc formation mechanism. To do so, we observed a sample of eight nearby edge-on galaxies with the MUSE integral field unit at the VLT. Six of the galaxies have a distinct thick disc. We derived thick disc velocities and velocity dispersions for the galaxies in our sample. We devise a formalism to estimate the fractions of retrograde material in the thick discs by using kinematical maps and thin/thick dis decompositions. None of the galaxies in our sample shows strong evidence for retrograde material at large distances from the centre. Including those found in the literature, there are seventeen thick discs with studied kinematics, with only one showing unambiguous signatures of retrograde material. Literature numerical studies of dynamical friction allow us to estimate that at the current cosmic time about one in six mergers for which the stars of the accreted galaxy ended in a thick disc were retrograde. This is in tension with the observed fraction of 1/17 of galaxies with a partly retrograde thick disc. We conclude that satellite accretion is not favoured by observations to be the main thick disk formation mechanism.