A monolithic collapse origin for the thin/thick disc structure of ESO 243-49

TL;DR

This study investigates the origin of the thin and thick disc structures in the galaxy ESO 243-49, suggesting they formed rapidly through a monolithic collapse or early dynamical heating, based on kinematic and stellar population analysis.

Contribution

It provides evidence supporting a monolithic collapse origin for the galaxy's thin and thick discs, challenging models that emphasize external accretion or prolonged formation.

Findings

Thick disc emits ~80% of light at heights >1.6 kpc.

Stars in the thick disc lag by 30-40 km/s in rotation velocity.

Both discs are old and metal-poor, with the thick disc being more metal-poor.

Abstract

ESO 243-49 is a high-mass (circular velocity $v_{\rm c}\approx200\,{\rm km\,s^{-1}}$) edge-on S0 galaxy in the Abell 2877 cluster at a distance of $\sim95\,{\rm Mpc}$. To elucidate the origin of its thick disc, we use MUSE science verification data to study its kinematics and stellar populations. The thick disc emits $\sim80\%$ of the light at heights in excess of $3.5^{\prime\prime}$ ($1.6\,{\rm kpc}$). The rotation velocities of its stars lag by $30-40\,{\rm km\,s^{-1}}$ compared to those in the thin disc, which is compatible with the asymmetric drift. The thick disc is found to be more metal-poor than the thin disc, but both discs have old ages. We suggest an internal origin for the thick disc stars in high-mass galaxies. We propose that the thick disc formed either ${\rm a)}$ first in a turbulent phase with a high star formation rate and that a thin disc formed shortly afterwards, or ${\rm b)}$ because of the dynamical heating of a thin pre-existing component. Either way, the star formation in ESO 243-49 was quenched just a few Gyrs after the galaxy was born and the formation of a thin and a thick disc must have occurred before the galaxy stopped forming stars. The formation of the discs was so fast that it could be described as a monolithic collapse where several generations of stars formed in a rapid succession.