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

Abstract

ESO 243-49 is a high-mass (circular velocity $v_{\mathrm{c}}\approx 200\mathrm{km}{\mathrm{s}}^{-1}$) edge-on S0 galaxy in the Abell 2877 cluster at a distance of $\sim 95\mathrm{Mpc}$. 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 $\sim 80\%$ of the light at heights in excess of $3.5^{\prime \prime }$ ($1.6\mathrm{kpc}$). The rotation velocities of its stars lag by $30-40\mathrm{km}{\mathrm{s}}^{-1}$ 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 $\mathrm{a})$ first in a turbulent phase with a high star formation rate and that a thin disc formed shortly afterwards, or $\mathrm{b})$ 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.

Accepted for publication in A&A. The reduced data-cube as well as the data necessary to build the kinematic and stellar population maps are available at https://etsin.avointiede.fi/dataset/urn-nbn-fi-csc-kata20160924142911632378

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