A Galactic Eclipse: The Small Magellanic Cloud is Forming Stars in Two, Superimposed Systems

TL;DR

This study reveals that the Small Magellanic Cloud consists of two superimposed, star-forming systems with distinct compositions, separated by about 5 kpc, based on kinematic and ISM structure analysis.

Contribution

The paper introduces a new model of the SMC showing it as two superimposed star-forming systems with different chemical properties, supported by kinematic and ISM observations.

Findings

SMC is composed of two structures with distinct chemical compositions.

The two systems are separated by approximately 5 kpc along the line of sight.

A simple model reproduces the observed superimposed structures.

Abstract

The structure and dynamics of the star-forming disk of the Small Magellanic Cloud (SMC) have long confounded us. The SMC is widely used as a prototype for galactic physics at low metallicity, and yet we fundamentally lack an understanding of the structure of its interstellar medium (ISM). In this work, we present a new model for the SMC by comparing the kinematics of young, massive stars with the structure of the ISM traced by high-resolution observations of neutral atomic hydrogen (HI) from the Galactic Australian Square Kilometer Array Pathfinder survey (GASKAP-HI). Specifically, we identify thousands of young, massive stars with precise radial velocity constraints from the Gaia and APOGEE surveys and match these stars to the ISM structures in which they likely formed. By comparing the average dust extinction towards these stars, we find evidence that the SMC is composed of two structures with distinct stellar and gaseous chemical compositions. We construct a simple model that successfully reproduces the observations and shows that the ISM of the SMC is arranged into two, superimposed, star-forming systems with similar gas mass separated by ~5 kpc along the line of sight.