Surprise! The “Small Magellanic Cloud” Is Actually Two Superimposed Galaxies

The structure of the Small Magellanic Cloud (SMC) has long puzzled astronomers. The irregular satellite galaxy of the Milky Way is assumed to be the remnant of a larger galaxy cannibalised by the Milky Way when it fell into the latter’s gravitational sphere of influence, but the structure of the SMC defied modeling of the gravitational dynamics of such an object. Now, precision measurements of the motion of stars and gas have revealed that what we’re seeing when looking at the SMC is actually two smaller, independent galaxies with similar mass superimposed on the sky, with around 5 kiloparsecs of separation between them along the line of sight.

The research paper is, “A Galactic Eclipse: The Small Magellanic Cloud is Forming Stars in Two, Superimposed Systems”. Here is the 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.