Meet the Neighbours The Sagittarius Dwarf Spheroidal Family

WE’VE KNOWN FOR A LONG TIME that big galaxies grow by cannibalising little galaxies, and their eating habits are quite messy. As they feast, they fling their scraps across the sky, forming immense stellar streams. In 1994, the team of Rodrigo Ibata, Mike Irwin and Gerry Gilmore (Institute of Astronomy at Cambridge University in the UK) discovered a vivid example of life at the galactic dinner table. During a spectroscopic study of the Milky Way’s central bulge, they noticed a group of stars moving together at a uniform velocity in a field of random motions. They had stumbled across our nearest neighbour, the Sagittarius Dwarf Spheroidal Galaxy.

The Milky Way’s strong gravitational grip is a relentless force of demolition. The shredded debris of the dwarf spheroidal is strung across the sky in two tidal tails that form a prominent substructure in the galactic halo called the Sagittarius Stream. Sloan Digital Sky Survey (SDSS) photometry first revealed this feature in 2002, and a study of M-giant halo stars in the Two Micron All-Sky Survey (2MASS) catalogue confirmed it. The leading and trailing tails of the stream wrap over 360° in looping rosettes around the Milky Way in a nearly vertical polar orbit, as shown in the diagram below. Astronomers refer to the primary wraps as L (leading) and T (trailing), while M represents the remnant core and its associated objects.

The Milky Way has grown via mergers with several dwarf galaxies, accreting stars, dark matter and globular clusters in the process. Each dwarf stellar population has a unique chemical signature, helping astronomers trace their streams across the sky. After the Sagittarius Dwarf Spheroidal Galaxy stream, the largest are the Gaia-Enceladus, Sequoia, Koala