Sagittarius Dwarf Spheroidal Galaxy

Lore & Background

Sgr dSph appears to be an older galaxy with little interstellar dust, composed largely of Population II stars, older and metal-poor, compared to the Milky Way. No neutral hydrogen gas related to Sgr dSph has been found. It has multiple stellar populations, ranging in age from the oldest globular clusters (almost as old as the universe itself) to trace populations as young as several hundred million years. It exhibits an age-metallicity relationship: its old populations are metal poor ([Fe/H] = −1.6 ± 0.1) while its youngest populations have super-solar abundances.

Further discoveries by astrophysics teams from the University of Virginia and the University of Massachusetts Amherst, drawing upon the 2MASS Two-Micron All Sky Infrared Survey data, revealed the entire loop-shaped structure. In 2003, with the aid of infrared telescopes and supercomputers, Steven Majewski, Michael Skrutskie, and Martin Weinberg helped create a new star map, picking out the full Sagittarius Dwarf presence, position, and looping shape from the mass of background stars, finding this smaller galaxy to be at a near right angle to the plane of the Milky Way.

Sgr dSph has at least nine known globular clusters. One, M54, appears to reside at its core, while three others reside within the main body of the galaxy: Terzan 7, Terzan 8, and Arp 2. Additionally, Palomar 12, Whiting 1, NGC 2419, NGC 4147, and NGC 5634 are found within its extended stellar streams. An analysis of VVV and Gaia EDR3 data has found at least twenty more globular clusters, which tend to be more metal-rich than previously known ones.

Reader's Guide

The Sagittarius Dwarf Spheroidal Galaxy is significant as one of the closest companion galaxies to the Milky Way, though its main parent cluster is on the opposite side of the Galactic Center from Earth, making it very faint despite covering a large area of the sky. Its looping, spiraling path has passed through the plane of the Milky Way several times in the past. Based on its current trajectory, the main cluster is about to pass through the galactic disc within the next hundred million years, and the extended loop-shaped ellipse is already extended around and through our local space and on through the Milky Way galactic disc, in the process of slowly being absorbed into the larger galaxy, calculated at 10,000 times the mass of Sgr dSph. The dissipation of the main cluster and its merger with the Milky Way stream is expected to be complete within a billion years. In 2018, the Gaia project showed that Sgr dSph had caused perturbations in a set of stars near the Milky Way's core, causing unexpected rippling movements triggered when it moved through the Milky Way between 300 and 900 million years ago. A 2020 study concluded that collisions between Sgr dSph and the Milky Way triggered major episodes of star formation in the latter. Some astronomers contend that Sgr dSph has been in orbit for some billions of years and has already orbited the Milky Way approximately ten times; its ability to retain coherence despite tidal strains suggests an unusually high concentration of dark matter within that galaxy.

Did You Know?

• Sgr dSph contains four globular clusters in its main body, with the brightest, NGC 6715 (M54), known well before the galaxy itself was discovered in 1994.
• It travels in a polar orbit at a distance of about 50,000 light-years from the Milky Way's core, roughly one third the distance of the Large Magellanic Cloud.
• In 2018, the Gaia project showed that Sgr dSph caused rippling movements in stars near the Milky Way's core when it passed between 300 and 900 million years ago.
• A 2019 study found evidence for two distinct populations in alpha abundances as a function of metallicity in Sgr dSph.