Which dwarf galaxy is currently merging with the Milky Way?

The dwarf galaxy currently merging with the Milky Way is the Sagittarius Dwarf Spheroidal Galaxy. Often shortened to Sagittarius dSph or Sgr dSph, it is a small satellite galaxy being pulled apart by the Milky Way’s gravity. Rather than remaining a compact system, many of its stars have been stretched into long tidal streams around our galaxy.

Astronomers discovered Sagittarius dSph in 1994, although its position behind dense regions of the Milky Way made it difficult to detect. It follows a steep, nearly polar orbit and has passed through the Milky Way’s disk several times. Gaia observations showed that earlier passages disturbed the motions of stars in the Milky Way, producing large-scale ripples in the galactic disk.

The globular cluster Messier 54 is believed to sit near the dwarf galaxy’s core. Sagittarius dSph is not the Large Magellanic Cloud, which is a much larger satellite galaxy, and the Canis Major Dwarf’s status as a separate galaxy remains disputed. The merger is expected to continue for hundreds of millions of years or longer as the dwarf’s remaining structure dissolves.