Among dwarf galaxies, the elliptical type is the most common type in the local universe. There is a general consensus that galaxies evolved to their present state through the hierarchical merging process. One of the key building blocks of that process is the group of dwarf galaxies which are thought to have added to the primordial galaxy through repeated mergers to achieve the structure in galaxies we see today. The Sagittarius dwarf galaxy represents a subclass of dwarfs known as nucleated dwarf elliptical galaxies. These galaxies possess a compact bright central region reminiscent of the nucleus of an elliptical galaxy or globular cluster. Curiously the origin of nucleated dwarf elliptical galaxies is believed to have occurred by one of two mechanisms; either the decay of the orbit of a pre-existing globular cluster or the in situ formation of a giant cluster.
The Sagittarius dwarf lies directly behind the central bulge of the Milky Way so it cannot be seen directly in images. It was detected serendipitously based on velocity measurements of stars. Its main body extends some 20 degrees of sky making it the largest apparent structure in the sky after the Milky Way itself. Its diameter is about 28,000 light years although its mass is only a thousandth of the Milky Way.
The accompanying image shows the globular cluster M56. M56 is associated with one of two bright knots in the center of the dwarf galaxy. The dwarf galaxy is so large and extended that it represents a barely perceptible increased stellar concentration from the sky foreground. The Sagittarius Dwarf Elliptical galaxy was discovered in 1994 and is believed to be a satellite galaxy currently being cannibalized by the Milky Way. The dwarf galaxy is the closest of about a dozen satellite galaxies and has an orbital period of about 700 million years. During each of its 10 previous orbits the dwarf has been stripped of mass leaving it with only a fraction of the mass and stars it once contained. The disruption of the dwarf galaxy continues today in the form of a gradual accretion and assimilation of the dwarf into the halo of the Milky Way. The tidal disruption is most evident in an enormous coherent stream of stars torn from the main body of the galaxy that extends over 150 degrees across the south galactic hemisphere. Known as the Sagittarius Stream, the stars have been tidally stripped by the Milky Way and stretch across the galactic halo at a distance ranging from 60,000 to 130,000 light years from the sun. The Sagittarius dwarf is a gasless galaxy similar to other elliptical galaxies which have exhausted their neutral hydrogen fuel and no longer make stars. Apparently the last vestige of gas was stripped from the Sagittarius dwarf along with its stars during the last 200 million years. Some neutral hydrogen has been detected within the Sagittarius Stream which likely represents the dwarf's last source of star forming fuel.
The Sagittarius system gives astronomers an unprecedented opportunity to study the mechanism of galactic mergers and also to estimate the mass and shape of the Milky Way halo. The Milky Way halo contains numerous streams of stellar debris which have contributed to its formation. The nature of the Sagittarius stream has enabled astronomers to estimate the Milky Way halo mass including its dark matter content necessary to produce the disruption.
The Sagittarius dwarf has had an impact on the structure and star forming history of the Milky Way. Astronomers estimate that as much as 10% of the Milky Way halo stars may have their origin in the dwarf galaxy, being captured during the last previous 8 billion years as it orbited our galaxy. The outer disk of the Milky Way is noted to have a well defined warp in the configuration of an integral sign. Warping is a common phenomenon among spiral galaxies and is believed to occur through tidal encounters with smaller galactic systems. The Sagittarius dwarf is believed to have passed through the outer disk of the Milky Way during the last 700 million years and the warp is most likely the outcome of this encounter. Our galaxy has most likely captured at least one globular cluster from the Sagittarius Dwarf, Palomar 12. Another globular cluster, M54 coincides with one of two bright knots within the dwarf galaxy and is in all likelihood an extragalactic cluster. In fact it may represent the surviving nucleus of the Sagittarius dwarf. Whether a true extragalactic globular cluster or the former nucleus of the Sagittarius dwarf, M54 will ultimately be captured and incorporated into the Milky Way halo.
One interesting theory about the formation of the Sagittarius dwarf has it arising from debris pulled from the Large Magellanic Cloud following a collision of the Magellanic clouds with each other or the Milky Way.