Discovery of a tidal extension of the Sagittarius dwarf spheroidal galaxy

We report the discovery of stars associated with the Sagittarius dwarf spheroidal galaxy (Sgr) located nearly 10 degrees from the center of the main body of the galaxy, far beyond the outer boundary defined by the star counts of Ibata and coworkers. The newly discovered stars in Sgr include three RR Lyrae stars found serendipitously behind the Galactic globular cluster M55, red horizontal branch stars, and main-sequence stars identified both in the M55 field and in another field located away from this cluster but still about 10 degrees from Sgr. The photometric properties of all of these stars are perfectly consistent with the previously determined distance, metallicity, and age of the Sgr galaxy. Our results indicate that Sgr is much larger than previously believed and that the stellar distribution in the outer parts of the galaxy is probably quite clumpy. We discuss the implications of our findings on the total luminosity and mass of Sgr and on the characteristics of its most recent past perigalacticon passages.