Progress of Detections of Stellar Tidal Streams around the Galaxy

Many stellar tidal streams in the outer halo of the Galaxy have been found since 1994. Some streams were formed from dwarf satellites of the Galaxy and others were from globular clusters. The streams of these two types of origin are usually called dwarf streams and tidal tails of clusters, respectively. Observational and theoretical studies of stellar tidal streams have been steadily accumulated throughout the past 20 years or so. Stellar tidal streams can be identified by using different luminous tracers, such as RR Layaes, red clump stars, blue horizontal-branch stars, main-sequence turnoff stars, M giant stars, and red giant branch stars. These stars are observable even if they are rather far away from the Sun and can be used to determine the heliocentric distances of streams. Various approaches can be used to analyze parameter distributions of these tracers to detect stellar tidal streams. The parameter distributions include distributions of positions, velocities, and Color-Magnitude values. Among the many streams discovered the Sagittarius (Sgr) stream is most spectacular. Since its discovery the Sgr dwarf spheroid galaxy (dSph) has been considered the illustrative example of the assimilation of a small galaxy into a larger one through tidal destruction. In 2003 two tidal sub-streams of the Sgr stream were discovered; one of them is leading the main body of the dSph galaxy and the other is trailing. A few years later, a bifurcation of the leading sub-stream (into branches A and B) starting at about the North Galactic Pole was identified. Recently, a similar bifurcation of the trailing sub-stream in the Southern Galactic Hemisphere was also identified. Since mid-1990s it has been known that most of the Galactic globular clusters show leading or trailing tidal tails out of their main bodies. Among these, tidal tails of the Palomar 5 (Pal 5) are the most closely studied. It has been known over a decade that the Pal 5 has two tails symmetrically stretching out from the main body with a total angular extension of at least 2.6°. Subsequent observations show that the extension of the tails can be even more than known previously, reaching up to 22.5° according to 2006 data. Studies of stellar tidal streams are important for several reasons. First, they yield details about tidal destruction and galaxy merging. Second, they may reveal interesting aspects of galaxy evolution in the presence of strong tidal forces. Third, tidal streams can be used to effectively probe the gravitational potential of the Galaxy (including the dark halo). On the last aspect, however, observations and models of the disruption of the Sgr stream have yielded ambiguous results, which are affected by different choices of data, analysis approaches, and the shape of the Galactic halo (which has been modeled to be oblate, protracted, spherical, or even triaxial).