Substructure of the outer Galactic halo from the 2-Micron All-Sky Survey

A pole-count analysis of the infrared 2-Micron All-Sky (2MASS) survey is presented, in order to identify faint stream-like great-circle structures within the halo of the Milky Way. Selecting stars with colours consistent with M-giant stars, we find a strong overdensity of sources on a stream with a pole (l=95°, b=13°), which corresponds to the pole of the orbit of the Sagittarius dwarf galaxy. This great-circle feature of width ∼12° contains ∼5 per cent of the late M-giants in the halo. No other stream-like structures are detected in M-giants in the 2MASS Second Incremental Data Release (2IDR), and in particular, we find no evidence for a stellar component to the Magellanic Stream. This suggests that the present accretion rate of low-mass satellites with a luminous component is very low, and the formation of the luminous component of the halo must have been essentially complete before the accretion of the Sagittarius dwarf galaxy, more than 3 Gyr ago. We also search for great-circle streams using almost all high-latitude (|b|>30°) sources in the 2IDR data set. No narrow great-circle streams of width 0.5°–2° were found, though we were only sensitive to relatively nearby (<17-kpc) remnants of massive (106-M⊙) globular clusters. If the Galactic potential is close to being spherical, as some recent observations suggest, the lack of observed great-circle streams is consistent with the presence of dark matter substructures in the halo. Although alternative explanations cannot be ruled out from our analysis of the 2IDR data set, future experiments with better statistics have the potential to reveal the heating effect of dark matter substructure on stellar streams.