Precession of the Sagittarius stream

Abstract

Using a variety of stellar tracers -- blue horizontal branch stars,\nmain-sequence turn-off stars and red giants -- we follow the path of the\nSagittarius (Sgr) stream across the sky in Sloan Digital Sky Survey data. Our\nstudy presents new Sgr debris detections, accurate distances and line-of-sight\nvelocities that together help to shed new light on the puzzle of the Sgr tails.\nFor both the leading and the trailing tail, we trace the points of their\nmaximal extent, or apo-centric distances, and find that they lie at $R^L$ =\n47.8 $\\pm$ 0.5 kpc and $R^T$ = 102.5 $\\pm$ 2.5 kpc respectively. The angular\ndifference between the apo-centres is 93.2 $\\pm$ 3.5 deg, which is smaller than\npredicted for logarithmic haloes. Such differential orbital precession can be\nmade consistent with models of the Milky Way in which the dark matter density\nfalls more quickly with radius. However, currently, no existing Sgr disruption\nsimulation can explain the entirety of the observational data. Based on its\nposition and radial velocity, we show that the unusually large globular cluster\nNGC 2419 can be associated with the Sgr trailing stream. We measure the\nprecession of the orbital plane of the Sgr debris in the Milky Way potential\nand show that, surprisingly, Sgr debris in the primary (brighter) tails evolves\ndifferently to the secondary (fainter) tails, both in the North and the South.\n