A fork in the Sagittarius trailing debris

Abstract

We take advantage of the deep and wide coverage of the VLT Survey Telescope (VST) ATLAS survey to study the line-of-sight structure of the Sagittarius (Sgr) stellar stream in the Southern hemisphere, only ∼40° away from the progenitor. We use photometrically selected subgiant branch (SGB) stars to reveal a complex debris morphology of the trailing arm and detect at least two clear peaks in the SGB distance modulus distribution. The separation between the two line-of-sight components is at least 5 kpc at the edge of the VST ATLAS footprint, but appears to change along the stream, which allows us to conclude that these detections correspond to two physically independent stellar structures, rather than a mix of co-distant stellar populations within a single stream. Our discovery of a fork in the Sgr trailing arm is verified using blue horizontal branch stars, and our distance measurements are calibrated using RR Lyrae stars from the Catalina Real-Time Transient Survey. Comparing with numerical simulations of the Sgr dwarf disruption, the more distant of the two components in the fork matches perfectly with the track of the trailing debris. However, no obvious counterpart exists in the simulation for the closer line-of-sight component.

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