A fork in the Sagittarius trailing debris

TL;DR

This study reveals a complex, bifurcated structure in the Sagittarius stellar stream's trailing arm using deep photometric data, identifying two distinct stellar components with different distances and origins.

Contribution

It provides the first observational evidence of a fork in the Sagittarius trailing debris, distinguishing two separate stellar structures along the stream's line of sight.

Findings

Detected at least two peaks in the SGB distance modulus distribution.

The more distant component aligns with the predicted trailing debris in simulations.

The closer component has no clear counterpart in existing models.

Abstract

We take advantage of the deep and wide coverage of the VST ATLAS survey to study the line-of-sight structure of the Sagittarius stellar stream in the Southern hemisphere, only ~40{\deg} away from the progenitor. We use photometrically selected Sub-Giant 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.