Detection of a 50° long Trailing Tidal Tail for the Globular Cluster M5

Abstract

Abstract Using photometry and proper motions from Gaia Data Release 2, we detect a 50° long stream of about 70 stars extending westward from the halo globular cluster M5. Based on the similarities in distance, proper motions, inferred color–magnitude distribution, and trajectory, we identify this stream as the trailing tidal tail of M5. While the surface density of stars is very low (≃1.5 star per square degree, or ≈35 mag per square arcsecond), selecting only stars with proper motions consistent with the orbit of the cluster yields a detection significance of ≈10σ. While we find a possible continuation of the stream to ≈85°, increasing foreground contamination combined with a greater predicted stream distance makes it difficult to detect with current data even if the stream continues unabated. The nonuniform distribution of stars in the stream appears to be consistent with episodic tidal stripping, with the most recently shed stars now trailing the cluster by tens of degrees. We provide a table of the highest-ranked candidate stream stars for ongoing and future spectroscopic surveys.