Evidence of Fanning in the Ophiuchus Stream

Branimir Sesar, Adrian M. Price-Whelan, Judith G. Cohen, Hans-Walter Rix, Sarah Pearson, Kathryn V. Johnston, Edouard J. Bernard, Annette M. N. Ferguson, Nicolas F. Martin, Colin T. Slater, Kenneth C. Chambers, Heather Flewelling, Richard J. Wainscoat, Christopher Waters

Research output: Contribution to journalArticlepeer-review


The Ophiuchus stellar stream presents a dynamical puzzle: its oldstellar populations (˜12 Gyr) cannot be reconciled with (1) itsorbit in a simple model for the Milky Way potential and (2) its shortangular extent, both of which imply that the observed stream formedwithin the last <1 {{Gyr}}. Recent theoretical work has shown thatstreams on chaotic orbits may abruptly fan out near their apparent ends;stars in these fans are dispersed in both position and velocity and maybe difficult to associate with the stream. Here we present the firstevidence of such stream-fanning in the Ophiuchus stream, traced by fourblue horizontal branch stars beyond the apparent end of the stream.These stars stand out from the background by their high velocities({v}{{los}}> 230 km s-1) against ˜40other stars: their velocities are comparable to those of the stream, butwould be exceptional if they were unrelated halo stars. Their positionsand velocities are, however, inconsistent with simple extrapolation ofthe observed cold, high-density portion of the stream. Theseobservations suggest that stream-fanning may be a real, observableeffect and, therefore, that Ophiuchus may be on a chaotic orbit. Theyalso show that the Ophiuchus stream is more extended and hencedynamically older than previously thought, easing the stellar populationversus dynamical age tension.
Original languageEnglish
JournalAstrophysical Journal Letters
Issue number1
Publication statusPublished - 1 Jan 2016


  • Galaxy: halo
  • Galaxy: kinematics and dynamics
  • Galaxy: structure
  • globular clusters: general


Dive into the research topics of 'Evidence of Fanning in the Ophiuchus Stream'. Together they form a unique fingerprint.

Cite this