Evidence of Fanning in the Ophiuchus Stream

Abstract

The Ophiuchus stellar stream presents a dynamical puzzle: its old stellar populations ($\sim 12$ Gyr) cannot be reconciled with (1) its orbit in a simple model for the Milky Way potential and (2) its short angular extent, both of which imply that the observed stream formed within the last $<1$ Gyr. Recent theoretical work has shown that streams on chaotic orbits may abruptly fan out near their apparent ends; stars in these fans are dispersed in both position and velocity and may be difficult to associate with the stream. Here we present the first evidence of such stream-fanning in the Ophiuchus stream, traced by four blue horizontal branch (BHB) stars beyond the apparent end of the stream. These stars stand out from the background by their high velocities ($v_{\mathrm{los}}>230$ km s${}^{-1}$) against $\sim 40$ other stars: their velocities are comparable to those of the stream, but would be exceptional if they were unrelated halo stars. Their positions and velocities are, however, inconsistent with simple extrapolation of the observed cold, high-density portion of the stream. These observations suggest that stream-fanning may be a real, observable effect and, therefore, that Ophiuchus may be on a chaotic orbit. They also show that the Ophiuchus stream is more extended and hence dynamically older than previously thought, easing the stellar population vs. dynamical age tension.

Accepted to ApJ Letters. This version corrects minor typos and references to bring it inline with the published version

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Cited by in corpus (10)

• Variations in the width, density, and direction of the Palomar 5 tidal tails
• Shepherding Tidal Debris with the Galactic Bar: The Ophiuchus Stream
• Spending too much time at the Galactic bar: chaotic fanning of the Ophiuchus stream
• Dispersal of tidal debris in a Milky-Way-sized dark matter halo
• Effects of chaos on the detectability of stellar streams
• The Extended Tidal Tails of NGC 7089 (M2)
• A Larger Extent for the Ophiuchus Stream
• The Ophiuchus stream progenitor: a new type of globular cluster and its possible Sagittarius connection
• Flipping of the tidal tails of the Ophiuchus stream due to the decelerating Galactic bar
• StarStream: Automatic detection algorithm for stellar streams