Shepherding Tidal Debris with the Galactic Bar: The Ophiuchus Stream

TL;DR

This paper explains how the rotating Galactic bar influences stellar stream dynamics, potentially creating shorter streams like the Ophiuchus stream, challenging traditional assumptions about halo structure.

Contribution

It introduces a novel explanation for stream length variations due to bar-induced torques, highlighting the impact of rotating barred potentials on stream evolution.

Findings

Rotating bars cause differential torques on stream stars.

Stream length can vary significantly depending on bar orientation.

The mechanism explains the short Ophiuchus stream near the Galactic bulge.

Abstract

The dynamics of stellar streams in rotating barred potentials is explained for the first time. Naturally, neighbouring stream stars reach pericentre at slightly different times. In the presence of a rotating bar, these neighbouring stream stars experience different bar orientations during pericentric passage and hence each star receives a different torque from the bar. These differing torques reshape the angular momentum and energy distribution of stars in the stream, which in turn changes the growth rate of the stream. For a progenitor orbiting in the same sense as the bar's rotation and satisfying a resonance condition, the resultant stream can be substantially shorter or longer than expected, depending on whether the pericentric passages of the progenitor occur along the bar's minor or major axis respectively. We present a full discussion of this phenomenon focusing mainly on streams confined to the Galactic plane. In stark contrast with the evolution in static potentials, which give rise to streams that grow steadily in time, rotating barred potentials can produce dynamically old, short streams. This challenges the traditional viewpoint that the inner halo consists of well phase-mixed material whilst the tidally-disrupted structures in the outer halo are more spatially coherent. We argue that this mechanism plays an important role in explaining the mysteriously short Ophiuchus stream that was recently discovered near the bulge region of the Milky Way.