Gaps and length asymmetry in the stellar stream Palomar 5 as effects of Galactic bar rotation

Abstract

The stellar stream emerging from the globular cluster Palomar 5 (Pal 5) is one of the few Galactic streams that is clearly associated with its progenitor system. Recent optical photometric data show that Pal 5’s leading arm appears truncated compared with the trailing arm, which is not expected from previous simulations. We demonstrate that inclusion of the rotating Galactic bar in the dynamical modelling of Pal 5 can reproduce the truncation. As the bar sweeps by, stream stars experience differences in net torques near their orbital pericentres. This leads to the formation of discontinuities in the energy distribution of stream members that in turn become apparent as ever-widening gaps in the stream’s spatial density. We conclude that only streams orbiting far from the Galactic Centre or streams on retrograde orbits can be used to unambiguously constrain dark matter subhalo interactions. Additionally, we expect that the Pal 5 leading-arm debris should reappear south of the density truncation. Rotation of the Galactic bar at the centre of the Milky Way can explain gaps and asymmetries in stellar stream Palomar 5. Similar streams close to the Galactic Centre are therefore unfit for probing the dark matter subhalo interactions in our Galaxy.