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


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.

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Fig. 1: The truncation in the leading arm of Pal 5 can be reproduced by introducing a prograde Galactic bar.
Fig. 2: Observables of the Pal 5 stream evolved in a potential with a prograde Galactic bar.
Fig. 3: Gap formation and evolution in a Pal 5-like stellar stream due to the spinning Galactic bar.
Fig. 4: Variations in the torque from the Galactic bar due to stream inclination (i) and pericentric distance (R p).
Fig. 5: The Galactic potential.


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We acknowledge A. Brown, the Gaia Project Scientist Support Team and the Gaia Data Processing and Analysis Consortium (DPAC) for making the PyGaia package open source. We thank the Flatiron Institute Center for Computational Astrophysics for providing the space to carry out this project. S.P. thanks J. Chanamé and E. Bernard for insightful discussions. K.V.J. and S.P. acknowledge support from National Science Foundation grant AST-1614743.

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S.P. led the modelling, analysis, figure production and writing of the paper. A.M.P.-W. assisted with the modelling, writing and with producing figures. All authors discussed the results, their interpretation and the presentation of the paper.

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Correspondence to Sarah Pearson.

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Pearson, S., Price-Whelan, A.M. & Johnston, K.V. Gaps and length asymmetry in the stellar stream Palomar 5 as effects of Galactic bar rotation. Nat Astron 1, 633–639 (2017).

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