Clumpy streams in a smooth dark halo: the case of Palomar 5

TL;DR

This study uses simulations to show that observed stellar stream substructures around Palomar 5 can be explained by stellar orbital dynamics alone, without requiring dark matter halo lumpiness.

Contribution

The paper demonstrates that stellar stream substructures can be reproduced by simple orbital dynamics, challenging the need for dark matter halo lumpiness in explaining these features.

Findings

Reproduced observed substructures without dark matter lumpiness

Overdensities caused by epicyclic motion of stars

Models match observed density contrasts in streams

Abstract

By means of direct N-body simulations and simplified numerical models, we study the formation and characteristics of the tidal tails around Palomar 5, along its orbit in the Milky Way potential. Unlike previous findings, we are able to reproduce the substructures observed in the stellar streams of this cluster, without including any lumpiness in the dark matter halo. We show that overdensities similar to those observed in Palomar 5 can be reproduced by the epicyclic motion of stars along its tails, i.e. a simple local accumulation of orbits of stars that escaped from the cluster with very similar positions and velocities. This process is able to form stellar clumps at distances of several kiloparsecs from the cluster, so it is not a phenomenon confined to the inner part of Palomar 5's tails, as previously suggested. Our models can reproduce the density contrast between the clumps and the surrounding tails found in the observed streams, without including any lumpiness in the dark halo, suggesting new upper limits on its granularity.