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

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.

A sharper view of Pal 5's tails: discovery of stream perturbations with a novel non-parametric technique

“…The star counts along the stream can have smallscale bumps and dips (see e.g. Küpper et al 2010;Yoon et al 2011;Mastrobuono-Battisti et al 2012), while the across-stream profile can become broader or narrower due to a range of effects, including distance variation, epicyclic feathering, interactions with small substructure and differential orbital plane precession (Koposov et al 2010;Erkal & Belokurov 2015a;Amorisco 2015;Erkal et al 2016a;Amorisco et al 2016).…”

“…The epicyclic overdensities have been studied extensively in the literature (see e.g. Combes et al 1999;Capuzzo Dolcetta et al 2005;Küpper et al 2008Küpper et al , 2010Mastrobuono-Battisti et al 2012;Amorisco 2015) and are the result of the similarity of the orbits of the unbound stars. As further confirmation, these observed peaks are closely aligned with the peaks seen in our simulation at φ1 = −0.7 • , +0.6 • (Fig.…”

Stellar streams as gravitational experiments

“…Apart from external perturbations, Newtonian simulations never produce such asymmetries. They can of course produce small length asymmetries, with a longer or shorter leading arm, due to the different orbital velocities of escaped stars when the orbit of the cluster is eccentric (Montuori et al 2007;Mastrobuono-Battisti et al 2012). However, the Newtonian orbits necessary to reproduce the Pal 5 stream never produce large asymmetries.…”

Gaps in globular cluster streams: giant molecular clouds can cause them too

“…These are armlets shed during recent pericentre passages. Their shape is a consequence of the orbital properties of the member stars (e.g., Küpper et al 2012;Mastrobuono-Battisti et al 2012;Amorisco 2015), and is unrelated to external disturbances. Panels Figure 2 shows 1-d density profiles for a selection of the streams displayed in Fig.…”