Title:Tidal Stream Morphology as an Indicator of Dark Matter Halo Geometry: the Case of 
Palomar 5

Abstract:
In this presentation an example where the morphology of a single stellar stream can be used to 
rule out a specific galactic potential form without the need for velocity information is presented. 
We investigated the globular cluster Palomar 5 (Pal 5), which is tidally disrupting into a cold, 
thin stream mapped over 22 degrees on the sky with a typical width of 0.7 degrees. We 
generated models of this stream by fixing Pal 5’s present-day position, distance and radial 
velocity via observations, while allowing its proper motion to vary. In a spherical dark matter halo 
we easily found models that fit the observed morphology. However, no plausible Pal 5 model 
could be found in the triaxial potential of Law & Majewski (2010), which has been proposed to 
explain the properties of the Sagittarius stream. In this case, the long, thin and curved 
morphology of the Pal 5 stream alone can be used to rule out such a potential configuration. Pal 
5 like streams in this potential are either too straight, missing the curvature of the observations, 
or show an unusual morphology which we dub stream-fanning: a signature sensitive to the 
triaxiality of a potential. We conclude that the mere existence of other thin tidal streams must 
provide broad constraints on the orientation and shape of the dark matter halo they inhabit.