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

This paper presents 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. We investigate the globular cluster Palomar5 (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 generate 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 find 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 Pal5 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.