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.
