Astro Lunch

What are the tidal streams constraining?

Cold stellar streams, remnants of tidally disrupted globular clusters,
have been employed as exquisite tracers of dark matter in the Milky
Way. Because of their different positions in phase space, different
ages, and different levels of observational scrutiny, different
streams tell us different things about the Galaxy. We employ a
Cramer--Rao or Fisher-matrix approach to understand the quantitative
information content in the known streams. In simple, static, analytic
models of the Milky Way, streams on eccentric orbits contain the most
information about the dark-matter shape. For any individual stream,
there are near-degeneracies between dark-matter halo properties and
parameters describing the Galactic bulge, disk and the stream
progenitor itself, but we find that simultaneous fitting of multiple
streams ought to constrain all parameters to a precision of a few
percent. At this level, simulated dark matter halos deviate from
analytic parametrizations, so we chart the way forward by discussing
constraints streams place on more flexible models of the Galactic
gravitational potential.