Stream-orbit misalignment - I. The dangers of orbit-fitting
Abstract
Tidal streams do not, in general, delineate orbits. A stream-orbit misalignment is expected to lead to biases when using orbit-fitting to constrain models for the Galactic potential. In this first of two papers, we discuss the expected magnitude of the misalignment and the resulting dangers of using orbit-fitting algorithms to constrain the potential. We summarize data for known streams which should prove useful for constraining the Galactic potential, and compute their actions in a realistic Galactic potential. We go on to discuss the formation of tidal streams in angle-action space, and explain why, in general, streams do not delineate orbits. The magnitude of the stream-orbit misalignment is quantified for a logarithmic potential and a multicomponent Galactic potential. Specifically, we focus on the expected misalignment for the known streams. By introducing a two-parameter family of realistic Galactic potentials we demonstrate that assuming that these streams delineate orbits can lead to order one errors in the halo flattening and halo-to-disc force ratio at the Sun. We present a discussion of the dependence of these results on the progenitor mass and demonstrate that the misalignment is mass independent for the range of masses of observed streams. Hence, orbit-fitting does not yield better constraints on the potential if one uses narrower, lower mass streams.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- August 2013
- DOI:
- 10.1093/mnras/stt806
- arXiv:
- arXiv:1305.1935
- Bibcode:
- 2013MNRAS.433.1813S
- Keywords:
-
- Galaxy: halo;
- Galaxy: kinematics and dynamics;
- Galaxy: structure;
- galaxies: kinematics and dynamics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 13 pages, 7 figures, accepted for publication in MNRAS