Stream-orbit misalignment I: The dangers of orbit-fitting

TL;DR

This paper investigates how tidal streams often do not follow orbits, which can cause significant biases in modeling the Galactic potential using orbit-fitting, emphasizing the need for caution in such methods.

Contribution

It quantifies the magnitude of stream-orbit misalignment in realistic Galactic potentials and highlights the potential errors in Galactic parameter estimation from orbit-fitting.

Findings

Stream-orbit misalignment can cause order one errors in halo shape estimates.

Assuming streams delineate orbits leads to significant biases in Galactic potential parameters.

Misalignment is independent of progenitor mass within observed ranges.

Abstract

Tidal streams don't, 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 multi-component 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 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.