Locating the orbits delineated by tidal streams

TL;DR

This paper introduces a method to determine the orbits of tidal streams in the Galaxy using velocity measurements, enabling precise predictions of distances and motions, and providing insights into the Galactic potential and dark matter halo.

Contribution

The paper presents a novel technique that accurately recovers Galactic orbits from tidal stream data, accounting for stream deviations and uncertainties, and constrains the Galactic potential and dark halo properties.

Findings

Accurate velocity data yields high-precision orbit predictions.

The method can exclude certain Galactic potential models.

Dark halo mass can be measured with 5% accuracy.

Abstract

We describe a technique that finds orbits through the Galaxy that are consistent with measurements of a tidal stream, taking into account the extent that tidal streams do not precisely delineate orbits. We show that if accurate line-of-sight velocities are measured along a well defined stream, the technique recovers the underlying orbit through the Galaxy and predicts the distances and proper motions along the stream to high precision. As the error bars on the location and velocities of the stream grow, the technique is able to find more and more orbits that are consistent with the data and the uncertainties in the predicted distances and proper motions increase. With radial-velocity data along a stream ~40deg long and <0.3deg wide on the sky accurate to ~1 km/s the precisions of the distances and tangential velocities along the stream are 4 percent and 5 km/s, respectively. The technique can be used to diagnose the Galactic potential: if circular-speed curve is actually flat, both a Keplerian potential and Phi(r) proportional to r are readily excluded. Given the correct radial density profile for the dark halo, the halo's mass can be determined to a precision of 5 percent.