Constraining the Solar Galactic Reflex Velocity Using Gaia Observations of the Sagittarius Stream

TL;DR

This paper uses Gaia data of the Sagittarius stream to precisely measure the solar reflex velocity and infer the Milky Way's circular speed at the solar radius, providing a model-independent approach.

Contribution

It introduces a novel method leveraging the Sagittarius stream's orientation to constrain the solar reflex velocity independently of the solar Galactocentric distance.

Findings

Solar reflex velocity constrained to 253 ± 6 km/s.

Implied Milky Way circular speed at solar radius is 229 ± 6 km/s.

Method reduces dependence on Sgr model assumptions.

Abstract

Because of its particular orientation around the Galaxy - i.e., in a plane nearly perpendicular to the Galactic plane and containing both the Sun and Galactic center - the Sagittarius (Sgr) stream provides a powerful means by which to measure the solar reflex velocity, and thereby infer the velocity of the Local Standard of Rest (LSR), in a way that is independent of assumptions about the solar Galactocentric distance. Moreover, the solar reflex velocity with respect to the stream is projected almost entirely into the proper motion component of Sgr stream stars perpendicular to the Sgr plane, which makes the inferred velocity relatively immune to most Sgr model assumptions. Using Gaia DR2 proper motions of ~2,000 stars identified to be Sgr stream candidates in concert with the Law and Majewski (2010) Sgr N-body models (which provide a good match to the Gaia observations) we constrain the solar reflex velocity induced by its orbital motion around the Galaxy to be $\Theta_{\odot} = 253 \pm 6$ km/s. Assuming a solar peculiar motion in the direction of orbital rotation of 12 km/s, and an LSR velocity of 12 km/s with respect to the local circular speed, the implied circular speed of the Milky Way at the solar circle is $229 \pm 6$ km/s.