Measuring the Sun's velocity using Gaia EDR3 observations of Stellar Streams

TL;DR

This paper measures the Sun's velocity relative to the Galactic halo using Gaia EDR3 stellar streams, providing a model-independent estimate that aligns with previous results but reveals a small difference in vertical motion.

Contribution

The study introduces a geometrical method to determine the Sun's velocity using stellar streams, independent of Galactic potential models and the Sun's galactocentric distance.

Findings

Measured Sun's velocity components with uncertainties.

Found agreement with past measurements in radial and rotational directions.

Detected a small but significant difference in vertical velocity component.

Abstract

We measure the Sun's velocity with respect to the Galactic halo using Gaia Early Data Release 3 (EDR3) observations of stellar streams. Our method relies on the fact that, in low-mass streams, the proper motion of stars should be directed along the stream structure in a non-rotating rest frame of the Galaxy, but the observed deviation arises due to the Sun's own reflex motion. This principle allows us to implement a simple geometrical procedure, which we use to analyse 17 streams over a $\sim 3-30$ kpc range. Our constraint on the Sun's motion is independent of any Galactic potential model, and it is also uncorrelated with the Sun's galactocentric distance. We infer the Sun's velocity as $V_{R,\odot}=8.88^{+1.20}_{-1.22}\,\rm{kms^{-1}}$ (radially towards the Galactic centre), $V_{\phi,\odot}=241.91^{+1.61}_{-1.73}\,\rm{kms^{-1}}$ (in the direction of Galactic rotation) and $V_{z,\odot}=3.08^{+1.06}_{-1.10}\,\rm{kms^{-1}}$ (vertically upwards), in global agreement with past measurements through other techniques; although we do note a small but significant difference in the $V_{z,\odot}$ component. Some of these parameters show significant correlation and we provide our MCMC output so it can be used by the reader as an input to future works. The comparison between our Sun's velocity inference and previous results, using other reference frames, indicates that the inner Galaxy is not moving with respect to the inertial frame defined by the halo streams.