Hidden deep in the halo: selection of a reduced proper motion halo catalogue and mining retrograde streams in the velocity space

TL;DR

This paper presents a large, parallax-independent catalogue of approximately 47 million halo stars using Gaia DR3 proper motions and photometry, enabling detailed studies of halo substructures and streams beyond Gaia's parallax limits.

Contribution

The authors introduce a novel halo star catalogue based on reduced proper motion, allowing for the detection and analysis of retrograde streams and halo features at greater distances than previous Gaia-based studies.

Findings

Resolved detailed structures in the GD-1 and Jhelum streams.

Demonstrated the catalogue's effectiveness in studying halo substructures.

Highlighted biases and limitations due to Gaia's scanning law.

Abstract

The Milky Way halo is one of the few galactic haloes that provides a unique insight into galaxy formation by resolved stellar populations. Here, we present a catalogue of $\sim$47 million halo stars selected independent of parallax and line-of-sight velocities, using a combination of Gaia DR3 proper motion and photometry by means of their reduced proper motion. We select high tangential velocity (halo) main sequence stars and fit distances to them using their simple colour-absolute-magnitude relation. This sample reaches out to $\sim$21 kpc with a median distance of $6.6$ kpc thereby probing much further out than would be possible using reliable Gaia parallaxes. The typical uncertainty in their distances is $0.57_{-0.26}^{+0.56}$ kpc. Using the colour range $0.45<(G_0-G_\mathrm{RP,0})<0.715$ where the main sequence is narrower, gives an even better accuracy down to $0.39_{-0.12}^{+0.18}$ kpc in distance. The median velocity uncertainty for stars within this colour range is 15.5 km/s. The distribution of these sources in the sky, together with their tangential component velocities, are very well-suited to study retrograde substructures. We explore the selection of two complex retrograde streams: GD-1 and Jhelum. For these streams, we resolve the gaps, wiggles and density breaks reported in the literature more clearly. We also illustrate the effect of the kinematic selection bias towards high proper motion stars and incompleteness at larger distances due to Gaia's scanning law. These examples showcase how the full RPM catalogue made available here can help us paint a more detailed picture of the build-up of the Milky Way halo.