Mapping Gaia parallax systematic errors over the sky with faint Milky Way stars

TL;DR

This paper empirically maps Gaia DR2 parallax systematic errors across the sky using faint Milky Way stars, revealing patterns like a waffle pattern and dust effects, and provides tools for future error quantification.

Contribution

It introduces a method to map Gaia parallax systematics over the sky using faint stars and Gaia simulations, revealing detailed spatial error patterns and their dependence on magnitude.

Findings

Identified a regular waffle pattern in parallax errors on ~1 degree scales.

Mapped the increase in systematic error amplitude with star brightness from G=13 to G=20.

Supported bias estimates through correlations with quasars and independent distance measurements.

Abstract

Parallaxes measured by the Gaia mission have huge significance for astronomy, but parallaxes in Gaia DR2 are known to have systematic errors that depend on the source position and other quantities. We use the abundant information in faint Milky Way stars, along with the GOG simulation of the Gaia catalog, to probe the spatial dependence of Gaia DR2 parallax systematic errors in an empirical way. The parallax signal, concentrated in thick disk turnoff stars with magnitude G ~ 17, is sufficient to construct maps of the parallax systematic error over the majority of the sky. These maps show a locally regular "waffle pattern" on ~1 degree scales following Gaia scan directions, stronger linear "scar" features, and coherent variations on larger scales. The parallax bias maps also retain traces of astrophysical effects such as dust clouds. The waffle pattern, known from earlier maps of the Magellanic Clouds, extends over the entire sky; its local rms amplitude averages 15 microarcsec and varies by about a factor of two. The strength of this pattern increases by a factor ~6 from magnitude G = 13 to G = 20. Correlations with parallaxes of quasars and of stars with independent distance estimates support our bias estimates. Using similar methods, we map systematic errors in the proper motion and examine the relationship with the parallax systematics. We provide a code package to access and query our bias maps. Similar tests on the general stellar population should be useful in quantifying systematic errors in future Gaia releases.