[Paper Review] Gaia Data Release 2. The astrometric solution

L. Lindegren, J. Hernández+88 more|arXiv (Cornell University)|Apr 25, 2018

Stellar, planetary, and galactic studies33 references891 citations

TL;DR

This paper presents the astrometric solution for Gaia Data Release 2 (Gaia DR2), which delivers five-parameter astrometry (position, parallax, proper motion) for 1.3 billion sources using 320 billion CCD centroid measurements. The solution achieves median uncertainties of 0.04 mas in parallax and 0.05 mas yr⁻¹ in proper motion for bright stars (G < 14 mag), with systematics below 0.1 mas and a global parallax zero-point offset of approximately -0.03 mas.

ABSTRACT

Gaia Data Release 2 (Gaia DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 22 months of its operational phase. We describe the input data, models, and processing used for the astrometric content of Gaia DR2, and the validation of these results performed within the astrometry task. Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion. For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G&lt;14 mag) sources, 0.1 mas at G=17 mag, and 0.7 mas at G=20 mag. In the proper motion components the corresponding uncertainties are 0.05, 0.2, and 1.2 mas/yr, respectively. The optical reference frame defined by Gaia DR2 is aligned with ICRS and is non-rotating with respect to the quasars to within 0.15 mas/yr. From the quasars and validation solutions we estimate that systematics in the parallaxes depending on position, magnitude, and colour are generally below 0.1 mas, but the parallaxes are on the whole too small by about 0.03 mas. Significant spatial correlations of up to 0.04 mas in parallax and 0.07 mas/yr in proper motion are seen on small (&lt;1 deg) and intermediate (20 deg) angular scales. Important statistics and information for the users of the Gaia DR2 astrometry are given in the appendices.

Motivation & Objective

• To develop and validate a high-precision astrometric solution for Gaia DR2 using 320 billion CCD measurements from the first 22 months of Gaia's mission.
• To estimate and characterize systematic and random errors in parallax and proper motion across the sky, magnitude, and color.
• To tie the Gaia optical reference frame to the International Celestial Reference System (ICRS) using quasars, ensuring alignment within 0.15 mas yr⁻¹.
• To identify and quantify systematic biases in the astrometric solution, particularly the parallax zero-point offset and spatial correlations.
• To provide users with accurate error estimates and data quality metrics through comprehensive validation and appendices.

Proposed method

• An astrometric global iterative solution (AGIS) was applied to 16 million selected sources using ~1% of the input data to determine satellite attitude and CCD calibration parameters.
• The primary solution was tied to the extragalactic ICRS via quasars, ensuring a non-rotating, ICRS-aligned optical reference frame.
• A two-step processing pipeline was used: first, the primary solution was computed; second, it was applied to all 1.693 billion sources to derive their astrometric parameters.
• Special validation solutions were designed to isolate and measure systematic errors in parallax and proper motion, including split-field and late/early solution comparisons.
• Cross-matching of detections to sources used an advanced clustering algorithm that accounts for linear motion, improving performance in dense fields and for high-proper-motion sources.
• Formal uncertainties were corrected for a DOF bug that previously underestimated errors, especially for bright sources (G < 13 mag).

Experimental results

Research questions

• RQ1 What is the precision and accuracy of the Gaia DR2 astrometric solution across the magnitude range G = 3 to G = 21?
• RQ2 How do systematic errors in parallax and proper motion vary with source position, magnitude, and color?
• RQ3 To what extent is the Gaia optical reference frame aligned with the ICRS, and what is its rotation rate relative to quasars?
• RQ4 What is the magnitude and spatial structure of the parallax zero-point offset, and how does it affect the astrometric solution?
• RQ5 How do data quality issues such as spurious detections and cross-matching errors impact the final astrometric results?

Key findings

• The median parallax uncertainty is 0.04 mas for bright sources (G < 14 mag), increasing to 0.1 mas at G = 17 mag and 0.7 mas at G = 20 mag.
• The median proper motion uncertainty is 0.05 mas yr⁻¹ for G < 14 mag, rising to 0.2 mas yr⁻¹ at G = 17 mag and 1.2 mas yr⁻¹ at G = 20 mag.
• The Gaia optical reference frame is aligned with ICRS and non-rotating relative to quasars to within 0.15 mas yr⁻¹.
• Systematic parallax errors depending on position, magnitude, and color are generally below 0.1 mas, but the overall parallax zero-point is too small by approximately 0.03 mas.
• Spatial correlations of up to 0.04 mas in parallax and 0.07 mas yr⁻¹ in proper motion are observed on angular scales of less than 1° and around 20°.
• Formal uncertainties for bright sources (G < 13 mag) were underestimated due to a DOF bug, corrected in the Gaia Archive, though faint quasars still show 8–12% underestimation.