Gaia Data Release 2: Photometric content and validation (1804.09368v3)

Abstract: Aims. We describe the photometric content of the second data release of the Gaia project (Gaia DR2) and its validation along with the quality of the data. Methods. The validation was mainly carried out using an internal analysis of the photometry. External comparisons were also made, but were limited by the precision and systematics that may be present in the external catalogues used. Results. In addition to the photometric quality assessment, we present the best estimates of the three photometric passbands. Various colour-colour transformations are also derived to enable the users to convert between the Gaia and commonly used passbands. Conclusions. The internal analysis of the data shows that the photometric calibrations can reach a precision as low as 2 mmag on individual CCD measurements. Other tests show that systematic effects are present in the data at the 10 mmag level.

• The paper presents a high-precision photometric catalog for 1.7 billion sources using Gaia DR2 data.
• It validates the photometry with precision down to 2 mmag and identifies systematic calibration effects at the 10 mmag level.
• Detailed passbands and color transformations are provided to align Gaia data with other standard astronomical systems.

Gaia Data Release 2: Photometric Content and Validation

The paper "Gaia Data Release 2: Photometric Content and Validation" presents a comprehensive analysis of the photometric data included in the second release of the Gaia mission. This is a crucial advancement in the field of observational astronomy, providing enhanced data quality and greater coverage of celestial bodies.

Summary of the Paper

Major Contributions

1. Photometric Catalog: The Gaia Data Release 2 (DR2) includes photometry in the $G$ band for approximately 1.7 billion sources, and in the integrated blue photometer (BP) and red photometer (RP) bands for about 1.4 billion sources. This significantly enhances the astrometric and photometric data available to researchers.
2. Calibration and Precision: The paper reports a precision as low as 2 millimagnitudes (mmag) on individual CCD measurements, with systematic effects identified at a 10 mmag level. The photometric data was subjected to rigorous validation processes including both internal analysis and external catalogue comparisons.
3. Passbands and Transformations: The publication provides detailed passbands for the Gaia photometric system, alongside color-color transformations to bridge Gaia data with other photometric systems. These are key for users interested in translating Gaia data into other standard systems used within the astronomical community.
4. Quality and Error Analysis: The authors detail the methods for quality assessment of photometric calibrations, using metrics such as unit-weight standard deviation and L1 norm metric for convergence checks of large-scale calibrations.

Numerical and Comparative Analysis

• Internal Calibration: It has been demonstrated that internal photometric calibrations reached a precision of ~2 mmag for the $G$ band, while the BP and RP data have slightly larger uncertainties of 3 to 5 mmag.
• Comparative Analysis: The paper utilizes comparisons with established external datasets such as Tycho, APASS, and SDSS to validate Gaia DR2 photometry, demonstrating systematic residuals at the 10 mmag level with faint-end deviations attributed to background subtraction challenges.

Implications

The data provided by the Gaia DR2 advances the capabilities of astronomers in terms of both precision and scale. By offering consistent, high-resolution photometric data, Gaia DR2 facilitates a wide range of astrophysical research, including stellar mass function studies, detailed star cluster analysis, and the mapping of the Milky Way's structural components.

Future Directions

Moving forward, the Gaia mission will further refine its photometric processing techniques, expanding upon the existing framework to handle complex calibration scenarios such as crowded fields and high-saturation zones. Subsequent data releases will likely incorporate epoch photometry for a more extensive selection of sources, alongside refined passband calibrations informed by spectroscopic data. The iterative calibration processes described in this paper lay the groundwork for enhanced future releases.

In conclusion, Gaia DR2 represents a significant leap forward in precision astrometry and photometry. The robustness of calibration and validation reported in this paper reflects a high standard in astronomical data processing, enabling future research that builds upon this extensive dataset. Researchers can anticipate further improvements and more detailed data in subsequent Gaia data releases, which will continue to elucidate the complexities of our galaxy and beyond.