Globular cluster fields are among the densest regions in the sky and therefore provide a reliable indication of the limits of the Gaia performances both in terms of astrometry and photometry. Their colour-magnitude diagrams are important instruments in the calibration of fundamental stellar parameters and in the testing of stellar evolution models.
The pictures on this page show a few examples of the first colour-magnitude diagrams of Galactic globular clusters: NGC 104 (47 Tuc), NGC 5139 (Omega Cen), NGC 6397 and NGC 6809 (M55). The photometric data at this stage is still very preliminary. The photometry is only internally calibrated and calibrations have not converged yet to a consistent internal system. This photometric data is not fully indicative of the quality of the data that will be contained in the first Gaia data release in the summer 2016. This data was produced during an internal test campaign and, due to the iterative nature of the processing and the addition of more recent data, the quality of the photometry included in the first data release is expected to be higher1.
Even with all these caveats in mind, the colour-magnitude diagrams on this page already show well-defined and narrow sequences, with populated red giant and horizontal branches typical of these low-metallicity and old clusters. The scientific requirements for the Gaia mission require that stars can generally be resolved in fields with densities up to 0.25 star per square arcsec. Although this seems to imply that a significant fraction of the Galactic globular clusters might not be observable in their core regions, thanks to the different scanning directions for different observations, the central cores are not totally lost and a large fraction of known Galactic globular clusters appears to be well populated in the Gaia photometric data.
Furthermore Gaia's astrometric data will enable us to vastly improve these colour-magnitude diagrams by separating the field stars from the cluster members kinematically. Astrometric measurements of the accuracy expected from the Gaia final catalogue will also allow us to study the internal dynamics of these clusters as well as to derive accurate distances and space motions.
(1) Note that the first Gaia data release (Gaia-DR1) announced for Summer 2016 will not contain colour information, which is derived from the data collected by the two Gaia spectro-photometers. The release of colours is planned for Summer 2017 (Gaia-DR2). The calibration of the low-resolution spectra, in particular the dispersion and geometric calibrations, as well as the correct treatment of crowded or contaminated spectra, require a good knowledge of the source positions, at an accuracy that only Gaia itself can provide. These plots are based on preliminary colour results required internally for the calibration of the G-band photometry. The production of the Gaia-DR2 will start later this year and will take advantage of improved accuracy in the Gaia data (thanks to the iterative nature of the cyclic processing) and the experience accumulated so far with these preliminary calibrations.