The Pristine survey XXIX. Data Release 2: Photometric metallicities for 15 millions stars down to G about 20.5

Tailored narrow-band photometric observations are an efficient way to measure the metallicity of Milky Way stars. The Pristine survey has been mapping the northern sky since 2015 using narrow-band CaHK photometry obtained with the MegaCam imager on the Canada-France-Hawaii Telescope (CFHT). Data Release 1 (DR1) provided a high-precision absolute calibration of the survey observed before October 2022 using DR3 XP synthetic photometry, yielding photometric metallicities over about 6,500 deg squared, but was limited in depth to the availability of spectro-photometry (G 17.65). We present the Pristine Data Release 2 (DR2) that includes all Pristine observations until January 2025, corresponding to an additional 1,500,deg squared of coverage, and including all faint sources down to G about20.5. This release aims to exploit the full depth of the Pristine observations to uncover the faint metal-poor halo. We process all images observed from the start of the survey in March 2015 through to January 2025 following the methodology presented in DR1. The core of the calibration utilizes the subset of bright stars with synthetic CaHK syn magnitudes to train the PhotCalib neural network. We then derive photometric metallicities using the established Pristine (CaHK, G, G BP, G RP) to [Fe/H] model. The resulting catalogue covers approximately about 8,000 deg squared and provides photometric metallicities for 15 millions FGK stars, which show good agreement with recently published, independent Dark Energy Spectroscopic Instrument (DESI) metallicities. With its increased coverage and access to a significantly fainter regime compared to DR1, DR2 significantly increases the volume of the survey. We report more than 200,000 and 6,500 very and extremely metal-poor star candidates, respectively.