Gaia Data Release 2. Cross-match with external catalogues - Algorithms and results

Context. Although the Gaia catalogue on its own is a very powerful tool, it is the combination of this high accuracy archive with other archives that will truly open up amazing possibilities for astronomical research. The advanced interoperation of archives is based on cross-matching, leaving the user with the feeling of working with one single data archive. The data retrieval should work not only across data archives but also across wavelength domains. The first step for a seamless data access is the computation of the cross-match between Gaia and external surveys. Aims. Describe the adopted algorithms and results of the pre-computed cross-match of Gaia Data Release 2 (DR2) catalogue with dense surveys (Pan-STARRS1, 2MASS, SDSS DR9, GSC2.3, URAT1, allWISE, PPMXL, APASS DR9) and sparse catalogues (Hipparcos2, Tycho-2, RAVE 5). Methods. A new algorithm is developed specifically for sparse catalogues. Improvements and changes with respect to the algorithm adopted for DR1 are described in details. Results. The outputs of the cross-match are part of the official Gaia DR2 catalogue. The global analysis of the cross-match results is also presented.