SIMBAD references

We describe the production of the Initial Gaia Source List (IGSL) used both as the starting point for observation-matching in the Gaia data processing and as a comparison catalog to trigger early science alerts. The IGSL was compiled from large scale public catalogs and a few specific catalogs provided by the Gaia Consortium. For all entries, a Gaia G magnitude is estimated and all objects with a G<21.0 were included. The catalog has 1222598530 entries with the J2000 ICRF positions, proper motions, classification (star/non-star), and estimates of the Gaia G, Gaia G_rvs, Guide Star Catalog red (R_F) and blue (B_J) magnitudes. It is available from the Centre de Donnees astronomiques de Strasbourg.