Internal number: pgc

All the objects in the database, galaxies, stars, clusters, ..., are indexed with their pgc number. Historically, the pgc numbering was created to unify the names of the galaxies when the database project started in 1983. The first effort was to cross-identify the early catalogues of galaxies, like the MCG, UGC ..., and it resulted in the publication of the Principal Catalogue of Galaxies (Paturel et al. 1989). This catalogue counted 73097 entries corresponding to what was believed to be galaxies. The project continued, new objects were identified and previous errors corrected. The main line of improvement was the precision of the astrometry, which evolved from about 1 arcmin in 1990 to about 1 arcsec now. With the publication of high quality catalogues, the necessity of maintening PGC names vanished in the 2000s, and we stopped to systematically publish PGC names. The database contains at present 1.7 million PGC names. Still, all the objects in the database (about 5 millions) have a pgc number which is used as a unique identification across the different tables. This number should not be used to build a name, since it would not be recognized by any database, including HyperLeda. To identify objects in publications, it is recommended to use (i) a name and (ii) a position. Hyperleda provides objname, which is the best suited name amongst all designations available for an object because it is the most likely to be widely recognized.
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