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In the framework of the AMIGA project we present a revision of theenvironment for galaxies in the Catalogue of Isolated Galaxies (CIG,Karachentseva 1973) using the ninth data release of the Sloan Digital SkySurvey (SDSS-DR9). The aims of this study are to refine the photometric-basedAMIGA sample of isolated galaxies and to provide an improvement of thequantification of the isolation degree with respect to previous works, usingboth photometry and spectroscopy. We developed an automatic method to searchfor neighbours within a projected area of 1 Mpc radius centred on each primarygalaxy to revise the CIG isolation criteria. Of the 636 CIG galaxies consideredin the photometric study, 426 galaxies fulfil the CIG isolation criteria within1 Mpc, taking into account projected neighbours. Of the 411 CIG galaxiesconsidered in the spectroscopic study, 347 galaxies fulfil the CIG isolationcriteria when a criterion about redshift difference is added. The availableredshifts allow us to reject background neighbours and thus improve thephotometric assessment. For the first time, the environment and the isolationdegree of AMIGA galaxies are quantified using digital data. The use of the SDSSdatabase permits one to identify fainter and smaller-size satellites than inprevious AMIGA works. About 50% of the neighbours considered as potentialcompanions in the photometric study are in fact background objects. We alsofind that about 92% of the neighbour galaxies that show recession velocitiessimilar to the corresponding CIG galaxy are not considered by the CIG isolationcriteria as potential companions, which may have a considerable influence onthe evolution of the central CIG galaxy.

The AMIGA sample of isolated galaxies XII. Revision of the isolation degree for AMIGA galaxies using the SDSS