The AMIGA sample of isolated galaxies XII. Revision of the isolation
  degree for AMIGA galaxies using the SDSS
  In the framework of the AMIGA project we present a revision of the
environment for galaxies in the Catalogue of Isolated Galaxies (CIG,
Karachentseva 1973) using the ninth data release of the Sloan Digital Sky
Survey (SDSS-DR9). The aims of this study are to refine the photometric-based
AMIGA sample of isolated galaxies and to provide an improvement of the
quantification of the isolation degree with respect to previous works, using
both photometry and spectroscopy. We developed an automatic method to search
for neighbours within a projected area of 1 Mpc radius centred on each primary
galaxy to revise the CIG isolation criteria. Of the 636 CIG galaxies considered
in the photometric study, 426 galaxies fulfil the CIG isolation criteria within
1 Mpc, taking into account projected neighbours. Of the 411 CIG galaxies
considered in the spectroscopic study, 347 galaxies fulfil the CIG isolation
criteria when a criterion about redshift difference is added. The available
redshifts allow us to reject background neighbours and thus improve the
photometric assessment. For the first time, the environment and the isolation
degree of AMIGA galaxies are quantified using digital data. The use of the SDSS
database permits one to identify fainter and smaller-size satellites than in
previous AMIGA works. About 50% of the neighbours considered as potential
companions in the photometric study are in fact background objects. We also
find that about 92% of the neighbour galaxies that show recession velocities
similar to the corresponding CIG galaxy are not considered by the CIG isolation
criteria as potential companions, which may have a considerable influence on
the evolution of the central CIG galaxy.
