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Description
Studies of the effects of environment on galaxy properties and evolution require well defined control samples. Such isolated galaxy samples have up to now been small or poorly defined. The AMIGA project (Analysis of the interstellar Medium of Isolated GAlaxies) represents an attempt to define a statistically useful sample of the most isolated galaxies in the local (z<0.05) Universe. A suitable large sample for the AMIGA project already exists, the Catalogue of Isolated Galaxies (CIG, Karachentseva 1973; 1050 galaxies, Cat. VII/82), and we use this sample as a starting point to refine and perform a better quantification of its isolation properties. Digitised POSS-I E images were analysed out to a minimum projected radius R<0.5Mpc around 950 CIG galaxies (those within Vr=1500km/s were excluded). We identified all galaxy candidates in each field brighter than B=17.5 with a high degree of confidence using the LMORPHO software. We generated a catalogue of approximately 54000 potential neighbours (redshifts exist for 30% of this sample). 666 galaxies pass and 284 fail the original CIG isolation criterion. The available redshift data confirm that our catalogue involves a largely background population rather than physically associated neighbours. We find that the exclusion of neighbours within a factor of 4 in size around each CIG galaxy, employed in the original isolation criterion, corresponds to {Delta}Vr~18000km/s indicating that it was a conservative limit. Galaxies in the CIG have been found to show different degrees of isolation. We conclude that a quantitative measure of this is mandatory. It will be the subject of future work based on the catalogue of neighbours obtained here.
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