Properties of Isolated Disk Galaxies

A new sample of northern isolated galaxies, which are defined by the physical criterion that they were not affected by other galaxies in their evolution during the last few Gyr, is presented. To find them we used the logarithmic ratio, f , between inner and tidal forces acting upon the candidate galaxy by a possible perturber. The analysis of the distribution of the f-values for the galaxies in the Coma cluster lead us to adopt the criterion f ≤ −4.5 for isolated galaxies. The candidates were chosen from the CfA catalog of galaxies within the volume defined by cz ≤5000 km/s, galactic latitude higher than 40 • and declination ≥ −2.5 •. The selection of the sample, based on redshift values (when available), magnitudes and sizes of the candidate galaxies and possible perturbers present in the same field is discussed. The final list of selected isolated galaxies comprises of 203 objects from the initial 1706. The list contains only truly isolated galaxies in the sense defined, but it is by no means complete, since all the galaxies with possible companions under the f-criterion but with unknown redshift were discarded. We also selected a sample of perturbed galaxies comprising of all the disk galaxies from the initial list with companions (with known redshift) satisfying f ≥ −2 and ∆(cz) ≤ 500 km/s, a total of 130 objects. The statistical comparison of both samples shows significant differences in morphology, sizes, masses, luminosities and color indices. Confirming previous results, we found that late spiral, Sc type galaxies are, in particular, more frequent among isolated galaxies, whereas Lenticular galaxies are more abundant among perturbed galaxies. Isolated systems appear to be smaller, 2 J. Varela et al.: Isolated Galaxies less luminous and bluer than interacting objects. We also found that bars are twice as frequent among perturbed galaxies compared to isolated galaxies, in particular for early Spirals and Lenticulars. The perturbed galaxies have higher LF IR/LB and M mol /LB ratios, but the atomic gas content is similar for the two samples. The analysis of the luminosity-size and mass-luminosity relations shows similar trends for both families, the main difference being the almost total absence of big, bright and massive galaxies among the family of isolated systems, together with the almost total absence of small, faint and low mass galaxies among the perturbed systems. All those aspects indicate that the evolution induced by interactions with neighbors, …