BVRI imaging of M51-type interacting galaxy pairs - III. Analysis of the photometric parameters

Photometric bulge and disc properties of a sample of 21 M51-type pairs obse rved in BVRI bands are analysed, and the derived parameters obtained by the R (1/4)-law and exponential bulge models are compared with the sample of n ormal galaxies reported by de Jong & van der Kruit in 1994. Some characteri stic properties of the intensity profiles are then interpreted by 3D N-body simulations, in which both components of the pair are described by an expo nential star + gas disc embedded in a live bulge and halo. The disc central surface brightness mu (0) was found to be similar for the M51-type and for normal galaxies, being 21.5 +/- 0.8B mag arcsec(-2), when the exponential bulge model was used. The main galaxies had generally almos t normal photometric properties, whereas for the companions secular evoluti on must have played an important role. Even though the mean scalelengths fo r the M51-type galaxies were smaller than for the comparison galaxies, thei r bulges were larger: especially the companions had extremely large bulge s izes relative to their disc scalelengths having [r(e)(bulge)/h] (B) = 0.37 +/- 0.10 in comparison with 0.15 +/- 0.09 for normal galaxies. Consequently , the bulge-to-disc luminosity ratios for the companions were also generall y larger than known for any of the Hubble types of normal galaxy. N-body simulations indicated that the inner disc can be steepened during th e interaction, and that this steepened disc can be erroneously interpreted as belonging to the bulge. Simultaneously material in the outer disc is red istributed, producing shallow outer profiles similar to those observed for typical M51-type galaxies. These processes were demonstrated by applying ph otometric bulge-to-disc decompositions for some of the simulated intensity profiles. The decompositions also showed that one must be cautious when app lying the R (1/4)-law bulge model, especially for interacting galaxies. A n early parabolic experiment showed that the redistribution of matter in the disc during the close passage can be long-lived, suggesting that galaxy int eractions even with small companions may play an important role in secular evolution for galaxies in clusters.