2001MNRAS.324..685L

Photometric bulge and disc properties of a sample of 21 M51-type pairs observed 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 normal galaxies reported by de Jong & van der Kruit in 1994. Some characteristic properties of the intensity profiles are then interpreted by 3D N-body simulations, in which both components of the pair are described by an exponential star+gas disc embedded in a live bulge and halo.

The disc central surface brightness µ₀ was found to be similar for the M51-type and for normal galaxies, being 21.5±0.8Bmag.arcsec^–2, when the exponential bulge model was used. The main galaxies had generally almost normal photometric properties, whereas for the companions secular evolution must have played an important role. Even though the mean scalelengths for the M51-type galaxies were smaller than for the comparison galaxies, their bulges were larger: especially the companions had extremely large bulge sizes 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 generally larger than known for any of the Hubble types of normal galaxy.

N-body simulations indicated that the inner disc can be steepened during the interaction, and that this steepened disc can be erroneously interpreted as belonging to the bulge. Simultaneously material in the outer disc is redistributed, producing shallow outer profiles similar to those observed for typical M51-type galaxies. These processes were demonstrated by applying photometric bulge-to-disc decompositions for some of the simulated intensity profiles. The decompositions also showed that one must be cautious when applying the R^1/4-law bulge model, especially for interacting galaxies. A nearly parabolic experiment showed that the redistribution of matter in the disc during the close passage can be long-lived, suggesting that galaxy interactions even with small companions may play an important role in secular evolution for galaxies in clusters.