1 Introduction

The light distributions of the disk and bulge are generally assumed to represent physically and dynamically distinct components. While the disks are rotationally supported, the bulges are pressure supported. Well known relations are the links between the bulge-to-disk (B/D) ratio with the Hubble type (Simien & de Vaucouleurs [1986]; Kent [1985]) and the near constancy of the central surface brightness (Freeman [1970]), although the latter has recently been suggested to be a selection effect (de Jong [1996a], [1996b]). Other fundamental parameters related with the Hubble type are the effective radius and surface brightness of the bulge and disk (Simien & de Vaucouleurs [1986]). All these correlations are generally suggested to manifest galaxy evolution governed by the intrinsic properties of the galaxies.

On the other hand, galaxy interactions modify galaxies affecting especially the gas distributions and the young stellar populations. For example, high surface brightness galaxies are more likely to have companions than the low surface brightness galaxies (Knezek [1993]) and the central surface brightnesses of interacting galaxies are on the average higher than those of the field galaxies (Reshetnikov et al. [1993]). Ellipticals located in high-density regions are characterized by smaller values of effective radii than galaxies located in the outer regions of the clusters (Strom & Strom [1978], [1979]). Tidal shearing may cause sharp outer cutoffs in the brightness profiles of interacting galaxies (Chromey et al. [1998]), or make the profiles systematically steeper in the outer parts with respect to the profiles of galaxies in low density regions (Bagget & Anderson [1992]). Indeed, galaxy interactions may play an important role in the evolution within the Hubble sequence.

Isolated M 51-type pairs are a distinct group of interacting galaxies, where the main galaxies have rather regular morphologies and therefore the structural parameters can be well quantified. With the goal of studying the effects of interaction on the evolution of M 51-type galaxies, we present the isophotal profiles and bulge-disk decompositions for a sample of 40 galaxies. Here we describe the derivation of the bulge and disk parameters, whereas the next paper in the series will focus on the analysis.