1 Introduction

Considering the fact that the majority of (spiral) galaxies is not completely isolated but located in an environment which enables repeated close encounters or even merging processes with small companions it seems to be meaningful to systematically investigate the properties of galaxies affected by such processes. The investigation of their structural and dynamical changes caused by tidal interactions or low-mass satellite infall - hence "minor merger'' - can help to clarify how far the evolution of disk galaxies was modified or even dominated by environmental effects.

Several N-body simulations were performed during the last decade in order to study the influence of minor mergers on galactic disks in greater detail (e.g. Quinn et al. [1993]; Mihos et al. [1995]; Walker et al. [1996]). It was possible to use more realistic, multiple-component models for the galaxy-satellite system - usually consisting of disk, bulge, and halo - as well as a large number of particles ( $n_{\rm disk} \geq 32 \, 000$). One of the main conclusions was that even merging processes in the range between $M_{\rm sat}/M_{\rm disk} \approx 0.05 - 0.2$ can cause a vertical thickening of the stellar disk component by a factor between 2 and 4, depending on the galactocentric distance. It was found that this vertical heating is due to a gain of kinetic energy of the disk stars by enhanced two-body relaxation. According to a series of papers on the frequency of these so called "soft merging'' events (e.g. Toth & Ostriker [1992]; Zaritsky [1995], [1996]) a large number of present-day (disk-) galaxies were affected by merging- or accretion processes of this magnitude since they have formed. As a consequence, interactions and minor mergers within this mass range might modify our picture of galaxy formation and evolution.

However, the enormous parameter space of such a complex scenario makes it difficult to derive general conclusions from a set of few specific simulations. The quantitative results still crucially depend on the chosen parameters such as the content and behaviour of gas in the disk, the mass ratio between bulge and disk, induced star formation, or the satellite orbit (Quinn et al. [1993]; Mihos et al. [1995]; Velazquez & White [1999]).

Statistical studies of galaxy interactions - based on optical photometry of disk galaxies (Reshetnikov & Combes [1996], [1997]) - focused on the effects of tidally-triggered disk thickening between systems of comparable mass. They found that the ratio $h/\mbox{$\space z_{0} $ }$ of the radial exponential scale length h to the constant scale height $\mbox{$\space z_{0} $ }$ is only about twice smaller for interacting galaxies - a lower value than derived from the minor mergers simulations. However, the small number of objects in their sample (7 non-interacting and 24 interacting galaxies) did not permit to study these questions in detail.

Therefore, we started a project based on a larger sample of edge-on disk galaxies in both optical and near infrared passbands. This combination offers a number of advantages:

First, observations in the near infrared particularly benefit from the much lower dust extinction near the galactic plane, i.e. at small z. Second, the presence of a dust lane along the major axis of most edge-on disk galaxies still presents one of the best methods to determine precise inclinations of the disks - two facts that will become very important in order to derive reliable scale parameters from a disk fitting procedure. Third, this combination enables us to make conclusions on disk populations of different ages.

The main questions of this study can be summarized as follows:

• Are interactions/minor mergers able to change the radial and vertical structure of affected galactic disks?

• Is there a substantial vertical disk thickening?

• Of which order are the differences and similarities in the disk parameter distribution for a sample of interacting/non-interacting galaxies, respectively?

• To what extent are the disk properties of galaxies in the local universe influenced by interactions/minor mergers?

Due to the complexity of these questions the paper is split into three parts: in this first part (Paper I) we present deep optical and near infrared photometric data of a total sample of 110 highly-inclined/edge-on disk galaxies. This sample consists of two subsamples of 61 non-interacting galaxies (control sample) and of 49 minor merging candidates. Additionally, 41 of these galaxies were observed in the near infrared. In Sect. 2 the criteria of the sample selection will be described briefly. Section 3 gives an overview on the observations and data reduction. The disk modelling- and fitting procedure applied to derive the disk parameters will be reviewed in Sect. 4. In Sect. 5 we summarize and conclude the paper.

In the second part (Schwarzkopf & Dettmar [2000], Paper II) the results of a detailed analysis of the structure of galactic disks will be presented.

The third part (Schwarzkopf & Dettmar in preparation, Paper III) will be focused on the influence of accompanying minor merger features - like disk "warping'' and "flaring'' - on the vertical disk structure.

 

 

Table 1: Nomenclature of morphological galaxy types (cross reference). Columns: (1) Source: ESO-LV = Lauberts & Valentijn ([1989]), Hubble = Hubble ([1926]), ESO-Upp = "old'' type in Lauberts & Valentijn ([1989]); (2) Designation of morphological types

Source	Morphological Type
(1)	(2)
ESO-LV	-5	-4	-3	-2	-1	0	1	2	3	4	5	6	7	8	9	10	11
Hubble	E	-	E-S0	S0	-	S0/a	Sa	Sa-b	Sb	Sb-c	-	Sc	-	Sc-Irr	-	IrrI	-
ESO-Upp	E	-	E-S0	S0	-	S0-a	Sa	Sa-b	Sb	Sb-c	S...	Sc, Sc-d	S.../Irr	Sd	-	Irr	unknown

Figure 1: a and b. The distribution of morphological types a) for the total galaxy sample and b) for both subsamples of non-interacting (normal) and interacting/merging galaxies (merger)