1 Introduction

The existence of non-axysimmetric components of the galactic potential has been frequently invoked as an efficient way to transport gas from the galaxy scale down to the nucleus to fuel the AGN. In particular, the shocks and gravitational torques induced by a galactic bar can make the gas loose angular momentum and therefore facilitate the fuelling mechanism (Shlosman et al. 1989). Nevertheless, it has been suggested that there is not a preference for Seyfert nuclei to occur in barred galaxies (Heckman 1980; Simkim et al. 1980). This result is confirmed by statistical analyses on the catalogued morphology of Seyfert galaxies (Moles et al. 1995) and through near-infrared imaging studies (McLeod & Rieke 1995; Alonso-Herrero et al. 1996; Mulchaey & Regan 1997).

We have started a large observational program aimed at studying the conditions for the onset of activity in galactic nuclei. Based on previous work (Moles et al. 1995), we search for detailed morphological and kinematical differences between active and non active galaxies of similar global morphology. We particularly pay attention to those that could facilitate the transport of gas towards the very central regions and the nucleus (for an example of the complete analysis of one of our sample galaxies see Pérez et al. 1999). For this purpose, we are obtaining optical and near-infrared images and long slit spectroscopy with the best possible spatial resolution.

We selected a sample of galaxies, out of which 18 host active nuclei and the rest form a control sample of non active galaxies (see Sect. 2).

In this paper we present the first data set: the infrared imaging data in the J and K' bands. Infrared imaging is particularly important because it allows us to trace the old stellar population, and to separate the various components (the bulge, disk, bar(s) and spiral arms) with the smallest contribution of the active nucleus and less contamination by dust absorption. In fact, various K band imaging studies have revealed that bars were present in galaxies classified as unbarred in the optical (McLeod & Rieke 1995; Mulchaey & Regan 1997), hence showing that the near-infrared is better suited for these purposes. The sample is described in Sect. 2. In Sect. 3 we present the near-infrared data we have obtained together with the HST archive images we used. In Sect. 4 we describe the different methods of analysis applied to the images. In Sects. 5 and 6 the results are described for each of the galaxies in both sets of active and non-active spirals respectively. The summary is given in Sect. 7.

A discussion on the NIR properties of these galaxies, together with the comparison of active versus non-active galaxies derived from these data will be presented in a companion paper (Márquez et al., in preparation).