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1. Introduction

An important aim of active galactic nuclei (AGN) research is to quantify the relative contributions of nonstellar and stellar (in some cases starburst) components within the nuclear regions. Based on emission line ratios in the extended narrow line region (ENLR), radio morphologies, and infrared (IR) images, it is known that some Seyfert nuclei have circumnuclear starburst regions (e.g. NGC 1068; Balick & Heckman 1985 and NGC 7469; Wilson et al. 1991). Part of the far-IR (FIR) luminosity of Seyferts is due to thermal reradiation of dust heated by hot OB stars (Rodriguez-Espinosa et al. 1987) and indeed, starbursts and Seyfert 2s have similar IRAS flux ratios and Htex2html_wrap_inline2601 luminosities (Dahari & DeRobertis 1988). Seyfert 2s in general have stronger mid-IR (MIR; Edelson et al. 1987; Maiolino et al.\ 1995), FIR (Dahari & De Robertis 1988), CO (Heckman et al. 1989) and radio emission (Ulvestad & Wilson 1989) than Seyfert 1s, indicating stronger star forming activity in their host galaxies.

Another important question in AGN research is whether observationally different types of AGN are intrinsically different or the same phenomenon. Recently, major effort has been put into unified models to explain the differences in terms of collimated radiation and orientation effects (e.g. Barthel 1989; Antonucci 1993). Specifically for Seyfert galaxies, it has long been been proposed that there is basically only one type of Seyfert nucleus with the optical spectroscopic division between types 1 and 2 dependent both on obscuration in and around the broad line region (BLR) and the luminosity of the central energy source (e.g. Lawrence & Elvis 1982). As we will discuss in Sect. 6.2, there is now compelling evidence that in Seyfert 1s we see the compact nucleus and BLR directly, whereas in Seyfert 2s optically thick material, in the form of a thick disk, torus or an outflowing wind, blocks our direct view, and the ENLR along the minor axis of the torus is lit up by the soft X-ray/UV radiation from the nucleus in two oppositely directed cones.

Although the nuclear torus is likely to be too small to be resolved, its outer parts e.g. in the form of a flattened dust distribution may be detectable. Because this material is expected to be redder than the surrounding stellar population, the geometry of the obscuring region should be apparent in colour maps. Evidence for this, in the form of a red distribution of continuum light perpendicular to the bipolar ENLR has been detected in NGC 5252 (Tadhunter & Tsvetanov 1989; Kotilainen & Prieto 1995) and NGC 5728 (Tsvetanov et al. 1996).

In Kotilainen & Ward (1994), we discussed the general properties of the host galaxies of a sample of hard X-ray selected AGN, which are mainly Seyfert 1 nuclei. We found that the host galaxy colours are usually redder than in normal spiral bulges. The most likely explanation is a combination of dust obscuration and/or reradiation from hot dust. We detected colour gradients with the nucleus being redder than the bulge. There is a good correlation between the AGN and host galaxy luminosity. Most of the emphasis in studies of the nuclear regions of Seyferts has concentrated on the emission line properties (since integral field spectroscopy and emission line imaging provide the best tools to study the ENLR). Except for the recent work of Pogge & De Robertis (1993), the continuum emission has been relatively neglected. In this paper we concentrate on the circumnuclear regions in Seyferts, rather than the integrated properties of the host galaxy. We present high resolution colour images of nine Seyfert galaxies (NGC 1068, NGC 3227, NGC 4151, NGC 7469, Mrk 3, Mrk 78, Mrk 348, Mrk 573 and NGC 1667). These images resolve the circumnuclear regions and reveal the existence of knots, elongations and rings not discernible in single filter images. We describe the observations in Sect. 2. In Sects. 3 and 4 we outline the data reduction procedures and methods used in the construction of the colour maps. In Sect. 5 we present the results for each galaxy, including comparison with available multiwavelength data. In Sect. 6 we discuss the relationship between Seyfert and starburst activity and the relevance of our results in the context of current unified AGN models. Conclusions are presented in Sect. 7. Throughout this paper we use Hubble constant of tex2html_wrap_inline2603 km stex2html_wrap_inline2605, and a deceleration parameter tex2html_wrap_inline2607.


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