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Astron. Astrophys. 364, 479-490 (2000)

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1. Introduction

A careful investigation of the inner parts in normal spiral galaxies with modern techniques reveals almost always a complicated structure which cannot be described by a classical two-component model consisting of a centrally concentrated, old, red, smooth de Vaucouleurs' bulge and a more extended, young, blue exponential disk with spiral arms. Among recent findings which may have serious implications for a global evolutionary picture we can mention papers on exponential bulges (Andredakis et al. 1995, Courteau et al. 1996), multi-tier disks - e.g. in NGC 5533 (Sil'chenko et al. 1997a) or in NGC 157 (Ryder et al. 1998), decoupled circumnuclear stellar disks (e.g. in NGC 4594, Emsellem et al. 1996), inner polar gaseous disks (e.g. in NGC 2841, Sil'chenko et al. 1997b), circumnuclear spiral arms (e.g. in NGC 5248, Laine et al. 1999, or in NGC 488, Sil'chenko 1999), etc. In some early-type spiral galaxies, e.g. in NGC 4138 (Jore et al. 1996) and in NGC 7217 (Merrifield & Kuijken 1994), the existence of counterrotating stars in their global disks was claimed. This may mean that discrete catastrophic events, such as minor mergers or gas accretion from a satellite, govern the evolution of even quite regular galaxies. But there may also exist some intrinsic evolutionary processes, such as bar formation and dissolution due to disk instabilities or mass concentration in the center, that can provide the same set of properties without any external action. We cannot yet prove either of these hypotheses because of the lack of detailed investigations of even nearby galaxies. So any new, careful analysis of the dynamics, structure, and stellar populations in normal spiral galaxies gives an additional chance to understand their evolution.

Here, we present a detailed study of the regular isolated Sab galaxy NGC 7217, the global parameters of which are given in Table 1. The galaxy has a prominent bulge and a truly flocculent spiral structure: unlike some other flocculent galaxies, e.g. NGC 2841 (Block et al. 1996) or NGC 5055 (Thornley & Mundy 1997) which unveil grand design in the NIR K-band, NGC 7217 preserves its non-wave appearance even at [FORMULA] (Elmegreen et al. 1999). But it has another puzzling morphological feature, namely, three stellar rings at radii of [FORMULA], [FORMULA], and [FORMULA], the innermost of which is accompanied by H[FORMULA] emission enhancement and the outermost also by HI concentration, so both are sites of intense star formation (Buta et al. 1995). Such star forming rings are usually treated as resonance loci of a bar. But morphologically NGC 7217 is a purely unbarred galaxy; besides, a bar should produce a wave spiral pattern which is not observed in this galaxy. Buta et al. (1995) tried to solve this problem, through a Fourier analysis of the I-band image of NGC 7217, and found a mode m=2; the conclusion was that the galaxy has a low-contrast bar aligned along [FORMULA]. Perhaps, this bar is observed in the late stage of dissolution: as Athanassoula (1996) has discussed, the rings are more long-lived than the bars, which may be destroyed by a central mass concentration. But the most striking peculiarity has been found by Merrifield & Kuijken (1994): they have claimed the existence of a counterrotating stellar disk. More exactly, from the analysis of stellar line-of-sight velocity distribution (LOSVD) data they noted a counterrotating "wing" over the full radius range, which they have studied, namely from [FORMULA] to [FORMULA]. This "wing" contains 30% of all stars and does not become weaker with radius; so they conclude that it is not a bulge, but a counterrotating fraction of the global disk. Immediately, the question has been raised about secondary gas infall or a minor merger, though the galaxy looks quite isolated.


Table 1. Global parameters of NGC 7217

We have already studied this interesting galaxy (Zasov & Sil'chenko 1997): from the ionized gas kinematics in the circumnuclear region and from an analysis of the central isophotes we have detected an inclined gaseous disk in the very center of NGC 7217 and reported a mass concentration of [FORMULA] in its nucleus. During the last years our understanding was growing on how the phenomena of chemically distinct galactic cores (Sil'chenko et al. 1992) and kinematically decoupled gaseous or stellar subsystems in disk galaxies may be related, both being produced by a secondary gas accretion or minor merger event. For example, in NGC 2841 we have found a chemically distinct nucleus, a circumnuclear polar disk of ionized gas with a radius of 200 pc, and counterrotating stars in the bulge (Sil'chenko et al. 1997b, Afanasiev & Sil'chenko 1999). The similarity between the gas and stellar kinematics in NGC 2841 and NGC 7217 has stimulated us to search for a chemically distinct nucleus in the latter galaxy. The results of this search, together with additional kinematical and photometric analyses, are presented in this paper.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001