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

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6. Conclusions and discussion

We have found unique structure in the center of NGC 7217. The continuum and emission-line isophotes within [FORMULA] (240 pc) are elongated approximately in the north-south direction, and the dynamical major axis of the ionized gas within [FORMULA] also has [FORMULA]; and this coincidence shows a planar circular character of the gas rotation. However, the plane of this rotation is nearly orthogonal to the global plane of the galaxy which has a line of nodes in [FORMULA]. Interestingly, a similar elongation in the north-south direction is seen in the contours of the Lick [FORMULA] index distribution, whereas the magnesium-line index [FORMULA] decreases quite axisymmetrically with radius. Therefore, we suggest the existence of a polar circumnuclear stellar structure, which is iron overabundant with respect to the solar abundance ratio.

Magnesium overabundant stellar systems are born when the epoch of their star formation is shorter than one billion years. Stellar systems with a solar magnesium-to-iron ratio are thought to have long and continuous star formation. To obtain iron overabundant stellar systems, chemical evolution modelists have come up with a plausible scenario: the star formation must be bursty, with pauses for a few Gyrs between the bursts. But this suggests that in the central, polarly elongated stellar structure several, at least two, discrete star formation bursts have occured. This can be related to the structure of the global disk of the galaxy, which also consists of several (three) exponential segments with different slopes. Lin & Pringle (1987) have shown that the exponential density profile of a stellar disk is a natural product of dynamical evolution when the characteristic time of the star formation is comparable to the viscous gas re-distribution time, and its exponential scalelength is completely determined by the initial radius of the gaseous disk. With this idea in mind, we can propose the following scenario for the evolution of NGC 7217: in the first stage, the primordial gaseous disk was rather extended and a first stellar disk was formed with a usual scalelength, some 3 kpc. But soon a catastrophique event occured, throwing the gas suddenly toward the center by external interaction or internal instability, and the formation of the next part of the disk resulted in a smaller scalelength. The cause of this catastrophique event cannot be diagnosed in the present epoch: though the galaxy looks isolated now, it may have had small satellites some Gyrs ago which may be fully accreted after playing a role of external disturber. After a few Gyrs of quiescent star formation in the global disk of NGC 7217 a second catastrophique event occured, again gas flows into the center, and the exponential scalelength of the next, third generation component of the stellar disk (bulge?) is even smaller than that of the second. Every abrupt re-distribution of the gas along the radius had to be accompanied by a star formation burst in the center; if the temporal separation of two bursts was two to three Gyrs, this would result in iron overabundance in the central stellar concentration.

But if the central gaseous and stellar structures were formed from the gas of the global disk, why does the circumnuclear gas now rotate in the polar plane? This may be a consequence of the intrinsic dynamical evolution of the viscous subsystem. Sofue & Wakamatsu (1994)proposed that gas rotating in the plane with non-axisymmetric perturbation (e.g., a bar) passes through shocks at the edges of the bar and loses its tangential momentum first of all; it then flows into the center and there loses its radial velocity component, and finally only its vertical velocity remains unaffected. This would result in the occurence of circumnuclear gaseous polar rings (disks) in barred galaxies. Anantharamaiah & Goss (1996) reported such a ring in NGC 253, and we have found several other examples: in the Sb galaxy NGC 2841 (Sil'chenko et al. 1997b) and in the S0/a galaxy NGC 6340 (Sil'chenko 2000) the presence of a compact circumnuclear gaseous polar disks may be caused by the triaxiality of their bulges; in NGC 2841 we directly observe large-scale shocks at [FORMULA] kpc (Afanasiev & Sil'chenko 1999). In the S0 galaxy NGC 7280, which also possesses a circumnuclear gaseous polar disk, we also detected an intermediate-scale high-contrast bar with an asymmetric dust lane - a true signature of a shock (Afanasiev & Sil'chenko 2000). In NGC 7217 the clear evidence for the present or past existence of a bar is its three rings. Buta et al. (1995) have found a low-contrast bar (more exactly, a triaxial structure) in [FORMULA] by applying a Fourier analysis to a deep I-band image and showed that even such a weak perturbation may produce rings. Perhaps, this bar was stronger earlier: mass accumulation in the center had to provoke its dissolution, making it only a low-contrast feature in the present epoch. At the time when it was stronger, it could have stimulated gas inflow, accumulating it in the polar plane around the nucleus.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001