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Astron. Astrophys. 330, 412-418 (1998)

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1. Introduction

According to Whitmore et al. (1990), polar-ring galaxies are mostly S0 galaxies with luminous rings whose projected major axes are nearly orthogonal to the galactic main body major axes. This special class of galaxies seems to provide additional possibilities to reveal three-dimensional structure of early-type flattened galaxies; however even for a nearby prototype of the polar-ring galaxies - the famous NGC 2685 - there are different points of view on its real spatial configuration. On the one hand, the exponential photometric profile (Makarov et al. 1989, Whitmore et al. 1990) and the rapid rotation of stars around the projected minor axis (Schechter & Gunn 1978) provide evidence for an ordinary axisymmetric disk structure; on the other hand, attempts of gas-dynamical modeling of the two long-lived stable rings, one polar and one equatorial as seen in NGC 2685, favour a prolate tumbling model (Peletier & Christodoulou 1993). An analysis of the two-dimensional velocity field may help to distinguish between these alternatives, but until now only long-slit cross-sections along the major and minor axes have been obtained. By measuring an emission line [OII] [FORMULA], Ulrich (1975) had found that there was seen a line-of-sight velocity trend along the minor axis, and Schechter & Gunn (1978) had confirmed this finding, detecting however strong variations of stellar velocities along the major axis. Shane (1980) had suggested that circumnuclear gaseous condensation is related to neutral hydrogen of the polar ring which circularly rotates in the plane orthogonal to the plane of the galactic disk. Recently a similar long-slit study is made for another polar-ring galaxy, IC 1689 (Hagen-Thorn & Reshetnikov 1997); and again, the maximum velocity gradients are found for ionized gas along the minor axis and for stars along the major axis. Though this picture does not contradict a concept of S0 galaxies with accreted gas in the polar planes, a absence of full two-dimensional kinematical analysis and a marginal presence of stellar line-of-sight velocity variations along the minor axis in NGC 2685 (Schechter & Gunn 1978) leave a chance for prolate or triaxial models of polar-ring galaxies.

We had one more reason to undertake 2D spectroscopy of the central parts of NGC 2685 and IC 1689. During our search for chemically decoupled stellar nuclei in nearby spiral galaxies we have found that in central 400 pc of the isolated regular Sb galaxy NGC 2841 possessing a chemically decoupled nucleus its ionized gas rotates orthogonally to rotation of the stellar component (Sil'chenko et al. 1997). It looks like a kind of gaseous polar disk. This finding has encouraged us to search for chemically decoupled nuclei in "bona fide" polar-ring galaxies. So NGC 2685 and IC 1689 were included into our observational program.

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998