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Astron. Astrophys. 325, 933-942 (1997)

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1. Introduction

Recent observations (and especially with HST) demonstrate that mutual interactions and mergers between galaxies at early stages of evolution of the Universe were probably among the main processes leading to the observed properties of galaxies (e.g., Keel 1996). Even at the present epoch, at least 5-10% of galaxies are members of interacting systems. Many other galaxies keep signs in their structure of past interactions and mergings (for example, elliptical and S0 galaxies with inclined gaseous disks, galaxies with faint shells and ripples, galaxies with kinematically decoupled nuclei, etc.).

Polar-ring galaxies (PRGs), consisting of large-scale rings of stars, gas and dust orbiting around major axes of early-type galaxies, may be considered as extreme samples of possible interaction relics. Indeed, in the case of PRGs, the remnants of merged galaxies are not mixed in one smooth object but stay separately in a quasi-steady state for a long time. PRGs are very rare objects. For example, the Polar Ring Catalogue by Whitmore et al. (1990) (PRC) lists only 6 classic kinematically-confirmed polar-ring galaxies.

The unique geometry of PRGs attracts the attention of astronomers trying to test the 3D shape of galactic potentials and to study the stability of inclined rings and disks (see recent review articles by Tohline 1990, Sackett 1991, Sparke 1991, Combes 1994, Cox & Sparke 1996). Such an important question as the origin of these peculiar objects it is still not adequately investigated. It is usually suggested that collapse of a single protogalactic cloud cannot create an object with two nearly-orthogonal large-scale systems (but see Curir & Diaferio 1994), and so some secondary event must occur in the history of PRGs.

Summarizing possible scenarios of polar-ring formation, one can enumerate the following: the accretion of matter from a nearby system or the capture and merging of a gas-rich companion; the delayed inflow of a primordial intergalactic cloud; the accretion of matter from the outskirts of the host galaxy itself; the polar-ring formation from the return of the tidal material during the merging of two gas-rich spirals (Toomre 1977, Shane 1980, Schweizer et al. 1983, Sackett 1991, Sparke 1991, Hibbard & Mihos 1995).

Probably, all the above mechanisms can create ring-like structures around early-type galaxies. To our mind, the most straightforward scenario is the first one. Recent observations of several binary interacting systems clearly demonstrate such rings in the making (for instance, NGC 7464/65 - Li & Seaquist 1994, NGC 3808A,B and NGC 6285/86 - Reshetnikov et al. 1996).

Another unclear question is the nature of central objects in PRGs and a possible correlation of host galaxy properties with characteristics of a surrounding polar ring.

In this paper, we present SPH simulations of polar ring formation around target galaxies of different structures due to gas accretion during the encounter with a comparable-mass spiral galaxy. In our simulations, we try to answer the following main questions: Does this mechanism work? What determines the size of the resulting ring, and what is its spatial structure? On what timescale does the ring form? What is the mass fraction of the gas captured into a ring?

The paper is organized as follows: in Sect. 2, we examine observational properties of all known kinematically confirmed PRGs and formulate some observational constraints on numerical simulations; in Sect. 3, we discuss previous attempts to model the PRGs formation, describe our modelling technique and results of simulations; and finally we give our conclusions in Sect. 4.

Throughout the paper, all distance-dependent quantities are calculated using [FORMULA] 75 km/s/Mpc.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998