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Astron. Astrophys. 326, 907-914 (1997)

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1. Introduction

The S0s with polar rings (Schweizer et al. 1983 ) are galaxies whose peculiarities have been originated by the accretion of matter from outside. They show a luminous ring, composed of gas, dust and stars, encircling the stellar body in polar orbits. The ring may exhibit a knotty appearance and blue colors (e.g. NGC 4650A) or a smooth aspect and red colors (e.g. UGC 7576). The latest and widest compilation of cases of polar ring galaxies has been made by Whitmore et al.  (1990). In this catalogue (here and after called PRC) they discuss the origin of such structures, estimating that about the 5% of all S0 galaxies has had a polar ring in the past, or has one now. The external origin of the ring is explained by the fact that there is no natural way for internal gas to set into a polar orbit and that the large quantities of HI detected ([FORMULA]) are very unusual for early-type galaxies.

The origin and stability of polar ring galaxies are still a matter of discussion. The current hypotheses foresee an origin linked to an environment which should be different from that of normal galaxies. The S0s could have "cannibalized" a gas-rich companion (Quinn 1991 , Steiman-Cameron 1991 ) or could have accreted cold gas on polar orbits from a massive disk galaxy, through a mass transfer during a close encounter (Toomre & Toomre 1972 ). Both these mechanisms are more frequent in an environment rich in satellites or in nearby galaxies. The PR may alternatively have accreted surrounding primeval gas, possibly from a gas cloud (Shane 1980 ). The stability of polar rings also shows different scenarios, as the present theories furnish different evolutionary times: some gas-dynamics simulations have calculated quite a fast evolution of the order of 108 yr (Steiman-Cameron 1991 ), while some N-body models foresee long formation time-scales and slow evolution (Rix & Katz 1991 ) or even dynamical equilibrium (Sparke 1986 , Arnaboldi & Sparke 1994 ). In the first cases the rings now observed must be all young, unstable structures; on the contrary if the evolutionary time scale is large or the ring is in equilibrium, most rings, or even all of them, may be old structures.

We present here a study oriented to point out the differences, if any, between the environment of PRs with respect to normal galaxies, in order to discriminate among the above different hypotheses. We engaged two kinds of approach: i) A statistical analysis of the objects detected in the sky region surrounding the polar ring; ii) A survey of the galaxies with similar magnitude and red-shift as the central object (PR or normal galaxy) in a wider region of sky.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998