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For refcode 1990A&A...236...25R:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1990A&A...236...25R The environment and the origin of twisting in early-type galaxies R. Rampazzo and L. M. Buson Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, I-35122 Padova, Italy Received September 11, 1989; accepted February 15, 1990 Abstract. We investigate the role of the environment on the origin of the isophotal twisting in early-type galaxies. To study the effect of random collisions in groups or clusters we firstly correlate the galaxy number density {rho}[Mpc^-3^] with the maximum twisting and ellipticity of each galaxy as deduced from the recent data in the literature. Secondly, we analyse stationary effects in a quasi-constant tidal field by studying the projected geometry of a set of isolated physical pairs. The main results of the former analysis are the following: (i) within formal errors, the percentage of twisted Es and S0s appears independent of the local density from field (log{rho} < - 2.5) to cluster objects (log{rho} > - 1.5); (ii) whatever the local density, the percentage of twisted Es is in fairly good agreement with the value (72%) given by Fasano and Bonoli (1989a) for isolated galaxies; (iii) whatever the environment there is an indication that S0s are less twisted than Es (iv) the relation found by Galletta (1980), showing that flattened objects are less twisted, is confirmed. The analysis of isolated, not hierarchical pairs suggests that the geometrical phenomenology observed is not merely explainable as the product of mutual tidal perturbations. In effect, though the size of our observed sample is exiguous in comparison with that one coming from the literature, the percentage of twisted objects in pairs is similar to that found in the more general sample. In addition, it is not unusual to find pairs in which twisting is negligible in one component, while it is conspicuous in the other. Finally, the two components of E + E pairs show the tendency to have a similar maximum ellipticity, as opposed to hierarchical pairs. Key words: galaxies: elliptical - galaxies: lenticular - galaxies: structure of
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