The intrinsic shapes of galactic discs

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Abstract
There are several pieces of evidence suggesting that the discs of spiral and S0 galaxies are generally slightly triaxial. Starting from the observed flattenings, in this paper we derive the distribution function f(ζ ≡ b/a) assuming a Gaussian distribution g(ξ ≡ ca), where a > b > c are the intrinsic axes. A volume-limited sample of 1064 objects, selected from the RC3 catalogue on the basis of the distances relative to the Virgo cluster published in the Kraan-Korteweg catalogue, is used. Distribution functions are derived respectively for S0 and spiral galaxies. The latter are also subdivided into two subsamples, ‘early- and late-type spirals’. Our main results can be summarized as follows: we find broad intrinsic distribution functions of f(ζ) for both lenticular and spiral galaxies, peaked at b/a in the range 0.8–0.9. In particular, the early-type spiral sample shows a peak at a b/a of almost 0.8, independently of the presence of a ‘disturbing’ barred component that is found to play a significant role in the late-type sample. Our distribution is shown to be consistent with that recently obtained by Bertola, Vietri & Zeilinger for a sample of spiral and S0 galaxies.

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