The ordered nature of elliptical galaxies - Implications for their intrinsic angular momenta and shapes

Abstract

The observations of rotation along the major and minor axes of 38 elliptical galaxies are analyzed to determine their intrinsic structure. Rotation along the minor axis occurs (i) as a result of projection effects in triaxial systems and/or (ii) because of real misalignment of the angular momentum axis from the intrinsic short axis, as is allowed in triaxial systems. The intrinsic angular momentum can point anywhere in the plane containing the short and the long axis. The distribution of apparent misalignments is a function only of the triaxiality T of a galaxy and its intrinsic misalignment, and is independent of the flattening of the galaxy in the plane containing the long and the short axis. The observed misalignment of the rotation axes and the short axes ranges from 0^deg^ to 90^deg^, with most ellipticals having misalignments that are small. This is not expected if ellipticals are generically triaxial systems with uniformly distributed angular momentum direction, and requires that particular constraints be imposed during the formation process. Some specific galaxies have dynamical subsystems with observed angular momenta that are offset 90^deg^ from the angular momenta of the main part of the galaxy. These galaxies are probably rotating about their long axis in the outer parts. The analysis of the observed misalignments requires us to derive the intrinsic ellipticity distribution for ellipticals. The resulting distribution, based on CCD data, shows a pronounced lack of round galaxies, unlike that previously derived from catalogs. The distribution of triaxiality and intrinsic misalignment of the angular momentum cannot be derived uniquely, since one observable (apparent misalignment) is used to determine two internal parameters (triaxiality and intrinsic misalignment). Various assumptions regarding the intrinsic shapes and the direction of the angular momentum were used. An exploration of the solution space showed a wide range of valid distributions, ranging from models with only nearly oblate shapes, oblate and prolate shapes, to distributions with only triaxial shapes, with varying distributions of intrinsic misalignments. However, all the models have a sizable fraction of galaxies (>= 35%) with intrinsic misalignments less than 15^deg^. The results were compared with predictions from several groups for hierarchical formation scenarios. The predicted distributions for halos are skewed significantly toward prolate-triaxial shapes. Our data show that the luminous parts of ellipticals do not have such high triaxialities. The shapes of the dark halos may well be different from the shapes of the luminous matter in ellipticals, despite their similar dynamics. This may result, for example, from the effect of gaseous dissipation.