The Atlas3D project - XXIV. The intrinsic shape distribution of
  early-type galaxies
  We use the Atlas3D sample to perform a study of the intrinsic shapes of
early-type galaxies, taking advantage of the available combined photometric and
kinematic data. Based on our ellipticity measurements from the Sloan Digital
Sky Survey Data Release 7, and additional imaging from the Isaac Newton
Telescope, we first invert the shape distribution of fast and slow rotators
under the assumption of axisymmetry. The so-obtained intrinsic shape
distribution for the fast rotators can be described with a Gaussian with a mean
flattening of q=0.25 and standard deviation sigma_q = 0.14, and an additional
tail towards rounder shapes. The slow rotators are much rounder, and are well
described with a Gaussian with mean q = 0.63 and sigma_q =0.09. We then checked
that our results were consistent when applying a different and independent
method to obtain intrinsic shape distributions, by fitting the observed
ellipticity distributions directly using Gaussian parametrisations for the
intrinsic axis ratios. Although both fast and slow rotators are identified as
early-type galaxies in morphological studies, and in many previous shape
studies are therefore grouped together, their shape distributions are
significantly different, hinting at different formation scenarios. The
intrinsic shape distribution of the fast rotators shows similarities with the
spiral galaxy population. Including the observed kinematic misalignment in our
intrinsic shape study shows that the fast rotators are predominantly
axisymmetric, with only very little room for triaxiality. For the slow rotators
though there are very strong indications that they are (mildly) triaxial.
