The MASSIVE Survey - X. Misalignment between kinematic and photometric axes and intrinsic shapes of massive early-type galaxies

Abstract

We use spatially resolved two-dimensional stellar velocity maps over a 107 x 107 arcsec(2) field of view to investigate the kinematic features of 90 early-type galaxies above stellar mass 10(11.5)M(circle dot) in the MASSIVE survey. We measure the misalignment angle psi between the kinematic and photometric axes and identify local features such as velocity twists and kinematically distinct components. We find 46 per cent of the sample to be well aligned (psi < 15 degrees), 33 per cent misaligned, and 21 per cent without detectable rotation (non-rotators). Only 24 per cent of the sample are fast rotators, the majority of which (91 per cent) are aligned, whereas 57 per cent of the slow rotators are misaligned with a nearly flat distribution of psi from 15 degrees to 90 degrees. 11 galaxies have psi greater than or similar to 60 degrees and thus exhibit minor-axis (’prolate’) rotation in which the rotation is preferentially around the photometric major axis. Kinematic misalignments occur more frequently for lower galaxy spin or denser galaxy environments. Using the observed misalignment and ellipticity distributions, we infer the intrinsic shape distribution of our sample and find that MASSIVE slow rotators are consistent with being mildly triaxial, with mean axis ratios of b/a = 0.88 and c/a = 0.65. In terms of local kinematic features, 51 per cent of the sample exhibits kinematic twists of larger than 20 degrees, and two galaxies have kinematically distinct components. The frequency of misalignment and the broad distribution of psi reported here suggest that the most massive early-type galaxies are mildly triaxial, and that formation processes resulting in kinematically misaligned slow rotators such as gas-poor mergers occur frequently in this mass range.