The MASSIVE Survey - X. Stellar Velocity Features and Misalignment between Kinematic and Photometric Axes in Massive Early-Type Galaxies

Irina Ene, Chung-Pei Ma, Melanie Veale, Jenny E. Greene, Jens Thomas, John P. Blakeslee, Jonelle L. Walsh, Jennifer Ito

Published 2018-01-31Version 1

We use spatially resolved two-dimensional stellar velocity maps over a $107''\times 107''$ field of view to investigate the kinematic features of 90 early-type galaxies above stellar mass $10^{11.5}M_\odot$ 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% of the sample to be well aligned ($\Psi < 15^{\circ}$), 33% misaligned, and 21% without detectable rotation (non-rotators). Only 24% of the sample are fast rotators, the majority of which (91%) are aligned, whereas 57% of the slow rotators are misaligned with a nearly flat distribution of $\Psi$ from $15^{\circ}$ to $90^{\circ}$. We find that 11 galaxies have $\Psi \gtrsim 60^{\circ}$ and thus exhibit minor-axis rotation (or "prolate" rotation) in which the rotation is preferentially around the photometric major axis. We find kinematic misalignment to occur more frequently for higher stellar mass, lower galaxy spin, lower ellipticity, or denser galaxy environments. In terms of local kinematic features, 51% of the sample exhibit kinematic twists of larger than $20^{\circ}$, and 2 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 likely to be at least mildly triaxial, and the formation processes resulting in kinematically misaligned slow rotators such as gas-poor mergers occur frequently in this mass range.