SDSS-IV MaNGA: The intrinsic shape of slow rotator early-type galaxies

By inverting the distributions of galaxies' apparent ellipticities and misalignment angles (measured around the projected half-light radius $R_{\rm e}$) between their photometric and kinematic axes, we study the intrinsic shape distribution of 189 slow rotator early-type galaxies with stellar masses $2\times 10^{11} M_{\odot}<M_\ast<2\times 10^{12} M_{\odot}$, extracted from a sample of about 2200 galaxies with integral-field stellar kinematics from the DR14 of the SDSS-IV MaNGA IFU survey. Thanks to the large sample of slow rotators, Graham+18 showed that there is clear structure in the misalignment angle distribution, with two peaks at both $0^{\circ}$ and $90^{\circ}$ misalignment (characteristic of oblate and prolate rotation respectively). Here we invert the observed distribution from Graham+18. The large sample allows us to go beyond the known fact that slow rotators are weakly triaxial and to place useful constraints on their intrinsic triaxiality distribution (around $1R_{\rm e}$) for the first time. The shape inversion is generally non-unique. However, we find that, for a wide set of model assumptions, the observed distribution clearly requires a dominant triaxial-oblate population. For some of our models, the data suggest a hint for a minor triaxial-prolate population, but a dominant prolate population is ruled out.