The SAMI Galaxy Survey: the intrinsic shape of kinematically selected galaxies

TL;DR

This study uses stellar kinematic maps from the SAMI Galaxy Survey to infer the intrinsic shapes of galaxies, revealing correlations between shape, rotational support, and galaxy formation histories.

Contribution

It introduces an optimized algorithm for determining galaxy shapes from kinematic data, improving upon previous imaging-based methods and applying it to new and existing datasets.

Findings

Most galaxies are oblate axisymmetric.

Low spin galaxies tend to be triaxial.

High spin galaxies are more flattened and axisymmetric.

Abstract

Using the stellar kinematic maps and ancillary imaging data from the Sydney AAO Multi Integral field (SAMI) Galaxy Survey, the intrinsic shape of kinematically-selected samples of galaxies is inferred. We implement an efficient and optimised algorithm to fit the intrinsic shape of galaxies using an established method to simultaneously invert the distributions of apparent ellipticities and kinematic misalignments. The algorithm output compares favourably with previous studies of the intrinsic shape of galaxies based on imaging alone and our re-analysis of the ATLAS3D data. Our results indicate that most galaxies are oblate axisymmetric. We show empirically that the intrinsic shape of galaxies varies as a function of their rotational support as measured by the "spin" parameter proxy Lambda_Re. In particular, low spin systems have a higher occurrence of triaxiality, while high spin systems are more intrinsically flattened and axisymmetric. The intrinsic shape of galaxies is linked to their formation and merger histories. Galaxies with high spin values have intrinsic shapes consistent with dissipational minor mergers, while the intrinsic shape of low-spin systems is consistent with dissipationless multi-merger assembly histories. This range in assembly histories inferred from intrinsic shapes is broadly consistent with expectations from cosmological simulations.