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Description
Recent cosmological hydrodynamical simulations suggest that integral field spectroscopy can connect the high-order stellar kinematic moments h_3_ (~skewness) and h_4_ (~kurtosis) in galaxies to their cosmological assembly history. Here, we assess these results by measuring the stellar kinematics on a sample of 315 galaxies, without a morphological selection, using two-dimensional integral field data from the SAMI Galaxy Survey. Proxies for the spin parameter ({lambda}R_e_) and ellipticity ({epsilon}_e_) are used to separate fast and slow rotators; there exists a good correspondence to regular and non-regular rotators, respectively, as also seen in earlier studies. We confirm that regular rotators show a strong h_3_ versus V/{sigma} anti-correlation, whereas quasi-regular and non-regular rotators show a more vertical relation in h_3_ and V/{sigma}. Motivated by recent cosmological simulations, we develop an alternative approach to kinematically classify galaxies from their individual h_3_ versus V/{sigma} signatures. Within the SAMI Galaxy Survey, we identify five classes of high-order stellar kinematic signatures using Gaussian mixture models. Class 1 corresponds to slow rotators, whereas Classes 2-5 correspond to fast rotators. We find that galaxies with similar {lambda}R_e_-{epsilon}_e_ values can show distinctly different h_3_-V/{sigma} signatures. Class 5 objects are previously unidentified fast rotators that show a weak h_3_ versus V/{sigma} anti-correlation. From simulations, these objects are predicted to be disk-less galaxies formed by gas-poor mergers. From morphological examination, however, there is evidence for large stellar disks. Instead, Class 5 objects are more likely disturbed galaxies, have counter-rotating bulges, or bars in edge-on galaxies. Finally, we interpret the strong anti-correlation in h_3_ versus V/{sigma} as evidence for disks in most fast rotators, suggesting a dearth of gas-poor mergers among fast rotators.
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