The SAMI Galaxy Survey: Revisitng galaxy classification through high-order stellar kinematics

• Published in: The Astrophysical Journal
• Main Authors: van de Sande, J, Bland-Hawthorn, Joss, Fogarty, Lisa M R, Cortese, L, D'Eugenio, Francesco, Croom, Scott M, Scott, Nicholas, Allen, James T, Brough, Sarah, Bryant, Julia J., Cecil, Gerald, Colless, Matthew, Groves, Brent Allan, Ho, I-Ting, Leslie, Sarah, Medling, Anne, Sharp, Rob, Sweet, Sarah
• Format: Article in Journal/Newspaper
• Language: English
• Published: IOP Publishing
• Subjects: Lambda; sami
• Online Access: http://hdl.handle.net/1885/218515; https://doi.org/10.3847/1538-4357/835/1/104

Recent cosmological hydrodynamical simulations suggest that integral field spectroscopy can connect the high-order stellar kinematic moments h3 (~skewness) and h4 (~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 (${\lambda }_{{R}_{{\rm{e}}}}$) and ellipticity (${\epsilon }_{{\rm{e}}}$) 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 h3 versus $V/\sigma $ anti-correlation, whereas quasi-regular and non-regular rotators show a more vertical relation in h3 and $V/\sigma $. Motivated by recent cosmological simulations, we develop an alternative approach to kinematically classify galaxies from their individual h3 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 ${\lambda }_{{R}_{{\rm{e}}}}\mbox{--}{\epsilon }_{{\rm{e}}}$ values can show distinctly different ${h}_{3}\mbox{--}V/\sigma $ signatures. Class 5 objects are previously unidentified fast rotators that show a weak h3 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 h3 versus $V/\sigma $ as evidence for disks in most fast rotators, suggesting a dearth of gas-poor mergers among fast rotators.