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

• Published in: The Astrophysical Journal
• Main Authors: Sande, JVD, Bland-Hawthorn, J, Fogarty, LMR, Cortese, L, d’Eugenio, F, Croom, SM, Scott, N, Allen, JT, Brough, S, Bryant, JJ, Cecil, G, Colless, M, Couch, WJ, Davies, R, Elahi, PJ, Foster, C, Goldstein, G, Goodwin, M, Groves, B, Ho, I-T, Jeong, H, Jones, DH, Konstantopoulos, IS, Lawrence, JS, Leslie, SK, López-Sánchez, ÁR, McDermid, RM, McElroy, R, Medling, AM, Oh, S, Owers, MS, Richards, SN, Schaefer, AL, Sharp, R, Sweet, SM, Taranu, D, Tonini, C, Walcher, CJ, Yi, SK
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Institute of Physics 2018
• Subjects: sami
• Online Access: https://doi.org/10.3847/1538-4357/835/1/104; https://ora.ox.ac.uk/objects/uuid:ff540780-9b73-4ea0-afdb-fe227984a931

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 (λRe) and ellipticity (∈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 h3versus V/σ anti-correlation, whereas quasi-regular and non-regular rotators show a more vertical relation in h3and V/σ. Motivated by recent cosmological simulations, we develop an alternative approach to kinematically classify galaxies from their individual h3versus V/σ 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 λRe- ∈evalues can show distinctly different h3- V/σ signatures. Class 5 objects are previously unidentified fast rotators that show a weak h3versus V/σ 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 h3versus V/σ as evidence for disks in most fast rotators, suggesting a dearth of gas-poor mergers among fast rotators.