THE SAMI GALAXY SURVEY: REVISITING GALAXY CLASSIFICATION THROUGH HIGH-ORDER STELLAR KINEMATICS

Abstract

Recent cosmological hydrodynamical simulations suggest that integral field\nspectroscopy can connect the high-order stellar kinematic moments h3\n(~skewness) and h4 (~kurtosis) in galaxies to their cosmological assembly\nhistory. Here, we assess these results by measuring the stellar kinematics on a\nsample of 315 galaxies, without a morphological selection, using 2D integral\nfield data from the SAMI Galaxy Survey. A proxy for the spin parameter\n($\\lambda_{R_e}$) and ellipticity ($\\epsilon_e$) are used to separate fast and\nslow rotators; there exists a good correspondence to regular and non-regular\nrotators, respectively, as also seen in earlier studies. We confirm that\nregular rotators show a strong h3 versus $V/\\sigma$ anti-correlation, whereas\nquasi-regular and non-regular rotators show a more vertical relation in h3 and\n$V/\\sigma$. Motivated by recent cosmological simulations, we develop an\nalternative approach to kinematically classify galaxies from their individual\nh3 versus $V/\\sigma$ signatures. We identify five classes of high-order stellar\nkinematic signatures using Gaussian mixture models. Class 1 corresponds to slow\nrotators, whereas Classes 2-5 correspond to fast rotators. We find that\ngalaxies with similar $\\lambda_{R_e}-\\epsilon_e$ values can show distinctly\ndifferent h3-$V/\\sigma$ signatures. Class 5 objects are previously unidentified\nfast rotators that show a weak h3 versus $V/\\sigma$ anti-correlation. These\nobjects are predicted to be disk-less galaxies formed by gas-poor mergers. From\nmorphological examination, however, there is evidence for large stellar disks.\nInstead, Class 5 objects are more likely disturbed galaxies, have\ncounter-rotating bulges, or bars in edge-on galaxies. Finally, we interpret the\nstrong anti-correlation in h3 versus $V/\\sigma$ as evidence for disks in most\nfast rotators, suggesting a dearth of gas-poor mergers among fast rotators.\n