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

Jesse Van De Sande, Joss Bland-Hawthorn, Lisa M. R. Fogarty, Luca Cortese, Francesco d'Eugenio, Scott M. Croom, Nicholas Scott, James T. Allen, Sarah Brough, Julia J. Bryant, Gerald Cecil, Matthew Colless, Warrick J. Couch, Roger Davies, Pascal J. Elahi, Caroline Foster, Gregory Goldstein, Michael Goodwin, Brent Groves, I-Ting HoHyunjin Jeong, D. Heath Jones, Iraklis S. Konstantopoulos, Jon S. Lawrence, Sarah K. Lee, Ángel López-Sánchez, Richard M. McDermid, Rebecca McElroy, Anne M. Medling, Sree Oh, Matt S. Owers, Samuel N. Richards, Adam L. Schaefer, Rob Sharp, Sarah M. Sweet, Dan Taranu, Chiara Tonini, C. Jakob Walcher, Sukyoung K. Yi

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    ecent 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 h3 versus V/σ anti-correlation, whereas quasi-regular and non-regular rotators show a more vertical relation in h3 and V/σ. Motivated by recent cosmological simulations, we develop an alternative approach to kinematically classify galaxies from their individual h3 versus 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-e values can show distinctly different h3-V/σ signatures. Class 5 objects are previously unidentified fast rotators that show a weak h3 versus 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 h3 versus V/σ as evidence for disks in most fast rotators, suggesting a dearth of gas-poor mergers among fast rotators.
    Original languageEnglish
    Article number104
    Pages (from-to)1-35
    Number of pages35
    JournalAstrophysical Journal
    Issue number1
    Publication statusPublished - 20 Jan 2017

    Bibliographical note

    Copyright The American Astronomical Society. First published in Astrophysical Journal, 835(1), 104. The original publication is available at, published by IOP Publishing. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.


    • cosmology: observations
    • galaxies: evolution
    • galaxies: formation
    • galaxies: kinematics and dynamics
    • galaxies: stellar content
    • galaxies: structure


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