The SAMI Galaxy Survey: Revisiting Galaxy Classification Through High-Order Stellar Kinematics

Abstract: 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 2D integral field data from the SAMI Galaxy Survey. A proxy 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 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. 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 h3-$V/\sigma$ signatures. Class 5 objects are previously unidentified fast rotators that show a weak h3 versus $V/\sigma$ anti-correlation. 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.

Revisiting Galaxy Classification Through High-Order Stellar Kinematics

The recent paper by van de Sande et al. presents an insightful exploration into galaxy classification using high-order stellar kinematic moments, specifically through the SAMI Galaxy Survey. The research utilizes the high-order moments $h_3$ (skewness) and $h_4$ (kurtosis), obtained from 315 galaxies without morphological bias, to examine the connection between these kinematics and the assembly history of galaxies, building on theories proposed by cosmological hydrodynamical simulations.

The authors effectively leverage the capabilities of the SAMI instrument, which provides two-dimensional integral field data. This approach allows for a nuanced classification of galaxies beyond the traditional morphology, instead using physical classifications such as spin parameter ($\lambda_{R_{\rm{e}}}$) and ellipticity ($\epsilon_{\rm{e}}$). The analysis segregates galaxies into fast and slow rotators and further examines their correspondence with regular and non-regular rotators.

A significant finding is the confirmation of a strong anti-correlation between $h_3$ and $V/\sigma$ for regular rotators, suggesting the presence of a disk. Non-regular rotators, however, show a more vertical $h_3$-$V/\sigma$ relation, indicating a different dynamical structure. Interestingly, the research identifies a novel classification of fast rotators—Class 5 objects—that exhibit a weak $h_3$-$V/\sigma$ anti-correlation. Such galaxies challenge the common narrative as they are predicted to be disk-less from simulations yet show signs of disks upon morphological examination.

Numerous model classes (Classes 1-5) are derived from high-order kinematic signatures, revealing complex dynamic patterns within the galaxy sample. These models extend the dialogue with cosmological models by demonstrating that even among fast rotators, there exists variation, some of which may be linked to late-stage gas-poor mergers, contrary to previous assertions.

Implications from this study impact both theoretical and observational astronomy. The findings suggest that high-order moments provide a richer understanding of a galaxy's merger history and dynamical state, which is pivotal in refining models of galaxy evolution. Moreover, it underscores the limitations of morphological classification and highlights the need for integral field spectroscopic surveys in comprehending the intricate nature of galaxies.

The study advocates for future investigations into binding high-order kinematic measurements with simulations that account for environmental dynamics and merger history more comprehensively. This approach could reduce discrepancies between observational data and theoretical predictions, refining the understanding of galaxy formation and evolution within the context of their large-scale structures.

In conclusion, van de Sande et al.'s work is a testament to the transformative role of integral field spectroscopy in astrophysics, pushing the boundaries of how galaxies are classified and understood within their cosmological context. This research stands as a foundation for further exploration into the kinematic intricacies of galaxy dynamics and their evolutionary pathways.