Morphology-spin connection in the SAMI Galaxy Survey

The spin parameter lambda is a proxy for the specific stellar angular momentum of galaxies and is a useful metric for classifying kinematic morphology. This study aims to quantify the relative importance of galaxy properties in explaining lambda, using data from the Sydney-AAO Multi-object Integral-field spectrograph (SAMI) Galaxy Survey. We apply partial correlation analysis and partial least squares regression to assess the relative contributions of different parameters in explaining lambda. We find that morphology indicators, bulge-to-total ratio within one effective radius (B/T) and ellipticity (epsilon), show the strongest correlations with lambda and play a leading role in the regression analysis. This result statistically confirms the established fast-rotator sequence, in which fast-rotating early-type galaxies form a continuous structural and kinematic sequence with spiral galaxies, with lambda decreasing as bulge prominence increases. The light-weighted age and stellar mass also exhibit significant correlations, but their contributions are secondary to the morphology indicators in multivariate analyses. We also examine whether the observed trends in lambda can be reproduced using galaxy properties alone. The morphology indicators (B/T, epsilon) reproduce the overall distribution of observed lambda with a scatter of about 0.12, while the inclusion of Age LW and M provides only modest additional improvement. However, these relations do not reproduce the slow-rotator regime well. Overall, our results show that photometric structural parameters best explain lambda and suggest that statistical inference of galaxy spin from non-IFS observables may become feasible with improved models and a broader set of parameters.