The MAGPI Survey: massive slow rotator population in place by z ∼ 0.3

We use the ‘Middle Ages Galaxy Properties with Integral field spectroscopy’ (MAGPI) survey to investigate whether galaxies have evolved in the distribution of their stellar angular momentum in the past 3–4 Gyr, as probed by the observational proxy for spin, λ_R. We use 2D stellar kinematics to measure λ_R along with detailed photometric models to estimate galaxy ellipticity. The combination of these measurements quantifies the kinematic classes of ‘fast rotators’ and the rarer ‘slow rotators’, which show no regular rotation in their line-of-sight velocity fields. We compare 51 MAGPI galaxies with log₁₀ (M_*/M_☉) > 10 to carefully drawn samples of MaNGA galaxies in the local Universe, selected to represent possible descendants of the MAGPI progenitors. The EAGLE simulations are used to identify possible evolutionary pathways between the two samples, explicitly accounting for progenitor bias in our results and the varied evolutionary pathways a galaxy might take between the two epochs. We find that the occurrence of slow rotating galaxies is unchanged between the MAGPI (z ∼ 0.3) and MaNGA (z ∼ 0) samples, suggesting the massive slow rotator population was already in place ∼4 Gyr ago and has not accumulated since. There is a hint of the MAGPI sample having an excess of high λ_R galaxies compared to the MaNGA sample, corresponding to more ordered rotation, but statistically the samples are not significantly different. The large-scale stellar kinematics, as quantified through the λ_R parameter, of galaxies at z ∼ 0.3 have already evolved into the diversity of structures seen today in the local Universe.