A massive, evolved slow-rotating galaxy in the early Universe

In today's Universe, most galaxies are rotationally supported against gravity. However, a small fraction of the most massive galaxies which are no longer forming stars (i.e., they are quiescent) are dispersion supported, and are termed 'slow-rotators.' These galaxies, which are highly evolved and often exist in dense cluster environments, are theorized to be formed by a history of merger activity that on average decreases the angular momentum of the merged system and disturbs any disk structure. At high redshift, such extreme slow-rotating systems are predicted to be far less common. While observations of massive high-redshift galaxies which enable kinematic studies are challenging to obtain, all previous data have revealed systems that are rotating rapidly. No slow-rotators have been confirmed from stellar kinematics beyond z~2. In this work we present results from James Webb Space Telescope near-infrared integral field spectroscopy of XMM-VID1-2075, a massive, quiescent galaxy at z=3.449 with a morphology exhibiting clear low-surface brightness asymmetries and with a measured spin parameter of $λ_{R_e} = 0.101 \pm 0.018$, consistent with the lack of rotation seen in slow rotators in the local Universe. Taken together, these lines of evidence suggest that merger activity played a key role in the formation and kinematic transformation of some of the most massive galaxies when the Universe was <2 Gyr old, much earlier than previously observed.