On the kinematic nature of apparent discs at high redshifts: Local counterparts are not dominated by ordered rotation but by tangentially anisotropic random motion

Abstract

It is not straightforward to physically interpret the apparent morphology of galaxies. Recent observations by James Webb Space Telescope (JWST) revealed a dominant galaxy population at high redshifts ($z>2$) that were visually classified as discs for their flattened shapes and/or exponential light profiles. The extensively accepted interpretation is that they are dynamically cold discs supported by bulk rotation. However, it is long known that flattened shapes and exponential profiles are not exclusive for rotating disc structure. To break degeneracy and assess the rotational support of typical high-$z$ galaxies in the JWST samples, those with active star formation and stellar masses $\mathrm{lg}(\mathcal{M}_{\star}/\mathcal{M}_{\odot})\sim9$, we study the kinematics of their equal-mass counterparts at $z=0$. While these local star-forming low-mass galaxies are photometrically similar to real dynamically cold discs, they are not supported by ordered rotation but primarily by random motion, and their flattened shapes result largely from tangential orbital anisotropy. Given the empirical and theoretical evidence that young galaxies are dynamically hotter at higher redshifts, our results suggest that the high-$z$ JWST galaxies may not be cold discs but are dynamically warm/hot galaxies with flattened shapes driven by anisotropy. While both having low rotational support, local low-mass galaxies possess oblate shapes, contrasting the prolate shapes (i.e. cigar-like) of low-mass systems at high redshifts. Such shape transition (prolate$\Rightarrow$oblate) indicates an associated change in orbital anisotropy (radial$\Rightarrow$tangential), with roots likely in the assembly of their host dark matter halos.