Prevalent elongated galaxies in the early Universe evidenced by stellar kinematics

TL;DR

This study uses stellar kinematics to show that early low-mass galaxies in the Universe were predominantly elongated prolate ellipsoids, indicating a fundamental morphological transition from spindle-like to disc-shaped galaxies over cosmic time.

Contribution

It provides the first decisive evidence supporting the prolate spindle morphology of high-redshift galaxies through stellar kinematics analysis.

Findings

Early Universe galaxies are mainly elongated prolate ellipsoids.

A dimensional transition from prolate to disc morphology occurred over cosmic time.

Supports the idea of morphological evolution in galaxy structure.

Abstract

The Universe is now extensively populated by discy galaxies with coherent galaxy-wise stellar rotation. This disc prevalence has been deemed a late-time phenomenon because the penetrating cold gaseous streams in the early Universe ($z\gtrsim 2$) fuel the star formation in galaxies too intensively to allow for thin disc formation. However, recent images taken by the James Webb Space Telescope (JWST) unveiled a prominent population of low-mass galaxies at high redshifts with flattened shapes, widely interpreted as early significance of discs given the well-established connection between flattening and discy morphology seen in the local Universe. It is noticed, on the other hand, that these galaxies show far more flattened systems than can be accounted for by randomly oriented oblate discs, and the axial ratio distributions are better explained by elongated prolate ellipsoids, an extremely rare spindle-like configuration at low redshifts. The true morphological nature of these early low-mass galaxies is fundamental to understanding the structure evolution of their discy descendants we see today, including our Milky Way. In this work, we discriminate the oblate disc and prolate spindle scenario by a decisive experiment with stellar kinematics at its core. The result clearly supports the prolate spindle scenario, and evidences an early Universe widely inhabited by linear stellar systems contrasting the current era dominated by planar discy galaxies, which suggests a dimensional transition in galactic structure over cosmic time.