Beyond the Disc--Spheroid Dichotomy at $0.6 < z < 8.0$: Evidence for a Morphological Indeterminacy Regime from JWST

Abstract

Galaxy morphology is a fundamental tracer of galaxy assembly, but its characterization becomes increasingly uncertain in the early Universe. We investigate morphological classifications across different methodologies over the redshift range $0.6 < z < 8.0$, using JWST observations of 15,567 galaxies with 6,099 synthetic galaxies from IllustrisTNG. To quantify morphology without relying on visual labels or prescribed light profiles, we use an unsupervised-supervised hybrid machine-learning approach based on the non-parametric MEGG system, which combines the second moment of the light distribution, Entropy, Gini, and Gradient Pattern Analysis. We find that parametric and non-parametric classifications remain broadly consistent at $z < 2.5$, but diverge systematically at higher redshift. We identify this epoch as the Morphological Indeterminacy Regime (MIR), in which classical disc and spheroidal categories no longer provide a unique description of the galaxy population. The fraction of mixed-interpretation galaxies increases with redshift, reaching $F_{\rm MIR} \sim 0.55$ at $z > 2.5$. Cross-domain and projection-invariance tests show that the classifications are stable and insensitive to data set and viewing angle. In the interval $2.5 < z < 8.0$, MIR galaxies occupy an intermediate stellar-mass regime (median; $\log(M_\star/M_\odot) \sim 9.7$), between low-mass discs (median; $\log(M_\star/M_\odot) \sim 9.5$) and massive spheroids (median; $\log(M_\star/M_\odot) \sim 10.1$), while irregular systems have stellar-mass distributions similar to those of MIR galaxies. We interpret these results as evidence that morphological indeterminacy is a characteristic feature of galaxy assembly in the early Universe, reflecting a population of dynamically young systems that have not yet settled into the classical Hubble sequence.