Galaxy morphology in CANDELS: addressing evolutionary changes across 0.2 ⩽ z ⩽ 2.4 with hybrid classification approach
I Kolesnikov, V M Sampaio, R. R. de Carvalho, Christopher J. Conselice · Monthly Notices of the Royal Astronomical Society · 2025
ABSTRACT Morphological classification of galaxies becomes increasingly challenging with redshift. We apply a hybrid unsupervised-supervised method to classify ${\sim} 14\,000$ galaxies in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey fields at $0.2 \le z \le 2.4$ into spheroid, disc, and irregular systems. Unlike previous works, our method is applied to redshift bins of width 0.2. Comparison between general versus bin-specific models applied to a wide redshift range and reveals significant differences in galaxy morphology beyond $z > 1$ and a consistent ${\sim} 25~{{\ \rm per\ cent}}$ disagreement. This suggests that using a single model across wide redshift ranges may introduce biases due to the large time intervals involved compared to galaxy evolution time-scales. Using the ferengi code to assess the impact of cosmological effects, we find that flux dimming and smaller angular scales may lead to the misclassification of up to 18 per cent of disc galaxies as spheroids or irregulars. Contrary to previous studies, we find an almost constant fraction of discs (${\sim} 60~{{\ \rm per\ cent}}$) and spheroids (${\sim} 30~{{\ \rm per\ cent}}$) across redshifts. We attribute discrepancies with earlier works, which suggested a decreasing fraction of discs beyond $z = 1$, to the biases introduced by visual classification. Our claim is further strengthened by the striking agreement to the results reported by Lee et al. using an objective, unsupervised method applied to JWST data. Exploring mass dependence, we observe an ${\sim} 40~{{\ \rm per\ cent}}$ increase in the fraction of massive ($M_{\rm stellar} \ge 10^{10.5}\ {\rm M}_{\odot }$) spheroids with decreasing redshift, well balanced with a decrease of ${\sim} 20~{{\ \rm per\ cent}}$ in the fraction of $M_{\rm stellar} \ge 10^{10.5}$ discs, suggesting that merging massive disc galaxies may form spheroidal systems.