The redshift evolution of the S0 fraction for $z<1$ in COSMOS

TL;DR

This study uses deep learning to classify galaxy morphologies in the COSMOS survey, revealing how the fraction of S0 galaxies evolves with redshift and mass, and identifying two distinct S0 populations.

Contribution

It introduces a transfer learning approach for high-redshift galaxy classification, providing new insights into S0 galaxy evolution and bimodal mass distribution.

Findings

S0 fraction increases as redshift decreases.

Two distinct S0 populations identified: high-mass red and low-mass blue.

High-mass S0s evolve earlier than low-mass S0s.

Abstract

Lenticular (S0) galaxies are galaxies that exhibit a bulge and disk component, yet lack any clear spiral features. With features considered intermediary between spirals and ellipticals, S0s have been proposed to be a transitional morphology, however their exact origin and nature is still debated. In this work, we study the redshift evolution of the S0 fraction out to $z \sim 1$ using deep learning to classify F814W ($i$-band) HST-ACS images of 85,378 galaxies in the Cosmological Evolution Survey (COSMOS). We classify galaxies into four morphological categories: elliptical (E), S0, spiral (Sp), and irregular/miscellaneous (IrrM). Our deep learning models, initially trained to classify SDSS images with known morphologies, have been successfully adapted to classify high-redshift COSMOS images via transfer learning and data augmentation, enabling us to classify S0s with superior accuracy. We find that there is an increase in the fraction of S0 galaxies with decreasing redshift, along with a corresponding reduction in the fraction of spirals. We find a bimodality in the mass distribution of our classified S0s, from which we find two separate S0s populations: high-mass S0s, which are mostly red and quiescent; and low-mass S0s, which are generally bluer and include both passive and star-forming S0s, the latter of which cannot solely be explained via the faded spiral formation pathway. We also find that the S0 fraction in high-mass galaxies begins rising at higher $z$ than in low-mass galaxies, implying that high-mass S0s evolved earlier.