Evolution of the early-type fraction in massive galaxies at $z<2$: how did early-type morphology form?

TL;DR

This study uses JWST and HST data to analyze the evolution of early-type galaxy fractions and morphology at redshifts 0.2 to 2.0, revealing their formation processes and structural changes over cosmic time.

Contribution

It provides new insights into the morphological evolution and formation history of massive early-type galaxies using high-resolution space telescope data.

Findings

Early-type galaxy fraction decreases with redshift, from ~70% at z~0.3 to ~20% at z~1.8.

Median R80 shows strong mass dependence and redshift evolution, unlike R20.

High-z early-type galaxies have younger stellar ages and evidence of rapid core formation followed by quenching.

Abstract

Using $JWST$/NIRCam data over a 0.28 deg$^{2}$ area from COSMOS-Web survey, together with $HST$/ACS data, we investigate early-type fraction of massive galaxies with $M_{star}>10^{10.5}M_{\odot}$ at $0.2<z<2.0$, and explore the formation of their early-type morphology. We measure concentration index $C$ ($=R_{80}/R_{20}$) and asymmetry index $A$, and select early-type galaxies with $C>C_{n=2.5}$ and $A_{cor}<0.2$. Here $C_{n=2.5}$ is the concentration expected for a Sersic profile with $n=2.5$ under the spatial resolution and depth of the data, and $A_{cor}$ is the asymmetry corrected for resolution effects. The fraction of early-type galaxies with $M_{star}>10^{11}M_{\odot}$ ($=10^{10.5}$-$10^{11}M_{\odot}$) decreases with increasing redshift from ~70% (~40-60%) at $z$ ~ 0.3 to ~20-25% (~15-25%) at $z$ ~ 1.8. We also examine the evolution of their $R_{20}$ and $R_{80}$, which enclose 20% and 80% of the total flux of the galaxy, respectively. The median $R_{80}$ shows strong mass dependence and significant redshift evolution, whereas the median $R_{20}$ shows little dependence on either stellar mass or redshift. In contrast, morphological differences are more pronounced in $R_{20}$ than in $R_{80}$: the median $R_{20}$ of early-type galaxies is smaller than that of late-type and irregular galaxies by 0.25-0.45 and 0.3-0.6 dex, respectively. The median SSFR of sample galaxies strongly correlates with $R_{20}$, and early-type galaxies have lower SSFRs by ~1 dex. We further find that early-type galaxies at $z>1.3$ have younger mass-weighted stellar ages of $t_{mw}<2$ Gyr than late-type and irregular ones. Their SSFRs, $t_{mw}$, and morphological properties suggest that these high-$z$ early-type galaxies experienced rapid formation of a dense stellar core through starburst, followed by quenching of star formation, and subsequently resumed star formation ~1-2 Gyr later.