Evolution of the early-type fraction in massive galaxies at z < 2: how did early-type morphology form?
Year: 2026
Type: article
Abstract: ABSTRACT Using James Webb Space Telescope/NIRCam data over a 0.28 deg$^{2}$ area from COSMOS-Web survey, together with Hubble Space Telescope/Advanced Camera for Surveys data, we investigate early-type fraction of massive galaxies with $M_{\rm star}\gt 10^{10.5}\,\mathrm{ M}_{\odot }$ at $0.2\lt z\lt 2.0$, 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 $C\gt C_{n=2.5}$ and $A_{\rm cor}\lt 0.2$. Here, $C_{n=2.5}$ is the concentration expected for a Sérsic profile with $n=2.5$ under the spatial resolution and depth of the data, and $A_{\rm cor}$ is the asymmetry corrected for resolution effects. The fraction of early-type galaxies with $M_{\rm star}\gt 10^{11}\, \mathrm{M}_{\odot }$ ($=10^{10.5}$–$10^{11}\, \mathrm{M}_{\odot }$) decreases with increasing redshift from $\sim$70 per cent ($\sim$40–60 per cent) at $z\sim 0.3$ to $\sim $20–25 per cent ($\sim$15–25 per cent) at $z\sim 1.8$. We also examine the evolution of their $R_{20}$ and $R_{80}$, which enclose 20 per cent and 80 per cent 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 (specific star formation rate) of sample galaxies strongly correlates with $R_{20}$, and early-type galaxies have lower SSFRs by $\sim 1$ dex. We further find that early-type galaxies at $z\gtrsim 1.3$ have younger mass-weighted stellar ages of $t_{\rm mw}\lesssim 2$ Gyr than late-type and irregular ones. Their SSFRs, $t_{\rm mw}$, 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 $\sim$1–2 Gyr later.
Author: Masaru Kajisawa
Institution: Ehime University
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $3,046
OpenAlex ID: W7141810175
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