Evolution of the early-type fraction in massive galaxies at <i>z</i> &amp;lt; 2: how did early-type morphology form?

Abstract Using JWST/NIRCam data over a 0.28 deg2 area from COSMOS-Web survey, together with HST/ACS data, we investigate early-type fraction of massive galaxies with Mstar &amp;gt; 1010.5M⊙ at 0.2 &amp;lt; z &amp;lt; 2.0, and explore the formation of their early-type morphology. We measure concentration index C (=R80/R20) and asymmetry index A, and select early-type galaxies with C &amp;gt; Cn = 2.5 and Acor &amp;lt; 0.2. Here Cn = 2.5 is the concentration expected for a Sérsic profile with n = 2.5 under the spatial resolution and depth of the data, and Acor is the asymmetry corrected for resolution effects. The fraction of early-type galaxies with Mstar &amp;gt; 1011 M⊙ (=1010.5–1011 M⊙) 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 R20 and R80, which enclose 20% and 80% of the total flux of the galaxy, respectively. The median R80 shows strong mass dependence and significant redshift evolution, whereas the median R20 shows little dependence on either stellar mass or redshift. In contrast, morphological differences are more pronounced in R20 than in R80: the median R20 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 R20, 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 tmw ≲ 2 Gyr than late-type and irregular ones. Their SSFRs, tmw, 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.