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

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.

Figures (21)

• Figure 1: Uncertainty of total stellar mass, $M_{\rm star}$ (left), SSFR in the past 40 Myr, SSFR$_{\rm 0-40Myr}$ (middle), and mass-weighted mean stellar age, $t_{\rm mw}$ (right) estimated in the SED fitting analysis for galaxies with $M_{\rm star} > 10^{10.5} M_{\odot}$ at $0.2<z<2.0$. The solid lines show median errors in logarithmic scale as a function of the property itself, and the dotted lines represent 16 and 84 percentiles of the errors. The different colours represent the different redshift ranges.
• Figure 2: Comparison between $t_{\rm mw}$ used in this study, which is estimated from the photometric SED, and that estimated from both the same photometric SED and optical spectrum from the LEGA-C survey (van21) for galaxies with $M_{\rm star} > 10^{10.5} M_{\odot}$ at $z =$ 0.5--1.0. The solid line shows the median values of $t_{\rm mw}$ estimated from only the photometric SED at a given $t_{\rm mw}$ estimated from both the photometric SED and LEGA-C spectrum, while the dashed lines represent 16 and 84 percentiles. The median 68% confidence intervals of $t_{\rm mw}$ estimated from the photometric SED at $t_{\rm mw} <$ 2 Gyr, 2--4 Gyr, and $>$ 4 Gyr are shown on the right side of the panel, while those of $t_{\rm mw}$ estimated from both the photometric SED and LEGA-C spectrum are shown at the bottom. The correlation coefficient between these two mass-weighted mean ages is also shown.
• Figure 3: $[3.6]$ magnitude vs. stellar mass for galaxies with $[3.6] < 26$ in the different redshift ranges. The blue dotted and solid lines represent 90 and 95 percentiles of [3.6] magnitude in each stellar mass bin with a width of $\pm 0.1$ dex. The vertical dotted-dashed line shows the stellar mass limit of $M_{\rm star} = 10^{10.5} M_{\odot}$.
• Figure 4: Distribution of SSFR$_{\rm 0-40Myr}$ for galaxies in the different stellar mass and redshift bins. The solid histograms show those galaxies morphologically classified with the $HST$ and $JWST$ data, namely, the F814W-band data at $0.2<z<0.7$, the F115W-band data at $0.7<z<1.3$, and the F150W-band data at $1.3<z<2.0$. The open histograms represent the parent sample from the COSMOS2020 catalogue over the entire COSMOS field. Note that the leftmost bin in each panel represents all galaxies with SSFR$_{\rm 0-40Myr} < 10^{-12}$ yr$^{-1}$. The probability that the both distributions for those galaxies with $HST$ and $JWST$ and the parent sample are extracted from the same distribution in the Kolmogorov-Smirnov test is shown at the bottom right of each panel.
• Figure 5: $C$ values for PSF-convolved Sérsic profiles with different Sérsic indices as a function of $\mu_{\rm lim}$/<$\mu_{80}$> (left) and FWHM$_{\rm PSF}$/$R_{20}$ (right). The red, green, and blue lines show those with $n=$ 1, 2.5, and 4, respectively. The dotted, solid, and dashed-dotted lines represent those for different FWHM$_{\rm PSF}$/$R_{20}$ (left) and $\mu_{\rm lim}$/<$\mu_{80}$> (right) values.