The stellar mass function of quiescent and star-forming galaxies and its dependence on morphology in COSMOS-Web

TL;DR

This study uses the COSMOS2025 catalog built from JWST COSMOS-Web imaging to measure the stellar mass function (SMF) of quiescent and star-forming galaxies from $0.2<z<5.5$, separately by morphology via bulge-to-total ratio ($B/T$) and rest-frame $NUVrJ$ color classification. The authors employ a $1/V_{ m max}$ estimator with mass-completeness and an explicit Eddington-bias correction, and fit the SMFs with Schechter functions using an MCMC framework. They find rapid, mass-dependent growth of the quiescent population, a knee at $\log(M_*/{\rm M}_\odot)\approx10.6$ in the star-forming SMF, and an upturn at $\log(M_*/{\rm M}_\odot)\lesssim9.5$ extending to $z\sim3.25$, with bulge-dominated quiescent galaxies dominating the high-mass end at all redshifts and disk-dominated systems contributing to the low-mass quiescent population. An empirical model links quenching rates, baryon conversion efficiency, and bulge formation, supporting a scenario where bulge growth precedes quenching in massive halos while environmental quenching drives the low-mass end, providing robust JWST-based benchmarks for galaxy formation theories.

Abstract

We study the stellar mass function (SMF) of quiescent and star-forming galaxies and its dependence on morphology in 10 redshift bins at $0.2<z<5.5$ using the COSMOS2025 catalog built from $0.54 \, {\rm deg}^2$ JWST imaging from COSMOS-Web. Galaxies are selected by type using the $NUVrJ$ rest-frame color diagram and classified morphologically by bulge-to-total light ratio ($B/T$). The quiescent SMF shows rapid early build-up, with the most massive systems (${\rm log}(M_{\star}/{\rm M_{\odot}})\gtrsim11$) assembled by $z\sim1$ and evolving little since. The star-forming SMF evolves more slowly, following a mass-evolution scenario where galaxies grow via star formation and quench at the characteristic mass $\log(M^{*}/{\rm M}_{\odot})\sim10.6$. Bulge systems ($B/T>0.6$) dominate the quiescent SMF at ${\rm log}(M_{\star}/{\rm M_{\odot}})>10$ at all redshifts, while disks ($B/T<0.2$) dominate at ${\rm log}(M_{\star}/{\rm M_{\odot}})<9$. However, most bulge-dominated galaxies are star-forming, with their fraction increasing with redshift and decreasing mass, consistent with being progenitors of quiescent bulges. We find evidence for environmental quenching onset at $z\sim3$ from the upturn in the quiescent SMF at ${\rm log}(M_{\star}/{\rm M_{\odot}})<9.5$, contributed by disk-dominated galaxies consistent with satellite quenching that retains disk morphologies. Number densities of ${\rm log}(M_{\star}/{\rm M_{\odot}})>10$ quiescent galaxies are lower than recent literature by $0.1-0.7$ dex, but agree well with simulations at $2<z<3$. At $z>3$, simulations increasingly underpredict observations. Finally, we build an empirical model describing galaxy number density evolution by parametrizing quenching rates, baryon conversion efficiency, and bulge formation. Our model supports a scenario where star-forming galaxies grow central bulges before quenching in massive halos.

Figures (12)

• Figure 1: Rest-frame $NUVrJ$ color diagram in 10 redshift bins used to select the samples of star-forming and quiescent galaxies separated by the dashed black line (Eq. \ref{['eq:nuvrj']}). The histogram is color-coded according to the $B/T$ ratio F115W at $z<2$, F150W at $2<z<3$ and F277W for $z>3$. It includes sources with $\log(M_{\star}/\Msun)>8.5$.
• Figure 2: Stellar mass function for quiescent (left) and star-forming (right) galaxies in 10 redshift bins from $z=0.2$ to $z=5.5$. The solid lines mark the measurements, while the filled areas envelop the $1\,\sigma$ confidence interval including Poisson, cosmic variance and SED-fitting errors. The SMFs are measured with a bin size of $\Delta \log M_{\star}=0.25$.
• Figure 3: Stellar mass function for quiescent (red) and star-forming (teal) galaxies. Each panel, corresponding to a different $z$-bin, shows the measurements (symbols) and best-fit intrinsic functions (solid lines and envelopes). The models are also shown extrapolated beyond the measured mass range. Comparison with recent literature works includes Weaver2023McLeod2021Santini2022Baker25b in gray symbols. This work indicates, for the first time, an upturn in the quiescent SMF at $\log(M_{\star}/{\rm M}_{\odot})$$\lesssim9.5$ out to $z\sim 3.25$.
• Figure 4: Stellar mass function for quiescent galaxies separated by the bulge-to-total ratio. Each panel, corresponding to a different $z$-bin, shows the measurements (symbols) and best-fit functions (solid lines and envelopes) for all quiescent (black), disk-dominated ($B/T<0.2$, blue), intermediate ($0.2<B/T<0.6$, orange) and bulge-dominated ($B/T>0.6$, dark red). The models are also shown extrapolated beyond the measured mass range.
• Figure 5: Stellar mass function for star-forming galaxies separated by the bulge-to-total ratio. Each panel, corresponding to a different $z$-bin, shows the measurements (symbols) and best-fit functions (solid lines and envelopes) for all quiescent (black), disk-dominated ($B/T<0.2$, blue), intermediate ($0.2<B/T<0.6$, orange) and bulge-dominated ($B/T>0.6$, dark red). The models are also shown extrapolated beyond the measured mass range.