COSMOS Web: Morphological quenching and size-mass evolution of brightest group galaxies from z = 3.7

TL;DR

The paper analyzes ~1700 Brightest Group Galaxies from z ≈ 3.7 to 0.08 using COSMOS-Web JWST imaging to measure rest-frame optical sizes and morphologies, classify galaxies as star-forming or quiescent via color and sSFR criteria, and quantify size–mass relations, size evolution, and star-formation surface density in group environments. It finds that quiescent BGGs are more compact with steeper size–mass slopes than star-forming BGGs, and that overall size growth follows $R_e \propto (1+z)^{-\alpha}$ with $\alpha$ around $1.31$ for SF and $1.40$ for QG. The study also shows that $\Sigma_{\mathrm{SFR}}$ increases with redshift and mass, with a pronounced structure–quenching link where bulge-dominated systems dominate the quiescent population, consistent with compaction and merger-driven growth in group halos. These results underscore the role of environment in shaping the structural and star-formation histories of central galaxies in groups and establish BGGs as key laboratories for baryonic assembly in group-scale halos, enabled by the unprecedented depth and resolution of JWST/COSMOS-Web.

Abstract

We present a comprehensive study of the structural evolution of Brightest Group Galaxies (BGGs) from redshift $z \simeq 0.08$ to $z = 3.7$ using the \textit{James Webb Space Telescope}'s 255h COSMOS-Web program. This survey provides deep NIRCam imaging in four filters (F115W, F150W, F277W, F444W) across $\sim 0.54~\mathrm{deg}^2$ and MIRI coverage in $\sim 0.2~\mathrm{deg}^2$ of the COSMOS field. High-resolution NIRCam imaging enables robust size and morphological measurements, while multiwavelength photometry yields stellar masses, SFRs, and Sérsic parameters. We classify BGGs as star-forming and quiescent using both rest-frame NUV--$r$--$J$ colors and a redshift-dependent specific star formation rate (sSFR) threshold. Our analysis reveals: (1) quiescent BGGs are systematically more compact than their star-forming counterparts and exhibit steeper size--mass slopes; (2) effective radii evolve as $R_e \propto (1+z)^{-α}$, with $α= 1.11 \pm 0.07$ (star-forming) and $1.40 \pm 0.09$ (quiescent); (3) star formation surface density ($Σ_{\mathrm{SFR}}$) increases with redshift and shows stronger evolution for massive BGGs ($\log_{10}(M_\ast/M_\odot) \geq 10.75$); (4) in the $Σ_*$--sSFR plane, a structural transition marks the quenching process, with bulge-dominated systems comprising over 80\% of the quiescent population. These results highlight the co-evolution of structure and star formation in BGGs, shaped by both internal and environmental processes, and establish BGGs as critical laboratories for studying the baryonic assembly and morphological transformation of central galaxies in group-scale halos.

Figures (24)

• Figure 1: The intrinsic richness, $\lambda*$, for the sample of detected groups and its trend with redshift, color-color classified BGGs as quiescent and star-forming are shown in red and blue points, respectively. Dashed horizontal orange line represents the level of $\lambda*$ limit we apply in this study.
• Figure 2: Selection and characterization of BGGs in the COSMOS-Web galaxy group catalog. The top panels show histograms of the number of groups as a function of log stellar mass $(log_{10}(M_*/M_\odot)$ and absolute magnitude ($M_R$), comparing selections based on two different aperture sizes: 250 kpc (blue) and 500 kpc (orange). The bottom panel displays a scatter plot of absolute magnitude ($M_R$) versus log stellar mass $(log_{10}(M_*/M_\odot)$, with points color-coded by selection method: mass-selected (blue circles), magnitude-selected (red squares), and hybrid-selected (green triangles). The solid lines represent the medians for each selection type: mass-selected (blue), magnitude-selected (red), and hybrid-selected (black). The shaded regions around the median lines show the corresponding confidence bands. The figure illustrates how the hybrid selection method balances stellar mass and luminosity to better capture the true BGG population, especially at the high-mass end.
• Figure 3: Redshift-dependent sSFR threshold used to classify BGGs as quiescent, defined as $\log_{10}(\mathrm{sSFR}) < \log_{10}(0.2 / t_{\mathrm{obs}})$. The bottom x-axis shows cosmic time in Gyr, while the top x-axis shows the corresponding redshift. The threshold decreases smoothly from early to late cosmic epochs, tracing the decline in global star formation efficiency.
• Figure 4: Rest-frame NUV--$r$ versus $r$--$J$ color--color diagram for BGGs across eight redshift bins from $z=0$ to $z=4$, each of width $\Delta z=0.5$. BGGs are color-coded by their $\log_{10}(\mathrm{sSFR}/\mathrm{yr}^{-1})$. Magenta lines delineate the quiescent region defined by $M_{\mathrm{NUV}} - M_r > 3(M_r - M_J) + 1$ and $M_{\mathrm{NUV}} - M_r > 3.1$. In each panel, we report the number of quiescent and star-forming BGGs based on sSFR-only, color-only, and the combined (color + sSFR) criteria. Open red and blue squares denote quiescent and star-forming BGGs, respectively, that meet both thresholds. A clear redshift trend emerges: the fraction of star-forming BGGs dominates at high redshift ($z \gtrsim 2$), while quiescent systems increase in prevalence at lower redshifts.
• Figure 5: Examples of JWST/NIRCam color composite of the BGGs in COSMOS-Web groups spanning redshifts from $z=0.22$ to $z=3.09$. The images display diverse morphologies and structural features, including compact spheroids, disturbed or merging systems, and prominent lensing arcs in massive group cores.