COSMOS-Web: JWST Extragalactic Survey

• COSMOS-Web is a comprehensive JWST extragalactic imaging survey that maps galaxy structure, star formation, and assembly over 12 billion years.
• It combines deep, high-resolution NIRCam and parallel MIRI imaging over a contiguous 0.54 deg² field to derive precise structural parameters and scaling relations.
• Observations reveal the evolutionary trends of Brightest Group Galaxies, highlighting changes in size, star formation surface density, and morphological quenching across cosmic time.

COSMOS-Web is a large-scale extragalactic imaging survey utilizing the James Webb Space Telescope (JWST) to probe galaxy evolution, structure, and assembly over more than 12 billion years of cosmic history. By exploiting deep, high-resolution Near-Infrared Camera (NIRCam) observations over a contiguous ∼0.54 deg² field (with parallel mid-infrared MIRI coverage), COSMOS-Web enables detailed mapping of rest-frame optical stellar light and internal structure for galaxies from the local universe out to redshift z = 3.7. A primary scientific focus is the structural and star formation evolution of Brightest Group Galaxies (BGGs)—the most luminous central galaxies in group-scale dark matter halos—serving as laboratories for the co-evolution of baryonic and structural properties within dense environments.

1. Program Scope, Instruments, and Survey Design

COSMOS-Web leverages the high spatial resolution and sensitivity of JWST's NIRCam, observing in four broadband filters (F115W, F150W, F277W, F444W) designed to sample the rest-frame optical emission of galaxies across a wide redshift range. The survey covers approximately 0.54 deg², reaching 5σ point-source depths of ~27.5–28.2 AB magnitudes (0.15″ apertures), with parallel MIRI imaging in F770W over ~0.2 deg² to facilitate identification of dust-obscured components. The contiguous, wide-area footprint minimizes cosmic variance and enables robust population statistics for rare massive systems and environmental studies.

This combination of depth and area, integrated with the COSMOS multiwavelength legacy, provides a foundational resource for mapping structural parameters (e.g., Sérsic index n, effective radius Rₑ), morphology, and star formation properties for large samples of both local and high-redshift BGGs. The survey's multi-epoch, carefully dithered observational strategy ensures high-quality mosaics suitable for precise photometric and structural measurements (Casey et al., 2022).

2. Methodology: Structural and Physical Parameter Measurement

BGG identification begins with robust group finding (e.g., AMICO algorithm) and assignment of central galaxies in group-scale halos (Toni et al., 15 Jan 2025). Morphological and structural characterization is achieved by fitting two-dimensional Sérsic profiles to NIRCam images using advanced tools (such as Galight), yielding effective radii, axis ratios, and bulge/disk decomposition. PSF modeling and Bayesian fitting frameworks are employed to ensure measurement fidelity across the redshift range.

BGGs are classified as star-forming or quiescent using two diagnostics: (i) rest-frame NUV–r–J color selection, accounting for dust and stellar population effects, and (ii) a redshift-dependent specific star formation rate (sSFR) threshold derived from spectral energy distribution (SED) fitting (e.g., CIGALE), incorporating multiwavelength (including MIRI) photometry.

Key derived parameters include:

• Stellar mass (M_*), SFR, and sSFR from SED fits,
• Effective radius (Rₑ) and Sérsic index (n) from profile fitting,
• Star formation surface density, $$\Sigma_{\mathrm{SFR}} = \frac{0.5\,\mathrm{SFR}}{\pi R_e^2}$$, where the factor 0.5 accounts for the half-light radius,
• Bulge-dominance and structural class (e.g., via n or bulge-to-total ratio).

Statistical analyses combine these measurements to construct scaling relations and evolutionary trends as a function of redshift and stellar mass.

3. Size–Mass Relation and Redshift Evolution

BGGs display a power-law relation between effective radius and stellar mass,

$$\log\left(\frac{R_e}{\text{kpc}}\right) = \log A + \alpha\,\left[\log\left(\frac{M_*}{5\times 10^{10}\,M_\odot}\right)\right],$$

where $A$ is a normalization and $\alpha$ is the slope. Quiescent BGGs are systematically more compact at fixed mass and exhibit steeper size–mass slopes than star-forming BGGs. The evolution at fixed $M_*$ is described by

$R_e \propto (1+z)^{-\alpha},$

with $\alpha = 1.11 \pm 0.07$ for star-forming and $\alpha = 1.40 \pm 0.09$ for quiescent BGGs (i.e., quiescent BGGs grow in size more rapidly over time), reflecting structural transformation as BGGs evolve from high-redshift compact, star-forming systems to larger, more spheroidal, quiescent galaxies at low redshift (Gozaliasl et al., 4 Jun 2025).

4. Star Formation Surface Density and Compactness

The paper quantifies how the intensity of star formation, $\Sigma_{\mathrm{SFR}}$, evolves with redshift and mass. For BGGs with $\log_{10}(M_*/M_\odot) \geq 10.75$, $\Sigma_{\mathrm{SFR}}$ rises with redshift, indicating that high-redshift BGGs are more intensely star-forming per unit area (a signature of high gas accretion rates and/or compaction). This parameter is calculated as

$$\Sigma_{\mathrm{SFR}} = \frac{0.5\,\mathrm{SFR}}{\pi R_e^2}.$$

At late times, $\Sigma_{\mathrm{SFR}}$ decreases and the population is dominated by massive, extended, low-sSFR systems.

A transition in the $\Sigma_*$–sSFR plane demarcates structural quenching: above a threshold in central stellar mass surface density (e.g., $\log_{10}\Sigma_* \sim 9.5$–$10\,M_\odot\,\mathrm{kpc}^{-2}$), BGGs are predominantly quiescent and bulge-dominated (Yang et al., 9 Apr 2025). The bulge-dominated mass fraction exceeds 80% for quiescent BGGs, indicating morphological transformation as a prerequisite for quenching.

5. Morphological Quenching and Coevolution

“Morphological quenching” refers to the process in which formation of a massive, centrally concentrated bulge stabilizes the gaseous disk, inhibiting gravitational fragmentation and thus star formation. Evidence in COSMOS-Web shows that bulge-dominated BGGs exhibit low sSFR and compact sizes, consistent with this mechanism. This is supported by the prevalence of bulge-dominated morphologies among quiescent BGGs, and the observation that quiescent BGGs are more structurally evolved (steeper size–mass slopes, higher $\Sigma_*$) than their star-forming counterparts at all redshifts probed.

This structural transition is seen as a sharp boundary in the $\Sigma_*$–sSFR diagram: as BGGs reach high central densities, quenching rapidly ensues, and their sizes inflate through subsequent minor, dry mergers, a scenario expected in hierarchical growth models of massive galaxies in group-scale halos.

6. Context: Environmental and Theoretical Implications

The coevolution of size, morphology, and star formation in BGGs reflects both internal/“secular” processes (bulge growth, morphological compaction, self-quenching) and external/environmental influences (e.g., minor mergers, dynamical interactions in the group potential). COSMOS-Web provides direct evidence for the rapidity of this evolutionary sequence, tracking BGGs from compact, high $\Sigma_{\mathrm{SFR}}$ objects at $z\gtrsim3$ through structural transition to predominately quiescent, extended spheroids by $z\lesssim1$.

BGGs thus serve as crucial testbeds for understanding baryonic assembly, the physical drivers of morphological transformation, and the connection between central galaxy buildup and group environment. The robust scaling relations and quenching thresholds derived from COSMOS-Web measurements inform semi-analytic and hydrodynamical models of galaxy evolution, directly connecting observed structural trends with simulations of group and cluster assembly.

7. Mathematical Framework and Analytical Expressions

Key quantitative relationships central to the analysis include:

• Size–mass relation for BGGs:

$$\log\left(\frac{R_e}{\mathrm{kpc}}\right) = \log A + \alpha \, \log\left(\frac{M_*}{5\times10^{10}\,M_\odot}\right)$$

• Size evolution with redshift at fixed mass:

$R_e \propto (1+z)^{-\alpha}$

with $\alpha$ measured separately for star-forming and quiescent populations.

• Star formation surface density:

$$\Sigma_{\mathrm{SFR}} = \frac{0.5\,\mathrm{SFR}}{\pi R_e^2}$$

where $R_e$ is in kpc and SFR in $M_\odot\,\mathrm{yr}^{-1}$.

These analytic forms underpin empirical findings and allow comparison with theoretical models and other observational datasets.

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The COSMOS-Web program's systematic, high-precision mapping of BGG structure and star formation as a function of redshift has established new benchmarks for studying the interplay of morphology, star formation, and environment in the evolution of central galaxies in group halos. The growing sample of well-characterized BGGs—including their transition through compaction, morphological quenching, and size growth—offers crucial constraints for galaxy formation and evolution models operating across a wide range of masses and cosmic epochs (Gozaliasl et al., 4 Jun 2025).