Photometrically Selected Protocluster Candidates at z~9-10 in the JWST COSMOS-Web field

TL;DR

This work targets protocluster formation during the Epoch of Reionisation by exploiting JWST COSMOS-Web DR0.5 to identify overdense regions at $z\sim 9-11$ using 366 $F115W$ dropout galaxies. It combines colour selection with SED-based photometric redshifts to construct redshift PDFs and then computes weighted overdensities within a $2.5\mathrm{cMpc}$ aperture, identifying seven robust protocluster cores (with linking length $7.5\mathrm{cMpc}$) and estimating their halo masses via extrapolated halo–stellar mass relations, $M_h\sim 10^{10.6}-10^{11.4}\,M_\odot$. The results demonstrate that COSMOS-Web’s large area enables a statistically meaningful census of early protoclusters, offering a testbed for structure formation models, though spectroscopic follow-up is essential to confirm membership and exclude interlopers. The study highlights the potential of future JWST spectroscopic campaigns (e.g., COSMOS-3D WFSS) to securely characterize these systems and map the emergence of the high-redshift large-scale structure.

Abstract

High-redshift protoclusters are crucial for understanding the formation of galaxy clusters and the evolution of galaxies in dense environments. The James Webb Space Telescope (JWST), with its unprecedented near-infrared sensitivity, enables the first exploration of protoclusters beyond $z>$10. Among JWST surveys, COSMOS-Web Data Release 0.5 offers the largest area $\sim$0.27 deg$^2$, making it an optimal field for protocluster searches. In this study, we searched for protoclusters at $z\sim$9-10 using 366 F115W dropout galaxies. We evaluated the reliability of our photometric redshift by validation tests with the JADES DR3 spectroscopic sample, obtaining the likelihood of falsely identifying interlopers as $\sim25\%$. Overdensities ($δ$) are computed by weighting galaxy positions with their photometric redshift probability density functions (PDF), using a 2.5 cMpc aperture and a redshift slice of $\pm$0.5. We selected the most promising core galaxies of protocluster candidate galaxies with an overdensity greater than the 95th percentile of the distribution of 366 F115W dropout galaxies. The member galaxies are then linked within an angular separation of 7.5 cMpc to the core galaxies, finding seven protocluster candidates. These seven protocluster candidates have inferred halo masses of $M_{\text{halo}} \sim 10^{11} M_{\odot}$. The detection of such overdensities at these redshifts provides a critical test for current cosmological simulations. However, confirming these candidates and distinguishing them from low-redshift dusty star-forming galaxies or Balmer-break galaxies will require follow-up near-infrared spectroscopic observations.

Figures (16)

• Figure 1: The two-colour diagram for the sources have $S/N > 2$ in F115W+F277W+F444W detection image of COSMOS-Web DR0.5 2023Casey. The green area shows colours that satisfy the F115W-dropout criteria from 2023Harikane. The colour is measured with a 0.3" diameter circular aperture. The numbers indicate how many sources there are in each subset.
• Figure 2: Spectral energy distribution (SED) of the spectroscopically confirmed galaxy GN-z11 at redshift z=10.60 (black line), overlaid with the 5$\sigma$ detection limits of various filters used in this study. The coloured upward arrows indicate 5$\sigma$ limiting depths in each band: F814W (purple), F115W (light blue), F150W (green), F277W (orange), and F444W (red). For each band, the fainter (transparent) arrows denote the shallower 5$\sigma$ depths reached in approximately 50% of the survey area, due to non-uniform coverage and exposure time. Fluxes are shown in nJy on the left y-axis, with the corresponding AB magnitudes on the right y-axis. Horizontal error bars represent the approximate width of each filter’s transmission curve. This figure illustrates the ability of the JWST NIRCam bands to probe the rest-frame UV-to-optical emission of galaxies at z>10.
• Figure 3: The two-colour diagram for sources from the JADES DR3 matching COSMOS-Web limiting magnitudes. Scattered grey dots represent all objects that have both NIRCam and NIRSpec data. The green polygon delineates our F115W-dropout selection criteria. coloured stars are spectroscopically confirmed galaxies with $8.0\leq z_{\text{spec}}\leq12.0$; the over-plotted numbers mark their spectroscopic redshifts. Red/Blue colours mark galaxies that satisfy/fail the colour criteria. Black triangles indicate $z_{\text{spec}}<8.0$ that satisfy the colour criteria sources. The text annotations quote a contamination rate of 25% (ratio between galaxies with $z_{\text{spec}}<8.0$ that satisfy the colour criteria (N=5) to the number of the source satisfy the colour criteria (N=20)) and a loss rate of 42% (fraction of $8.0\leq z_{\text{spec}}\leq12.0$ galaxies that do not satisfy the colour criteria (N=11) to the number of all $8.0\leq z_{\text{spec}}\leq12.0$ galaxies (N=26).
• Figure 4: The relative percentage deviation of photo-z derived from CIGALE for the best-fit model and the Bayesian estimation. The colourmap indicates the number of sources in each hexagonal bin. Red scatter points represent the median of certain redshifts and the corresponding standard deviation. The Bayesian estimation does not deviate $> 7.5\%$ from the best-fit model across 8 $\leq z \leq$ 12 for most sources.
• Figure 5: Comparison between photometric and spectroscopic redshifts from CIGALE/EAZY and spectroscopic redshifts from JADES DR3. Grey points represent all galaxies with both photometry and spectroscopic redshifts in the JADES field. Note that we did not use filters not available in the COSMOS-Web, and the JADES photometry is downgraded to the COSMOS-Web quality. Green stars denote JADES galaxies that satisfy our F150W-dropout colour criteria. The solid black line indicates the one-to-one correspondence ($z_{\mathrm{spec}} = z_{\mathrm{phot}}$), and the red points highlight sources within the dashed lines of 10% deviation. For the high-redshift subset ($z_{\mathrm{spec}} > 8$), we find a significantly improved normalised median absolute deviation (NMAD) and outlier fraction. This figure demonstrates the performance of photometric redshift estimation under COSMOS-Web-like conditions.