[Paper Review] COSMOS-Web: Intrinsically Luminous z$\gtrsim$10 Galaxy Candidates Test Early Stellar Mass Assembly

Caitlin M. Casey, Hollis B. Akins+40 more|arXiv (Cornell University)|Aug 21, 2023

Astronomy and Astrophysical Research10 citations

TL;DR

The paper reports the discovery of 15 exceptionally luminous z>10 galaxy candidates in the first 0.28 deg2 of JWST/NIRCam imaging from COSMOS-Web, with 12 likely genuine high-z sources and 3 probable low-z contaminants, implying massive early stellar reservoirs and rapid star formation.

ABSTRACT

We report the discovery of 15 exceptionally luminous $10\lesssim z\lesssim14$ candidate galaxies discovered in the first 0.28 deg$^2$ of JWST/NIRCam imaging from the COSMOS-Web Survey. These sources span rest-frame UV magnitudes of $-20.5>M_{ m UV}>-22$, and thus constitute the most intrinsically luminous $z\gtrsim10$ candidates identified by JWST to-date. Selected via NIRCam imaging with Hubble ACS/F814W, deep ground-based observations corroborate their detection and help significantly constrain their photometric redshifts. We analyze their spectral energy distributions using multiple open-source codes and evaluate the probability of low-redshift solutions; we conclude that 12/15 (80%) are likely genuine $z\gtrsim10$ sources and 3/15 (20%) likely low-redshift contaminants. Three of our $z\sim12$ candidates push the limits of early stellar mass assembly: they have estimated stellar masses $\sim5 imes10^{9}\,M_\odot$, implying an effective stellar baryon fraction of $ε_{\star}\sim0.2-0.5$, where $ε_{\star}\equiv M_{\star}/(f_{b}M_{halo})$. The assembly of such stellar reservoirs is made possible due to rapid, burst-driven star formation on timescales $<$100\,Myr where the star-formation rate may far outpace the growth of the underlying dark matter halos. This is supported by the similar volume densities inferred for $M_\star\sim10^{10}\,M_\odot$ galaxies relative to $M_\star\sim10^{9}\,M_\odot$ -- both about $10^{-6}$ Mpc$^{-3}$ -- implying they live in halos of comparable mass. At such high redshifts, the duty cycle for starbursts would be of order unity, which could cause the observed change in the shape of the UVLF from a double powerlaw to Schechter at $z\approx8$. Spectroscopic redshift confirmation and ensuing constraints of their masses will be critical to understanding how, and if, such early massive galaxies push the limits of galaxy formation in $Λ$CDM.

Motivation & Objective

• Motivate the search for intrinsically luminous galaxies in the epoch of reionization using JWST data.
• Characterize the brightest z>10 galaxy candidates in COSMOS-Web and assess their redshift robustness.
• Estimate physical properties (stellar mass, SFR, UV luminosity) to test early stellar mass assembly scenarios.
• Evaluate the likelihood of low-redshift contaminants and the implications for the UV luminosity function at z>10.

Proposed method

• Assemble a multiwavelength photometric catalog combining JWST NIRCam data with HST, Subaru/HSC, UltraVISTA, and Spitzer COSMOS data.
• Perform model-based photometry (SourceXtractor++ SE++) and aperture photometry (SE classic) on PSF-homogenized images.
• Fit spectral energy distributions with multiple codes (EAzY, LePhare, Bagpipes) using FSPS templates and variant Lyα implementations.
• Use flat redshift priors (0<z<20) and assess low-z solutions by running each code with alternative priors and template sets.
• Quantify goodness of fit with a normalized chi-squared, Delta chi-squared criteria, and cross-code comparison to identify robust z>10 candidates.
• Cross-check for brown dwarf contamination and ensure spatially resolved detections to support extragalactic nature.

Experimental results

Research questions

• RQ1 How many intrinsically luminous z>10 galaxy candidates can be identified in the first COSMOS-Web data release?
• RQ2 What are the redshift probabilities and robustness of these candidates against low-z solutions?
• RQ3 What are the derived physical properties (M*, SFR, MUV, β, AV) of the luminous z>10 candidates, and do they imply rapid early stellar mass assembly?
• RQ4 What is the estimated contamination rate by low-redshift interlopers among the bright z>10 sample?

Key findings

• Identified 15 luminous z>∼10 galaxy candidates in 0.28 deg2, with rest-frame UV magnitudes around −21 to −22.
• 12 of the 15 candidates are likely genuine z>10 sources, while 3 are probable low-redshift contaminants.
• Three z∼12 candidates have stellar masses of ~5×10^9 M⊙, implying an effective stellar baryon fraction ε★ of ~0.2–0.5.
• The inferred rapid, burst-driven star formation can yield SFRs that may outpace dark halo growth on <100 Myr timescales, enabling substantial stellar reservoirs early on.
• Volume densities for M★∼10^10 M⊙ and M★∼10^9 M⊙ galaxies are similar (~10^−6 Mpc^−3), suggesting comparable host halo masses and high-redshift starburst duty cycles near unity.
• Spectroscopic confirmation and precise mass assembly constraints are critical to assess whether such massive systems push the limits of ΛCDM galaxy formation.