Context. How galaxies form, assemble, and cease their star formation is a central question within the modern landscape of galaxy evolution studies. These processes are indelibly imprinted on the galaxy stellar mass function (SMF), and its measurement and understanding is key to uncovering a unified theory of galaxy evolution. Aims. We present constraints on the shape and evolution of the galaxy SMF, the quiescent galaxy fraction, and the cosmic stellar mass density across 90% of the history of the Universe from z = 7.5 → 0.2 as a means to study the physical processes that underpin galaxy evolution. Methods. The COSMOS survey is an ideal laboratory for studying representative galaxy samples. Now equipped with deeper and more homogeneous near-infrared coverage exploited by the COSMOS2020 catalog, we leverage the large 1.27 deg2 effective area to improve sample statistics and understand spatial variations (cosmic variance) – particularly for rare, massive galaxies – and push to higher redshifts with greater confidence and mass completeness than previous studies. We divide the total stellar mass function into star-forming and quiescent subsamples through NUVrJ color-color selection. The measurements are then fit with single- and double-component Schechter functions to infer the intrinsic galaxy stellar mass function, the evolution of its key parameters, and the cosmic stellar mass density out to z = 7.5. Finally, we compare our measurements to predictions from state-of-the-art cosmological simulations and theoretical dark matter halo mass functions. Results. We find a smooth, monotonic evolution in the galaxy stellar mass function since z = 7.5, in general agreement with previous studies. The number density of star-forming systems have undergone remarkably consistent growth spanning four decades in stellar mass from z = 7.5 → 2 whereupon high-mass systems become predominantly quiescent (“downsizing”). Meanwhile, the assembly and growth of low-mass quiescent systems only occurred recently, and rapidly. An excess of massive systems at z ≈ 2.5 − 5.5 with strikingly red colors, with some being newly identified, increase the observed number densities to the point where the SMF cannot be reconciled with a Schechter function. Conclusions. Systematics including cosmic variance and/or active galactic nuclei contamination are unlikely to fully explain this excess, and so we speculate that they may be dust-obscured populations similar to those found in far infrared surveys. Furthermore, we find a sustained agreement from z ≈ 3 − 6 between the stellar and dark matter halo mass functions for the most massive systems, suggesting that star formation in massive halos may be more efficient at early times.
This paper presents a new search for z ≥ 7.5 galaxies using the COSMOS2020 photometric catalogues. Finding galaxies at the reionisation epoch through deep imaging surveys remains observationally challenging. The larger area covered by ground-based surveys such as COSMOS enables the discovery of the brightest galaxies at these high redshifts. Covering 1.4 deg 2 , our COSMOS catalogues were constructed from the latest UltraVISTA data release (DR4) combined with the final Spitzer/IRAC COSMOS images and the Hyper-Suprime-Cam Subaru Strategic Program DR2 release. We identified 17 new 7.5 < z < 10 candidate sources, and confirm 15 previously published candidates. Using deblended photometry extracted by fitting surface brightness models on multiband images, we selected four candidates which would be rejected using fixed aperture photometry. We tested the robustness of all our candidates by comparing six different photometric redshift estimates. Finally, we computed the galaxy UV luminosity function in three redshift bins centred at z = 8, 9, 10. We find no clear evolution of the number density of the brightest galaxies M UV < −21.5, in agreement with previous works. Rapid changes in the quenching efficiency or attenuation by dust could explain such a lack of evolution between z ∼ 8 and z ∼ 9. A spectroscopic confirmation of the redshifts, already planned with JWST and the Keck telescopes, will be essential to confirm our results.
We report the discovery of a compact group of galaxies, CGG-z5, at z ∼ 5.2 in the EGS field covered by the JWST/CEERS survey. CGG-z5 was selected as the highest overdensity of galaxies at z > 2 in recent JWST public surveys and it consists of six candidate members lying within a projected area of 1.5 ′′ × 3 ′′ (10×20 kpc 2 ). All group members are HST/F435W and HST/F606W dropouts while securely detected in the JWST/NIRCam bands, yielding a narrow range of robust photometric redshifts 5.0 < z < 5.3. The most massive galaxy in the group has a stellar mass log(M * /M ⊙ ) ≈ 9.8, while the rest are low-mass satellites (log(M * /M ⊙ ) ≈ 8.4-9.2). While several group members were already detected in the HST and IRAC bands, the low stellar masses and the compactness of the structure required the sensitivity and resolution of JWST for its identification. To assess the nature and evolutionary path of CGG-z5, we searched for similar compact structures in the Eagle simulations and followed their evolution with time. We find that all the identified structures merge into a single galaxy by z = 3 and form a massive galaxy (log(M * /M ⊙ ) >11) at z ∼ 1. This implies that CGG-z5 could be a "proto-massive galaxy" captured during a short-lived phase of massive galaxy formation.
We present a new infrared survey covering the three Euclid deep fields and four other Euclid calibration fields using Spitzer Space Telescope's Infrared Array Camera (IRAC). We combined these new observations with all relevant IRAC archival data of these fields in order to produce the deepest possible mosaics of these regions. In total, these observations represent nearly 11 % of the total Spitzer Space Telescope mission time. The resulting mosaics cover a total of approximately 71.5 deg 2 in the 3.6 and 4.5 µm bands, and approximately 21.8 deg 2 in the 5.8 and 8 µm bands. They reach at least 24 AB magnitude (measured to 5σ, in a 2 . 5 aperture) in the 3.6 µm band and up to ∼ 5 mag deeper in the deepest regions. The astrometry is tied to the Gaia astrometric reference system, and the typical astrometric uncertainty for sources with 16 < [3.6] < 19 is 0 . 15. The photometric calibration is in excellent agreement with previous WISE measurements. We extracted source number counts from the 3.6 µm band mosaics, and they are in excellent agreement with previous measurements. Given that the Spitzer Space Telescope has now been decommissioned, these mosaics are likely to be the definitive reduction of these IRAC data. This survey therefore represents an essential first step in assembling multi-wavelength data on the Euclid deep fields, which are set to become some of the premier fields for extragalactic astronomy in the 2020s.
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