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Abstract:

We report a galaxy overdensity candidate at z ≈ 10.5 in the JWST Advanced Deep Extragalactic Survey. This overdensity contains 18 galaxies with consistent photometric redshifts within 8 comoving Mpc in projection. The galaxy number density is 4 times higher than the field expectation, accounting for one-third of comparably bright galaxies and nearly 50% of the total star formation rate (SFR) at 10 < zphot < 12 in the GOODS-S field. Galaxies in the overdensity more frequently have close companions or substructure, with one-third showing such features within 1 kpc at consistent photometric redshifts, implying enhanced interactions. Most galaxies have stellar masses of 0.6–3 × 108 M⊙, half-light radii of ∼200 pc, and SFRs of ∼5 M⊙ yr−1. Their stellar masses and SFRs are slightly higher than those of field galaxies, but remain broadly consistent with typical high-redshift scaling relations. Two compact objects show possible Balmer breaks, suggestive of evolved stellar populations or little red dots. We find tentative evidence for a spatially varying Lyα transmission inferred photometrically, consistent with an emerging ionized bubble. This overdensity provides a rare opportunity for probing the environmental impact on galaxy evolution and the onset of cosmic reionization within the first 500 Myr.

Abstract:

Our understanding of the physical properties of star-forming galaxies during the Epoch of Reionization (EoR; at z > 6) suffers from degeneracies among the apparent properties of the stars, nebular gas, and dust. These degeneracies are most prominent with photometry, which has insufficient (1) spectral resolution and (2) rest-frame spectral coverage. We explore ways to break these degeneracies with a sample of N = 22 high-redshift star-forming galaxies at 7 < zphot ≤ 9, using some of the deepest existing imaging from JWST/NIRCam and JWST/MIRI with JADES. Key to this study is the imaging from JWST/MIRI at 7.7 μm, which provides coverage of the rest-frame I band at the observed redshifts. We infer stellar population properties and rest-frame colors using a variety of filter sets and star formation history (SFH) assumptions to explore the impact of these choices. Evaluating these quantities both with and without the 7.7 μm data point shows that dense spectral coverage with JWST/NIRCam (eight or more filters, including at least one medium-band at λobs ≈ 4–5 μm) can compensate for lacking the rest-frame I-band coverage for the vast majority (≈80%) of our sample. Furthermore, these galaxy properties are most consistently determined by assuming the delayed-tau SFH, which provides the smallest offsets and scatters around these offsets when including JWST/MIRI. Within extragalactic surveys like JADES and CEERS, our findings suggest that robust characterization of the stellar population properties and rest-frame colors for typical high-redshift star-forming galaxies (i.e., galaxies with MUV ≈ −20 and βUV ≈ −2) is possible with JWST/NIRCam alone at z ≈ 8.

Abstract:

Ultracool T (Teff ≈ 500–1200 K) and Y dwarfs (Teff ⪅ 500 K) have historically been found only a few hundred parsecs from the Sun. The sensitivity and wavelength coverage of the NIRCam instrument on board the James Webb Space Telescope offer a unique method for finding low-temperature brown dwarfs in deep extragalactic datasets out to multiple kiloparsecs. Here we report on the selection of a sample of 41 brown dwarfs and brown dwarf candidates across the JWST Advanced Deep Extragalactic Survey in the GOODS-S and GOODS-N regions. We introduce a new open-source Bayesian tool, Near-Infrared Fitting for T and Y dwarfs (NIFTY), to derive effective temperatures, metallicities, and distances from JWST photometry. We find that 31 candidates have fits consistent with T dwarf temperatures out to 5–6 kpc, and 10 candidates have fits consistent with Y dwarf temperatures out to 1–2 kpc. The majority of the sources are best fit with subsolar metallicity models. We report proper motions for 10 brown dwarf candidates (three are newly presented), and calculate the number density of T and Y dwarfs as a function of temperature and distance above the Milky Way midplane. We further discuss how Y dwarfs can serve as contaminants in the search for ultra-high-redshift galaxies. Together, these results demonstrate the power of deep JWST extragalactic imaging to probe the coldest substellar populations far beyond the solar neighborhood, providing new constraints on the Milky Way’s structure and brown dwarf demographics.