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Abstract:

© 2024 The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We report the discovery of an extreme galaxy overdensity at $z = 5.4$ in the GOODS-S field using JWST/NIRCam imaging from JADES and JEMS alongside JWST/NIRCam wide field slitless spectroscopy from FRESCO. We identified potential members of the overdensity using HST+JWST photometry spanning $\lambda = 0.4-5.0\ \mu\mathrm{m}$. These data provide accurate and well-constrained photometric redshifts down to $m \approx 29-30\,\mathrm{mag}$. We subsequently confirmed $N = 81$ galaxies at $5.2 < z < 5.5$ using JWST slitless spectroscopy over $\lambda = 3.9-5.0\ \mu\mathrm{m}$ through a targeted line search for $\mathrm{H} \alpha$ around the best-fit photometric redshift. We verified that $N = 42$ of these galaxies reside in the field while $N = 39$ galaxies reside in a density around $\sim 10$ times that of a random volume. Stellar populations for these galaxies were inferred from the photometry and used to construct the star-forming main sequence, where protocluster members appeared more massive and exhibited earlier star formation (and thus older stellar populations) when compared to their field galaxy counterparts. We estimate the total halo mass of this large-scale structure to be $12.6 \lesssim \mathrm{log}_{10} \left( M_{\mathrm{halo}}/M_{\odot} \right) \lesssim 12.8$ using an empirical stellar mass to halo mass relation, which is likely an underestimate as a result of incompleteness. Our discovery demonstrates the power of JWST at constraining dark matter halo assembly and galaxy formation at very early cosmic times.Peer reviewe

Abstract:

Massive, starbursting galaxies in the early Universe represent some of the most extreme objects in the study of galaxy evolution. One such source is HFLS3 (z ∼ 6.34), which was originally identified as an extreme starburst galaxy with mild gravitational magnification (μ ∼ 2.2). Here, we present new observations of HFLS3 with the JWST/NIRSpec integral field unit in both low (PRISM/CLEAR; R ∼ 100) and high spectral resolution (G395H/290LP; R ∼ 2700), with high spatial resolution (∼0.1″) and sensitivity. Using a combination of the NIRSpec data and a new lensing model with accurate spectroscopic redshifts, we find that the 3″ × 3″ field is crowded, with a lensed arc (C, z = 6.3425 ± 0.0002), two galaxies to the south (S1 and S2, z = 6.3592 ± 0.0001), two galaxies to the west (W1, z = 6.3550 ± 0.0001; W2, z = 6.3628 ± 0.0001), and two low-redshift interlopers (G1, z = 3.4806 ± 0.0001; G2, z = 2.00 ± 0.01). We present spectral fits and morpho-kinematic maps for each bright emission line (e.g. [OIII]λ5007, Hα, and [NII]λ6584) from the R2700 data for all sources except G2 (whose spectral lines fall outside the observed wavelengths of the R2700 data). From a line ratio analysis, we find that the galaxies in component C are likely powered by star formation, though we cannot rule out or confirm the presence of active galactic nuclei in the other high-redshift sources. We performed gravitational lens modelling, finding evidence for a two-source composition of the lensed central object and a magnification factor (μ = 2.1 − 2.4) comparable to findings of previous work. The projected distances and velocity offsets of each galaxy suggest that they will merge within the next ∼1 Gyr. Finally, we examined the dust extinction-corrected SFRHα of each z > 6 source, finding that the total star formation (510 ± 140 M⊙ yr−1, magnification-corrected) is distributed across the six z ∼ 6.34 − 6.36 objects over a region of diameter ∼11 kpc. Altogether, this suggests that HFLS3 is not a single starburst galaxy, but instead a merging system of star-forming galaxies in the epoch of reionisation.

Abstract:

We present NIRSpec IFS observations of a galaxy group around the massive GS_4891 galaxy at z ∼ 3.7 in GOODS-South that includes two other two systems, GS_4891_n to the north and GS_28356 to the east. These observations, obtained as part of the GTO Galaxy Assembly – NIRSpec IFS (GA-NIFS) program, allow us to study for the first time the spatially resolved properties of the interstellar medium (ISM) and the ionised gas kinematics of a galaxy at this redshift. Leveraging the wide wavelength range spanned with the high-dispersion grating (with resolving power R = 2700) observations, covering from [O  II ] λλ 3726, 29 to [S  II ] λλ 6716, 31, we explore the spatial distribution of the star formation rate, nebular attenuation, and gas metallicity, together with the mechanisms responsible for the excitation of the ionised gas. GS_4891 presents a clear gradient of gas metallicity (as traced by 12 + log(O/H)) by more than 0.2 dex from the southeast (where a star-forming clump is identified) to the northwest. The gas metallicity in the less massive northern system, GS_4891_n, is also higher by 0.2 dex than at the centre of GS_4891, suggesting that inflows of lower-metallicity gas might be favoured in higher-mass systems. The kinematic analysis shows that GS_4891 presents velocity gradients in the ionised gas consistent with rotation. The region between GS_4891 and GS_4891_n does not present high gas turbulence, which, together with the difference in gas metallicities, suggests that these two systems might be in a pre-merger stage. Finally, GS_4891 hosts an ionised outflow that extends out to r out = 1.5 kpc from the nucleus and reaches maximum velocities, v out , of approximately 400 km s −1 . Despite entraining an outflowing mass rate of Ṁ out ∼ 4 M ⊙ yr −1 , the low associated mass-loading factor, η ∼ 0.04, implies that the outflow does not have a significant impact on the star formation activity of the galaxy