Overdense fireworks in GOODS-N: Unveiling a record number of massive dusty star forming galaxies at z$\sim$5.2 with the N2CLS

TL;DR

This study identifies a remarkable overdensity of eight dusty star-forming galaxies at $z\sim5.2$ in GOODS-N using the NIKA2 survey, NOEMA CO line measurements, and JWST/FRESCO data. One new DSFG (N2GN_1_13) is spectroscopically confirmed at $z=5.182$, with a second JWST-identified member at $z=5.201$, while five additional members have spectroscopic redshifts or photometric redshifts consistent with the structure; six lie in overdense filaments. These galaxies are extremely massive and dust-obscured, with a median $M_\star \sim 9\times10^{10}\,M_\odot$, $\langle A_V\rangle \sim 3.3$, and SFRs of several hundred to ~thousand $M_\odot$ yr$^{-1}$, yielding short gas depletion times ($\tau_{dep} \sim 0.12$–0.23 Gyr) and high stellar baryon conversion efficiencies ($\epsilon_* > 0.2$). Their contribution to the cosmic SFRD at $z\sim5.2$ is substantial, dominating the obscured component within the overdensity and challenging current galaxy formation models to reproduce such extreme early growth in dense environments. The results imply accelerated evolution in protocluster regions, with implications for the formation of massive quiescent galaxies by $z \sim 4.5$ and for the interpretation of high-$z$ dusty populations in future surveys.

Abstract

As part of the N2CLS Survey, we have identified a remarkable overdensity of eight bright dusty star-forming galaxies at z$\sim$5.2 in the GOODS-N field. Three of these galaxies, N2GN_1_01, 06, and 23 (known as GN10, HDF850.1, and S3, respectively), had previously been spectroscopically confirmed as members of the exceptional large-scale structure at z$\sim$5.1-5.3, which is notably elongated along the line of sight, spanning 30 cMpc. We present the spectroscopic confirmation of N2GN_1_13 at z$_{\rm spec}$=5.182, a massive dusty star-forming galaxy identified through targeted NOEMA observations, and N2GN_1_61 at z$_{\rm spec}$=5.201, revealed using JWST/FRESCO data. In addition to these five spectroscopically confirmed members, we identify three further candidates with photometric redshifts consistent with the overdense structure. These galaxies are massive (with a median stellar mass of 9x 10$^{10}$ M$_{\odot}$) and highly obscured (with a median A$_V$ of 3.3), caught in a short-lived yet extreme starburst phase at z$\sim$5.2. Their high SFRs (with a median of 590 M$_{\odot}$ yr$^{-1}$), efficient baryon to stellar mass conversion ($ε_{\star}>$20%), substantial gas reservoir and dust content, suggest rapid evolution and imminent quenching. Six of these galaxies reside in overdense filaments; the remaining two may trace new distinct structures which will have to be spectroscopically confirmed. These few dusty galaxies dominate the star formation within the overdensity, contributing more than the numerous H$_α$ emitters, and surpassing the cosmic average star formation rate density for this epoch. Their properties suggest an accelerated evolution that current models and simulations have difficulty reproducing.

Figures (9)

• Figure 1: N2GN_1_13. Left: False colour image produced using HST data, with F814W, F125W, and F160W images in blue, green, and red, respectively. Overlaid in red are the NOEMA continuum contours at 2 mm, ranging from 4$\sigma$ to 14$\sigma$ in steps of 2. The white contours are JWST F444W brightness. The scale bar in the bottom left has a length of 1". Middle: NOEMA CO(4-3) and CO(5-4) line spectra. The red line shows the Gaussian fit to the line profile (including the continuum). Notice that the CO(5-4) line lies precisely at the interface between two basebands, resulting in the partial fragmentation of the spectral profile. Right: NOEMA 3.5 mm continuum map. Contours are displayed with a step of 12 $\mu$Jy/beam (corresponding to 2$\sigma$).
• Figure 2: Position of our eight N2CLS DSFGs in the SFR--M$_\star$ plane at $z = 5.2$, with sources at spectroscopic and photometric redshifts shown in red and blue, respectively. The SFMS from popesso2023 and speagle2014 is shown, with the $1\sigma$ scatter indicated by the light grey lines. For comparison, we include DSFGs from the GOODS-N field identified by Xiao2024 using JWST at $5.1 < z < 5.4$ (excluding HDF850.1, GN10, and S3, which are part of our N2CLS sample). Galaxies from the 2 deg$^2$ simulated infrared dusty extragalactic sky (SIDES) simulation Bethermin2017, selected in a similar way as N2CLS with $S_{1.2\,\mathrm{mm}} > 0.7$ mJy at $5.1 < z < 5.4$, are shown as downward orange triangles. The blue area indicates the location of the z=5.2 DSFGs from kumar2025, selected based on their 850 $\mu$m flux, using a flux threshold consistent with our N2CLS sample.
• Figure 3: Stellar masses of our galaxies at $z\sim5.2$ compared to expectations from the simple relation that bounds the total stellar content of a dark matter halo to $M_{\star} \le \epsilon_{\star}\,f_b\,M_{halo}$, where $\epsilon_{\star}$ (which should be $\leq 1$) is the efficiency with which baryons are converted into stars and $f_b$ is the cosmic baryonic fraction. We indicate the maximum stellar mass ($M_{\star}^{\rm max}$) calculated from the maximum halo mass given our survey volume (160 arcmin$^2$, z=5.2$\pm$0.5) for $\epsilon_{\star}$=0.2 (dotted line) and $\epsilon_{\star}$=1 (dashed grey line). We also show $M_{\star}^{\rm max}$ for $\epsilon_{\star}$=0.2 using the full sky area (dotted line).
• Figure 4: Contribution of the dusty galaxies in the GOODS-N overdensity to the cosmic SFRD, $\psi$, at z$\sim$5.2. The red star represents the SFRD derived from spectroscopically confirmed galaxies, while the blue star also includes the contribution from galaxies with photometric redshifts. The purple curves show the total SFRD as a function of redshift: the dashed line corresponds to the Madau2014 result, obtained by correcting the UV SFRD for dust attenuation, and the solid line to fujimoto2024, obtained by summing the obscured (IR) and unobscured (UV) contributions. At z$\sim$5, the dust-obscured SFRD estimates from Traina2024 and sun2025 are shown as an orange diamond and circle, respectively. The orange and blue shaded areas represent the obscured and unobscured SFRD, respectively, from Zavala2021.
• Figure 5: Large-scale structure of the overdense environment in the GOODS-N field at $z = 5.17-5.40$. Galaxy positions are shown relative to HDF850.1, in units of comoving Mpc. All galaxies are colour-coded by redshift. The eight N2CLS galaxies are indicated by crosses (for spectroscopic redshifts, $z_{\mathrm{spec}}$) and upward pointing triangles (for photometric redshifts, $z_{\mathrm{phot}}$); their NIKA2 identifiers are labelled in red, with source 20 discussed in Appendix \ref{['App:ID20']}. Galaxy groups and filaments are taken from the JADES survey Sun2024, with galaxies in less dense regions shown as transparent symbols. Additional galaxies, shown as dots, are drawn from the FRESCO survey Herard-Demanche2024.