JWST observes the assembly of a massive galaxy at z~4

Aayush Saxena, Roderik A. Overzier, Catarina Aydar, Jianwei Lyu, George H. Rieke, Victoria Reynaldi, Montserrat Villar-Martín, Krisztina Éva Gabányi, Kenneth J. Duncan, Sándor Frey, Andrew Humphrey, George Miley, Laura Pentericci, Krisztina Perger, Huub Röttgering, Philip Best, Sarah E. I. Bosman, Gyorgy Mező, Masafusa Onoue, Zsolt Paragi, Bram Venemans

First listed 2025-11-17 | Last updated 2026-03-23

Abstract

We present JWST observations of the radio galaxy TGSSJ1530+1049, spectroscopically confirmed at $z=4.0$. NIRCam images and NIRSpec/IFU spectroscopy (R=2700) show that TGSSJ1530+1049 is part of one of the densest-known structures of continuum and line-emitting objects found at these redshifts. NIRCam images show a number of distinct continuum objects and evidence of interactions traced by diffuse emission, and the NIRSpec IFU cube reveals further strong line emitting regions. We identify six continuum and four additional strong H$α$ emitting sources with weaker or no underlying continuum within the 3x3 arcsec IFU field. From spatial alignment with high-resolution radio data and emission line profiles, the radio AGN host galaxy is clearly identified. The bright H$α$ emission (but not the optical components) is distributed remarkably linearly along the radio axis, suggestive of a biconical illumination zone by a central obscured AGN. The emission line kinematics indicate jet-gas interactions on scales of a few kpc. However, due to large relative velocities and presence of underlying continuum, the alignment with the radio structure appears to be, at least partly, caused by a particular configuration of interacting galaxies. At least four objects within a 10x10 (projected) kpc$^2$ area which includes the radio source have high stellar masses ($\log(M_\star/M_\odot)>10.3$) and star formation rates in the range $70-163\,M_\odot$ yr$^{-1}$. Using a stellar mass-based analysis, we predict a total dark matter halo mass of $\approx10^{13} M_\odot$. Based on the physical separations and velocity differences between the galaxies, it is expected that these galaxies will merge to form a massive galaxy within a few Gyr. The system qualitatively resembles the forming brightest cluster galaxies in cosmological simulations that form early through a rapid succession of mergers.

Short digest

JWST NIRCam plus NIRSpec/IFU (R≈2700) dissect the z=4.0 radio galaxy TGSS J1530+1049, revealing one of the densest small-scale assemblies with six continuum sources and four additional strong Hα emitters within a 3×3 arcsec field. The Hα (but not the optical continuum) lies strikingly linearly along the radio axis, identifying C2 as the radio AGN host and pointing to biconical illumination by an obscured nucleus with few‑kpc jet–gas interactions. At least four galaxies within a 10×10 kpc² region have log M⋆/M⊙>10.3 and SFRs of 70–163 M⊙ yr⁻¹, implying a ≈10¹³ M⊙ halo that is likely to merge into a massive galaxy within a few Gyr. A noted caveat is that part of the radio–line alignment may arise from the specific configuration and relative velocities of interacting galaxies.

Key figures to inspect

• Figure 1: Compare NIRCam continuum clumps and diffuse emission to the Hα+[N II] map to see mergers/interactions versus the remarkably linear line emission aligned with the VLBI radio axis, and how SED variations appear across F210M–F430M.
• Figure 2: Inspect the Hα+[N II] versus F300M maps with C1–C6 and L1–L4 apertures to understand which clumps are line-only, their spatial ordering along the putative bicone, and the compact 21 kpc-scale environment.
• Figure 3: Use extracted spectra of C1–C6 and L1–L4 to spot the broad Hα component in C2 that pins down the AGN host, compare line centroids/widths among clumps, and gauge the few‑kpc jet–gas interaction kinematics.
• Figure 4: Overlay of F300M, Hα+[N II], and e‑MERLIN/L‑band contours to judge how radio hotspots and axis relate to the linear Hα chain and where alignment is AGN/jet‑driven versus merger‑configured.