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

TL;DR

This work uses JWST/NIRSpec IFU spectroscopy and NIRCam imaging, complemented by HST data, to map a complex, merger-rich environment around TGSSJ1530+1049 at $z=4.0$. Through spatially resolved emission-line analysis, kinematics, and SED fitting with BAGPIPES, the study identifies six continuum sources and four line-emitting regions within a 21 kpc scale, inferring a total stellar mass of $\sim1.5\times10^{11}\,M_\odot$ and a combined SFR of $\sim555\,M_\odot\mathrm{yr}^{-1}$. The H$\alpha$ emission aligns with the radio axis, indicating AGN illumination and jet–gas interactions, while line broadening and [S III] diagnostics reveal AGN- and merger-driven outflows in a multi-component system that is likely to merge into a massive galaxy within a few Gyr. The inferred halo mass of ~$10^{13}\,M_\odot$ places TGSSJ1530 in a rare, dense proto-cluster-like environment at $z\approx4$, offering a direct glimpse into the early assembly of brightest cluster galaxy–type systems and the role of mergers in rapid mass build-up. Overall, TGSSJ1530 provides a concrete observational window into early massive galaxy formation within a dense environment, consistent with rapid, merger-driven growth and protocluster assembly in the early Universe.

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 Halpha emitting sources with weaker or no underlying continuum within the 3''x3'' IFU field. From spatial alignment with high-resolution radio data and emission line profiles, the radio AGN host galaxy is clearly identified. The bright Halpha 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.

Figures (9)

• Figure 1: Top-left to bottom-right: Median stacked HST/ACS images (F606W, F775W, F850LP), HST/WFC3 images (F105W, F125W, F160W), JWST/NIRCam images (F210M, F300M, F430M) and the JWST/NIRSpec IFU image of H$\alpha$+[N ii]. In all panels, the radio VLBI contours at 1.4 GHz from gabanyi25 are shown. The NIRCam images show a highly complex morphology including multiple clumps and diffuse (continuum) emission that may be indicative of mergers and interactions. The H$\alpha$+[N ii] image on the other hand shows a remarkable linear configuration of strong line-emitting sources that tend to be faint or absent in the continuum images. The variation in brightness over the larger wavelength range shows that the SEDs of the individual sources in this complex system vary substantially. The diameter of the maps correspond to a projected physical size of about 21 kpc at $z=4$.
• Figure 2: H$\alpha$+[N ii] 2D image (left) and F300M map (right) of the system, with apertures for the spectral extraction indicated (circles). The extraction is performed for six clear continuum sources (C1-C6) identified from the NIRCam image, and four further emission line regions (L1-L4) that show up prominently in the H$\alpha$+[N ii] map but appear absent or less prominent in the NIRCam images. Note that one of the brightest line-emitting clumps, at the Southern edge of the image, corresponds to a continuum source that had already been labeled C6. The width of the maps corresponds to a projected physical size of about 21 kpc at $z=4$.
• Figure 3: Extracted 1D spectra from continuum sources identified in NIRCam imaging (C1--C6) and line emitting sources identified from NIRSpec (L1--L4), with the locations of prominent emission lines marked. The continuum source C2 is the presumed radio AGN host based on its co-location with the radio emission seen from this system as well as the presence of a broad component in its H$\alpha$+[N ii] complex.
• Figure 4: Composite image showing NIRCam F300M in the background, with H$\alpha$+[N ii] map overlaid, along with the radio VLBI e-MERLIN L-band contours from Gabanyi2018gabanyi25. This composite highlights the different morphologies seen in the emission line and continuum maps. The radio contours appear to originate from the continuum source C2, which we propose to be the radio AGN host.
• Figure 5: Best-fitting line profiles for the H$\alpha$+[N ii] complex seen across all the identified continuum (C1-C6) and line (L1-L6) emitting regions in the data cube, with per pixel uncertainties shown in purple shaded regions. A large variety of line profiles and kinematics are seen across the system, with certain line emitting regions showing the presence of both broad (orange) and narrow (green) line components, tracing disturbed kinematics and the presence of outflows.