Leviathan: a very massive, extended sub-millimetre galaxy spectroscopically confirmed at z = 4.05

Abstract

We present Leviathan, a sub-millimetre galaxy (SMG) spectroscopically confirmed at z=4.051 with JWST/NIRSpec. Modelling 31 bands of photometry from the rest-frame ultraviolet to the millimetre, we find a stellar mass of log(M_⋆/M_⊙)=11.34^+0.15_−0.22, making Leviathan one of the most massive spectroscopically confirmed galaxies at z>4, yet consistent with ΛCDM for a star-formation efficiency of approximately 20%. Although SMGs are often thought to be building the dense cores of compact quiescent galaxies, Leviathan is extended, with a rest-frame 0.9 μm half-light radius of 4.3 kpc. Spatially resolved SED fitting gives a half-mass radius of 3.1±0.4 kpc, consistent with its size in rest-frame 1.5 μm imaging. This is three times larger than quiescent galaxies of the same mass, but consistent with the size-mass relation of star-forming galaxies at 4<z<5 extrapolated to M_⋆=10^11.3M_⊙. Leviathan forms stars on the main sequence (approximately 400 M_⊙ yr⁻¹) with dust spread across its disc. Both its stellar mass and dust are more extended than those of z≈3 SMGs. Alongside GN20.2b, which has a similar mass and star-formation rate but is three times smaller, these show that massive dusty galaxies at z≈4 can have very different stellar structures, and that not all have their stellar mass concentrated in a compact core.

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BibTeX RIS

BibTeX

@misc{baker2026leviathan,
  title = {{Leviathan: a very massive, extended sub-millimetre galaxy spectroscopically confirmed at $\rm z = 4.05$}},
  author = {William M. Baker and Francesco Valentino and Jens Hjorth and Gabriel Brammer and Ugnė Dudzevičiūtė and Kei Ito and Antonio Pensabene and Pengpei Zhu},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.27101},
  eprint = {2610.27101},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.27101},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - Leviathan: a very massive, extended sub-millimetre galaxy spectroscopically confirmed at z = 4.05
AU  - William M. Baker
AU  - Francesco Valentino
AU  - Jens Hjorth
AU  - Gabriel Brammer
AU  - Ugnė Dudzevičiūtė
AU  - Kei Ito
AU  - Antonio Pensabene
AU  - Pengpei Zhu
PY  - 2026
DA  - 2026/10/19
PB  - arXiv
DO  - 10.48550/arxiv.2610.27101
UR  - https://arxiv.org/abs/2610.27101
AB  - We present Leviathan, a sub-millimetre galaxy (SMG) spectroscopically confirmed at z=4.051 with JWST/NIRSpec. Modelling 31 bands of photometry from the rest-frame ultraviolet to the millimetre, we find a stellar mass of log(M_⋆/M_⊙)=11.34^+0.15_−0.22, making Leviathan one of the most massive spectroscopically confirmed galaxies at z>4, yet consistent with ΛCDM for a star-formation efficiency of approximately 20%. Although SMGs are often thought to be building the dense cores of compact quiescent galaxies, Leviathan is extended, with a rest-frame 0.9 μm half-light radius of 4.3 kpc. Spatially resolved SED fitting gives a half-mass radius of 3.1±0.4 kpc, consistent with its size in rest-frame 1.5 μm imaging. This is three times larger than quiescent galaxies of the same mass, but consistent with the size-mass relation of star-forming galaxies at 4<z<5 extrapolated to M_⋆=10^11.3M_⊙. Leviathan forms stars on the main sequence (approximately 400 M_⊙ yr^−1) with dust spread across its disc. Both its stellar mass and dust are more extended than those of z≈3 SMGs. Alongside GN20.2b, which has a similar mass and star-formation rate but is three times smaller, these show that massive dusty galaxies at z≈4 can have very different stellar structures, and that not all have their stellar mass concentrated in a compact core.
ER  -

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