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

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 $\log(M_\star/M_\odot)=11.34^{+0.15}_{-0.22}$, making Leviathan one of the most massive spectroscopically confirmed galaxies at $z>4$, yet consistent with $\Lambda$CDM 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 $0.9\,\mu\mathrm{m}$ half-light radius of 4.3 kpc. Spatially resolved SED fitting gives a half-mass radius of $3.1\pm0.4$ kpc, consistent with its size in rest-frame $1.5\,\mu\mathrm{m}$ 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 $M_\star=10^{11.3}M_\odot$. Leviathan forms stars on the main sequence (approximately $400\,M_\odot\,\mathrm{yr}^{-1}$) with dust spread across its disc. Both its stellar mass and dust are more extended than those of $z\approx3$ 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 $z\approx4$ can have very different stellar structures, and that not all have their stellar mass concentrated in a compact core.