Leviathan: a very massive, extended sub-millimetre galaxy spectroscopically confirmed at 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 (M/M)=11.34+0.15-0.22, making Leviathan one of the most massive spectroscopically confirmed galaxies at z>4, yet consistent with Λ 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,μ m half-light radius of 4.3 kpc. Spatially resolved SED fitting gives a half-mass radius of 3.1 plus or minus0.4 kpc, consistent with its size in rest-frame 1.5,μ 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=10 11.3 M. 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 about3 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 about4 can have very different stellar structures, and that not all have their stellar mass concentrated in a compact core.