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

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 $\log (M_{\star }/M_{\odot })=11.34_{-0.22}^{+0.15}$, 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 $3.1\pm 0.4$ 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 $400M_{\odot }{\mathrm{yr}}^{-1}$) with dust spread across its disc. Both its stellar mass and dust are more extended than those of $z\approx 3$ 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 $z\approx 4$ can have very different stellar structures, and that not all have their stellar mass concentrated in a compact core.

arXiv comment4 pages, 3 figures, submitted as a A&A letter