Extended Data Fig. 9: Compact spheroid-like gas distribution from hydro-simulations of major mergers.

Reanalysis of recent very-high-resolution simulations of mergers of turbulent clumpy disks⁴². a, Maps (2 × 2 kpc) of the central gas in three different mergers, showing the flattest projection for these systems observed at 12 Myr from coalescence, that is, these systems are 3D spheroidal structures, not face-on disks. b,c, Evolution of SFR surface density and disk thickness C/A over time after merger coalescence for three merger orbits, respectively. Panel b distinguishes the nuclear regions of the mergers in which submm emission is bright from the wider outskirts. Time analysis shows that the spheroidal shape of the gas can be maintained over approximately 30–50 Myr. This is compatible with the inferred timescales for the submm-bright bursts based on observations. After the intensely star-forming spheroid-like star formation, lacking further turbulent energy injection back into the system, the residual gas flattens into a disk (panel c). However, the earlier phase led to the formation of a stellar spheroid.