A tale of two clump masses: a new way to study clump formation in simulations

We present a new method to study the characteristic scales of collapse and fragmentation in galactic discs. Clump formation is seeded in simulations via controlled perturbations with a specified wavelength and velocity. These are applied to otherwise quiet gas discs ranging from analogues of present day spirals to gas-rich, high-redshift galaxies. The results are compared to linear theory, turbulently perturbed discs and observations. The results reflect the expectations of linear, non-axisymmetric theory with a finite window for growth into a bound clump. We identify two new modes of clump formation: rotation-driven fission and fragmentation of tidal tails, though both are expected to rarely contribute to clump formation in observed discs. We find that bound clumps are generally much smaller than the commonly used Toomre mass. The preferred scale for fragmentation increases with the disc gas mass but cannot produce bound objects larger than ∼109 M⊙. The most likely bound clump mass increases from 3 × 106 in low-mass discs up to 5 × 108 M⊙. We conclude that observed massive stellar and gaseous clumps on 1 kpc scales at high redshift are most likely aggregates of many initially distinct bound clumps.