Sub-millimeter galaxies in hierarchical models: revisiting the need for a top-heavy stellar initial mass function with Bayesian optimisation

Abstract The properties of high-redshift sub-millimetre galaxies (SMGs) remain controversial within hierarchical structure formation models. We revisit whether a top-heavy stellar initial mass function (IMF) in starbursts is required to reproduce both SMG observations and local galaxy properties. Using Bayesian optimisation, we perform an extensive search of the 15-dimensional parameter space of the GALFORM semi-analytical model. This efficient approach converges to optimal parameter values in fewer than 200 model evaluations, representing orders of magnitude fewer runs than traditional methods. We test whether GALFORM can simultaneously match three key observational constraints: the z = 0 K-band luminosity function, the SMG number counts at 850 μm, and the SMG redshift distribution. We consider two model variants: one with a universal solar neighbourhood IMF for all star formation, and another allowing the IMF slope in starbursts to vary as a free parameter. When assuming a universal Chabrier IMF, we find no parameter combination that simultaneously reproduces all three datasets. The model either matches the SMG constraints while grossly overpredicting the local K-band luminosity function, or matches the local luminosity function while severely underpredicting SMG counts by factors of 3–100. In contrast, allowing a top-heavy IMF in starbursts enables excellent simultaneous fits to all constraints. The best-fitting model prefers an IMF slope parameter x ≈ 0.7 (where dn/dlnm∝m−x). Our comprehensive parameter space exploration confirms that, given the implementation of galaxy formation physics within GALFORM, a top-heavy IMF in starbursts is necessary to reconcile high-redshift dusty star-forming galaxies with local galaxy populations.