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Semi-analytic models (SAMs) systematically predict higher stellar-massscatter at a given halo mass than hydrodynamical simulations and most empiricalmodels. Our goal is to investigate the physical origin of this scatter byexploring modifications to the physics in the SAM Dark Sage. We design twoblack hole formation models that approximate results from the IllustrisTNG300-1 hydrodynamical simulation. In the first model, we assign a fixed blackhole mass of $10^{6}\, \mathrm{M}_{\odot}$ to every halo that reaches$10^{10.5}\, \mathrm{M}_{\odot}$. In the second model, we disregard any blackhole growth as implemented in the standard Dark Sage model. Instead, we forceall black hole masses to follow the median black hole mass-halo mass relationin IllustrisTNG 300-1 with a fixed scatter. We find that each model on its owndoes not significantly reduce the scatter in stellar mass. To do this, wereplace the native Dark Sage AGN feedback model with a simple model where weturn off cooling for galaxies with black hole masses above $10^{8}\,\mathrm{M}_{\odot}$. With this additional modification, the SMBH seeding andfixed conditional distribution models find a significant reduction in thescatter in stellar mass at halo masses between $10^{11-14}\,\mathrm{M}_{\odot}$. These results suggest that AGN feedback in SAMs acts in aqualitatively different way than feedback implemented in cosmologicalsimulations. Either or both may require substantial modification to match theempirically inferred scatter in the Stellar Mass Halo Mass Relation (SMHMR).

Why do semi-analytic models predict higher scatter in the stellar mass-halo mass relation than cosmological hydrodynamic simulations?