Forming stars on a viscous time‐scale: the key to exponential stellar profiles in disc galaxies?

We argue for implementing star formation on a viscous time‐scale in hydrodynamical simulations of disc galaxy formation and evolution. Modelling two‐dimensional isolated disc galaxies with the Bhatnagar–Gross–Krook (BGK) hydrocode, we verify the analytic claim of various authors that if the characteristic time‐scale for star formation is equal to the viscous time‐scale in discs, the resulting stellar profile is exponential on several scalelengths whatever the initial gas and dark matter profile. This casts new light on both numerical and semi‐analytical disc formation simulations that either (a) commence star formation in an already exponential gaseous disc, (b) begin a disc simulation with conditions known to lead to an exponential, i.e. the collapse of a spherically symmetric nearly uniform sphere of gas in solid‐body rotation under the assumption of specific angular momentum conservation, or (c) in simulations performed in a hierarchical context, tune their feedback processes to delay disc formation until the dark matter haloes are slowly evolving and without much substructure so that the gas has the chance to collapse under conditions known to give exponentials. In such models, star formation follows a Schmidt‐like law, which for lack of a suitable time‐scale, resorts to an efficiency parameter. With star formation prescribed on a viscous time‐scale, however, we find gas and star fractions after ∼12 Gyr that are consistent with observations without having to invoke a ‘fudge factor’ for star formation. Our results strongly suggest that despite our gap in understanding the exact link between star formation and viscosity, the viscous time‐scale is indeed the natural time‐scale for star formation.