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Resolution and Gas Smoothing Length

CosmicGrain's dust predictions are coupled to the resolved gas density field. A numerical gas smoothing-length floor can therefore propagate into star formation, enrichment, and dust production even though MinGasHsml is not a dust-physics parameter.

Current implementation

The SPH density solver applies MinGasHsml only when its value is greater than zero. A value of zero therefore disables the explicit floor; the normal neighbor solution and other hydrodynamic/gravitational limits still apply.

The current source also contains an optional diagnostic for particles that hit the floor, which is useful when establishing whether the parameter is actually active in a run.

CosmicGrain resolution tests

Dedicated CG10 tests showed that this parameter can be scientifically important:

  • at 512³-equivalent resolution, MinGasHsml=1.0 produced floor hits and a substantially lower present-day SFR than 0.5, 0.25, or 0.0; the latter three runs were effectively identical in the tested diagnostics;
  • at 1024³-equivalent resolution, lowering the floor from 1.0 to 0.25 removed the floor-limited gas population and increased the high-density tail, SFR, and surviving dust mass;
  • a 1024³ run with MinGasHsml=0.0 is therefore used as the primary CG10 analysis realization while higher-resolution behavior continues to be checked explicitly.

These results should not be interpreted as a universal physical calibration of MinGasHsml. They demonstrate that an imposed smoothing-length floor can become an active resolution limit and must be audited rather than inherited unchanged between resolutions.

For each production resolution:

  1. record the minimum and percentile distribution of gas smoothing lengths;
  2. report the fraction of relevant gas at the explicit floor;
  3. compare the high-density tail (nH, SFR-producing gas) between candidate floor values;
  4. compare stellar mass, SFR, and dust mass at matched outputs; and
  5. only then adopt a production value.

A floor that is never reached should not change the solution. A floor that is frequently reached is part of the effective numerical resolution and should be reported with the simulation setup.