Alternative Dark Matter

Cold dark matter is the default, but Galacticus can follow warm, fuzzy, self-interacting, and decaying dark matter through the whole pipeline. The dark matter particle class declares the candidate and its properties (a thermal relic mass, an axion mass, a self-interaction cross-section, a decay lifetime and kick velocity). The transfer function then carries the small-scale cutoff (Bode et al. (2001) for thermal warm dark matter, the ETHOS fitting form of Cyr-Racine et al. (2016), Murgia et al. (2017) or Passaglia and Hu (2022) for fuzzy dark matter, or a direct axionCAMB computation), and the critical overdensity and merger tree branching classes have implementations that account for the suppressed collapse of small halos so that tree building reproduces the reduced abundance of low-mass progenitors.

        flowchart LR
   Particle[<a href='https://galacticus.readthedocs.io/en/latest/physics/darkMatterParticle.html' style='text-decoration: none'>Dark Matter Particle</a>]
   Transfer[<a href='https://galacticus.readthedocs.io/en/latest/physics/transferFunction.html' style='text-decoration: none'>Transfer Function</a>]
   Critical[<a href='https://galacticus.readthedocs.io/en/latest/physics/criticalOverdensity.html' style='text-decoration: none'>Critical Overdensity</a>]
   Branching[<a href='https://galacticus.readthedocs.io/en/latest/physics/mergerTreeBranchingProbability.html' style='text-decoration: none'>Branching Probability</a>]
   Profile[<a href='https://galacticus.readthedocs.io/en/latest/physics/darkMatterProfileDMO.html' style='text-decoration: none'>Halo Profile</a>]
   Heating[<a href='https://galacticus.readthedocs.io/en/latest/physics/darkMatterProfileHeating.html' style='text-decoration: none'>Profile Heating</a>]
   Trees([Merger trees])
   Subhalos([Subhalo evolution])
   Particle --> Transfer
   Transfer --> Critical
   Critical --> Branching
   Branching --> Trees
   Particle --> Profile
   Heating --> Profile
   Profile --> Subhalos
    

Halo structure follows: the dark matter-only profile class provides soliton-cored profiles for fuzzy dark matter, isothermal and parametric cores for self-interacting dark matter, profiles depleted by decays, and profiles heated by tidal shocks, impulsive outflows, decay kicks, or two-body relaxation. Because subhalo evolution (see Subhalo Evolution) reads the profile through the same interface, tidal stripping and dynamical friction respond to the modified structure without further changes. Dedicated node operators handle the model-specific physics that has no cold dark matter analog, such as gravothermal core collapse in self-interacting models and mass loss through decays.