Dissipative Dark Matter on FIRE: Structural and kinematic properties of dwarf galaxies and observational constraints

Despite its veiled nature, dark matter is considered the main driver of structure formation in the Universe. The ΛCDM model has been successful in describing the large scale structures in the Universe, but faces significant challenges in matching observations at small scales. Self-interacting dark matter (SIDM) is an important category of alternative dark matter models proposed to solve small-scale problems. In most astrophysical studies, only elastic scattering of SIDM is considered. However, dissipative self-interaction channels are more ubiquitous in particle physics models and the cosmological structure formation in this scenario has not been studied.

In this series of papers, we present the first set of cosmological hydrodynamical simulations of galaxies including dissipative SIDM (dSIDM). These simulations utilize the Feedback In Realistic Environments (FIRE-2) galaxy formation physics, but allow dissipative dark matter self-interactions parameterized by the self-interaction cross-section and the dimensionless degree of dissipation. The simulation suite includes from low-mass dwarf galaxies to Milky Way-mass galaxies. We find a universal cuspy central density profile in simulated dSIDM dwarfs with the power-law slope asymptoting to about -1.5. In large cross-section models, haloes develop significant coherent rotation of dark matter, accompanied by halo deformation. The bulk properties of galaxies (halo/stellar masses) do not show appreciable difference from CDM, but dark matter kinematics and halo concentrations/shapes can differ.

In terms of observables, we find more compact stellar cores and more prominent stellar/neutral gas disks developed in dSIDM dwarfs, but they are still consistent with the observationally inferred size-mass relation. In addition, as a result of the cuspy central density profiles in dSIDM, the sub-kpc circular velocities of dwarf galaxies are enhanced by about a factor of two, which are still consistent with the kinematic measurements of Local Group satellites but in tension with the HI rotation curve measurements of field dwarfs. For satellites of Milky Way-mass host galaxies, the rotation curves are marginally affected by dSIDM physics, but dSIDM has the implication in explaining the missing compact dwarf populations in CDM. In conclusion, the dSIDM models with cross-section >1 cm²/g are effectively ruled out at the mass scale of bright dwarfs. However, models with lower values of cross-section or velocity-dependent cross-sections are still viable and can give rise to non-trivial observational signatures, especially in isolated dwarfs.