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We study the effects of inelastic dark matter self-interactions on theinternal structure of a simulated Milky Way (MW)-size halo. Self-interactingdark matter (SIDM) is an alternative to collisionless cold dark matter (CDM)which offers a unique solution to the problems encountered with CDM onsub-galactic scales. Although previous SIDM simulations have mainly consideredelastic collisions, theoretical considerations motivate the existence ofmulti-state dark matter where transitions from the excited to the ground stateare exothermic. In this work, we consider a self-interacting, two-state darkmatter model with inelastic collisions, implemented in the Arepo code. We findthat energy injection from inelastic self-interactions reduces the centraldensity of the MW halo in a shorter timescale relative to the elastic scale,resulting in a larger core size. Inelastic collisions also isotropize theorbits, resulting in an overall lower velocity anisotropy for the inelastic MWhalo. In the inner halo, the inelastic SIDM case (minor-to-major axis ratio $s\equiv c/a \approx 0.65$) is more spherical than the CDM ($s \approx 0.4$), butless spherical than the elastic SIDM case ($s \approx 0.75$). The speeddistribution $f(v)$ of dark matter particles at the location of the Sun in theinelastic SIDM model shows a significant departure from the CDM model, with$f(v)$ falling more steeply at high speeds. In addition, the velocity kicksimparted during inelastic collisions produce unbound high-speed particles withvelocities up to 500 km s$^{-1}$ throughout the halo. This implies thatinelastic SIDM can potentially leave distinct signatures in direct detectionexperiments, relative to elastic SIDM and CDM.

The impact of inelastic self-interacting dark matter on the dark matter structure of a Milky Way halo