Not so Fast: Revised Calibration of SIDM Core Collapse

Abstract

Self-interacting dark matter (SIDM) haloes undergo gravothermal evolution whose late-time behaviour is central to predictions for dense substructure and black-hole seed formation. We revisit the calibration of this evolution using a new spherical Direct Simulation Monte Carlo code, SphDSMC, which follows spherically symmetric SIDM systems without force softening and to substantially higher central densities than standard N-body simulations. We compare SphDSMC calculations with conventional N-body simulations and with one-dimensional gravothermal fluid models. We show that N-body results converge to SphDSMC only for softening lengths significantly smaller than those normally adopted for collisionless simulations, and that allowing multiple scatterings per particle per time step does not introduce significant errors when implemented appropriately. Calibrating the gravothermal fluid equations against SphDSMC, we find that the commonly used harmonic interpolation between the long- and short-mean-free-path conductivities is inaccurate. A gentler transition, however, yields excellent agreement for NFW haloes over a range of concentrations and truncation radii, with a single long-mean-free-path conductivity parameter. The revised calibration gives a long-mean-free-path collapse time that is somewhat shorter than previous estimates, but we show that collapse slows dramatically once the core becomes optically thick. We provide fitting functions for the late-time evolution and argue that black-hole seed formation by SIDM gravothermal collapse has generally been underestimated in duration, making it unlikely to explain the massive high-redshift black holes inferred from JWST observations without additional physics.