Cosmological structure formation in scalar field dark matter with repulsive self-interaction: the incredible shrinking Jeans mass

Scalar field dark matter (SFDM) comprised of ultralight (greater than or similar to 10(-22) eV) bosons is an alternative to standard, collisionless cold dark matter (CDM) that is CDM-like on large scales but inhibits small-scale structure formation. As a Bose-Einstein condensate, its free-field ('fuzzy') limit (FDM) suppresses structure below the de Broglie wavelength, similar to lambda(deB), creating virialized haloes with central cores of radius similar to lambda(deg), surrounded by CDM-like envelopes, and a halo mass function (HMF) with a sharp cut-off on small scales. With a strong enough repulsive self-interaction (SI), structure is inhibited, instead, below the Thomas-Fermi (TF) radius, RTF (the size of an SI-pressure-supported (n = 1)-polytrope), when R-TF > lambda(deB). Previously, we developed tools to describe SFDM dynamics on scales above lambda(deB) and showed that SFDM-TF haloes formed by Jeans-unstable collapse from non-cosmological initial conditions have RTF-sized cores, surrounded by CDM-like envelopes. Revisiting SFDM IF in the cosmological context, we simulate halo formation by cosmological infall and collapse, and derive its transfer function from linear perturbation theory to produce cosmological initial conditions and predict statistical measures of structure formation, such as the HMF. Since FDM and SFDM-TF transfer functions both have small-scale cut-offs, we can align them to let observational constraints on FDM proxy for SPDM-TF, finding FDM with particle masses 1 less than or similar to m/(10(-22) eV/c(2)) less than or similar to 30 corresponds to SFDM-TF with 10 greater than or similar to R-TF/(1 pc) greater than or similar to 1, favouring subgalactic (sub-kpc) core size. The SFDM-TF IIMF cuts off gradually, however, leaving more small-mass haloes: Its Jeans mass shrinks so fast that scales filtered early can still recover and grow!

Automated summary

Scalar field dark matter (SFDM) comprised of ultralight bosons can inhibit small-scale structure formation while behaving like standard cold dark matter (CDM) on large scales. By simulating halo formation and deriving transfer functions, researchers can study SFDM dynamics in cosmological contexts.