Self-interactions of ULDM to the rescue?

TL;DR

This paper investigates how ultralight dark matter with self-interactions can satisfy galactic rotation curves and soliton-halo relations, suggesting that even tiny repulsive self-couplings can reconcile ULDM with cosmological observations.

Contribution

It revisits constraints on ULDM by incorporating self-interactions into the soliton-halo relation, showing that small repulsive couplings can align ULDM with observational data.

Findings

Repulsive self-interactions can satisfy galactic rotation curves and SH relations.

Tiny self-coupling constants (~10^{-90}) are sufficient for compatibility.

ULDM remains a viable dark matter candidate with self-interactions considered.

Abstract

One of the most important questions in cosmology is concerning the fundamental nature of dark matter (DM). DM could consist of spinless particles of very small mass i.e. $m \sim 10^{-22}\ \text{eV}$. This kind of ultralight dark matter (ULDM) would form cored density profiles (called "solitons") at the centre of galaxies. In this context, recently it has been argued that (a) there exists a power law relation between the mass of the soliton and mass of the surrounding halo called the Soliton-Halo (SH) relation, and, (b) the requirement of satisfying observed galactic rotation curves as well as SH relations is so stringent that ULDM is disfavoured from comprising $100\%$ of the total cosmological dark matter. In this work, we revisit these constraints for ULDM particles with non-negligible quartic self-interactions. Using a recently obtained soliton-halo relation which takes into account the effect of self-interactions, we present evidence which suggests that, for $m = 10^{-22}\ \text{eV}$, the requirement of satisfying both galactic rotation curves as well as SH relations can be fulfilled with repulsive self-coupling $\lambda \sim \mathcal{O}(10^{-90})$.