Collapse of spherical overdensities in superfluid models of dark matter

TL;DR

This paper investigates how superfluid dark matter models influence the formation and collapse of small-scale cosmic structures, revealing that superfluid properties can enhance collapse efficiency compared to non-superfluid models.

Contribution

It introduces a numerical analysis of superfluid dark matter collapse in an expanding universe, highlighting the effects of superfluid fractions, temperature, and interactions on structure formation.

Findings

Superfluid dark matter can collapse more efficiently than expected.

Core regions become more superfluid during collapse but eventually lose superfluidity.

Formation of superfluid halos surrounded by normal dark matter is unlikely.

Abstract

We intend to understand cosmological structure formation within the framework of superfluid models of dark matter with finite temperatures. Of particular interest is the evolution of small-scale structures where the pressure and superfluid properties of the dark matter fluid are prominent. We compare the growth of structures in these models with the standard cold dark matter paradigm and non-superfluid dark matter. The equations for superfluid hydrodynamics were computed numerically in an expanding $\Lambda$CDM background with spherical symmetry; the effect of various superfluid fractions, temperatures, interactions, and masses on the collapse of structures was taken into consideration. We derived the linear perturbation of the superfluid equations, giving further insights into the dynamics of the superfluid collapse. We found that while a conventional dark matter fluid with self-interactions and finite temperatures experiences a suppression in the growth of structures on smaller scales, as expected due to the presence of pressure terms, a superfluid can collapse much more efficiently than was naively expected due to its ability to suppress the growth of entropy perturbations and thus gradients in the thermal pressure. We also found that the cores of the dark matter halos initially become more superfluid during the collapse, but eventually reach a point where the superfluid fraction falls sharply. The formation of superfluid dark matter halos surrounded by a normal fluid dark matter background is therefore disfavored by the present work.