Scalar Field Dark Matter: Helping or Hurting Small-Scale Problems in Cosmology?

TL;DR

This paper models ultra-light scalar field dark matter (SFDM) haloes, analyzing their density profiles and comparing them to observations, revealing that SFDM can both alleviate and exacerbate small-scale cosmological problems depending on halo mass.

Contribution

It provides a comprehensive model for SFDM halo density profiles as a function of mass and scalar field mass, highlighting the need for baryonic feedback in SFDM scenarios.

Findings

Lower core densities in $10^{10} M_\odot$ haloes alleviate TBTF problem.

Higher-mass SFDM haloes are denser than CDM at small radii, worsening cusp/core issues.

SFDM exacerbates small-scale problems unless baryonic feedback is included.

Abstract

Building upon results of cosmological simulations of ultra-light scalar field dark matter (SFDM), we present a comprehensive model for the density profiles of SFDM haloes as a function of halo virial mass $M_{\rm h}$ and scalar field mass $m$. The central regions of SFDM haloes are dominated by solitons with characteristic densities that increase with increasing halo mass and asymptote to CDM-like profiles at large radii. For scalar field masses $m \sim 10^{-22}$ eV, consistent with large-scale structure observations, $M_{\rm h} \sim 10^{10} \,M_\odot$ haloes have lower core densities than their Cold Dark Matter (CDM) counterparts and this alleviates the Too Big to Fail problem (TBTF) in a regime where feedback is less effective. However, higher-mass SFDM haloes with $M_{\rm h} \sim 10^{11} \,M_\odot$ are denser than their CDM counterparts at small, observationally relevant radii. We use rotation curves of $V \sim 100$ km s$^{-1}$ galaxies from the SPARC database to show that SFDM exacerbates the cusp/core and central density problems seen in CDM at this scale. We conclude that if the conventional cosmological SFDM scaling relations are correct, then baryonic feedback is required to lower densities in SFDM haloes even more so than in CDM. This motivates cosmological and self-consistent hydrodynamic simulations of SFDM to determine whether central soliton structure can be altered by realistic feedback implementations.