Constraints on the dark matter sound speed from galactic scales: the cases of the Modified and Extended Chaplygin Gas

TL;DR

This paper uses galaxy rotation curves to tightly constrain the dark matter sound speed, providing new limits on certain dark energy models and explaining why previous cosmological constraints were less restrictive.

Contribution

It presents the most stringent local constraint on dark matter sound speed from galactic data and analyzes limitations of extended Chaplygin Gas models.

Findings

Dark matter sound speed constrained to less than 10^{-4} c

Galaxy rotation curves provide tighter constraints than CMB and BAO

Limitations of Extended Chaplygin Gas discussed

Abstract

We show that the observed rotation curves of spiral galaxies constrain the sound speed of the dark matter to be $c_s < 10^{-4} c$, where $c$ is the speed of light in vacuum. Using the Modified Chaplygin Gas as a representative example of a class of unified dark energy models incorporating an effective dark matter component with a non-zero sound speed, we determine the most stringent constraint to date on the value of the constant contribution to the equation of state parameter in this class of models. Finally, we explain the reason why previous constraints using the Cosmic Microwave Background and Baryonic Acoustic Oscillations were not as competitive as the one presented in this paper and discuss the limitations of the recently proposed Extended Chaplygin Gas.