Constraints on Dark Matter-Electron Scattering from Molecular Cloud Ionization

TL;DR

This paper uses ionization rates in molecular clouds caused by dark matter to set new constraints on dark matter-electron interactions, especially for strongly interacting dark matter that is hard to detect otherwise.

Contribution

It introduces a novel astrophysical method using molecular cloud ionization to constrain dark matter-electron scattering cross sections, complementing existing detection techniques.

Findings

Dark matter with mass > 4 MeV can ionize H2 in molecular clouds.

Constraints on dark matter-electron cross section are derived from observed ionization rates.

Future experiments could further tighten these astrophysical bounds.

Abstract

We demonstrate that ionization of $\text{H}_2$ by dark matter in dense molecular clouds can provide strong constraints on the scattering strength of dark matter with electrons. Molecular clouds have high UV-optical attenuation, shielding them from ultraviolet and X-ray photons. Their chemical and thermal evolution are governed by low-energy cosmic rays. Dark matter with mass $\gtrsim 4$ MeV can ionize $\text{H}_2$, contributing to the observed ionization rate. We require that the dark matter-induced ionization rate of $\text{H}_2$ not exceed the observed cosmic ray ionization rate, $\zeta^{\text{H}_2}$, in diffuse molecular clouds as well as dense molecular clouds such as L1551 in the Taurus cloud complex. This allow us to place strong constraints on the DM-electron cross section, $\bar{\sigma}_e$, that complement existing astrophysical constraints and probe the strongly interacting parameter space where terrestrial and underground direct detection experiments lose sensitivity. We show that constraints from molecular clouds combined with planned balloon and satellite-based experiments would strongly constrain the fractional abundance of dark matter that interacts strongly with electrons. We comment on future modeling and observational efforts that may improve our bounds.