Soft Scattering Evaporation of Dark Matter Subhalos by Inner Galactic Gases

TL;DR

This paper investigates how dark matter subhalos can be evaporated by interactions with galactic gases, especially considering dark matter's electromagnetic dipole moments, affecting the distribution of low-mass subhalos in galaxies.

Contribution

It provides a detailed calculation of dark matter kinetic decoupling, protohalo mass, and subhalo evaporation rates considering electromagnetic dipole moments, highlighting their impact on subhalo survival.

Findings

Subhalos lighter than 10^{-5} solar masses can evaporate in the galactic center.

Dark matter's electromagnetic dipole moments below current detection limits influence subhalo evaporation.

Soft scattering with ionized gas and cosmic rays affects low-mass subhalo survival.

Abstract

The large gap between a galactic dark matter subhalo's velocity and its own gravitational binding velocity creates the situation that small subhalos can be evaporated before dark matter thermalize with baryons due to the low binding velocity. In case dark matter acquires an electromagnetic dipole moment, the survival of low-mass subhalos requires stringent limits on the photon-mediated soft scattering. The current stringent direct detection limits indicate for a small dipole moment, which lets DM decouple early and allows small subhalos to form. We calculate the DM kinetic decoupling temperature in the Early Universe and evaluate the smallest protohalo mass. In the late Universe, low-mass subhalos can be evaporated via soft collision by ionized gas and accelerated cosmic rays. We calculate the subhalos evaporation rate and show that subhalos lighter than $10^{-5}M_{\odot}$ in the gaseous inner galactic region are subject to evaporation via dark matter's effective electric and magnetic dipole moments below current direct detection limits, which potentially affects the low-mass subhalos distribution in the galactic center.