Spheroidal galactic halos and mirror dark matter

TL;DR

The paper addresses the problem that dissipative mirror dark matter would radiate away energy and collapse unless heated. It estimates the cooling time of a typical mirror halo via virial arguments and computes the halo mirror photon luminosity under an isothermal hydrostatic model with a central cutoff. The results give $t_{cool} \sim 3\times 10^{8}$ years and $L_{halo} \sim (3\mathrm{kpc}/R_1)\times 10^{44}$ erg s$^{-1}$, implying heating at order $\sim 10^{43}$ erg s$^{-1}$ is needed to balance losses; heating could come from mirror supernovae or ordinary supernovae via photon–mirror-photon kinetic mixing with $\epsilon \sim 5\times 10^{-9}$. This macroscopic asymmetry between sectors offers a plausible mechanism for the observed disk–halo dichotomy and yields testable implications for the structure and energetics of galactic halos with mirror dark matter.

Abstract

Mirror matter has been proposed as a dark matter candidate. It has several very attractive features, including automatic stability and darkness, the ability to mimic the broad features of cold dark matter while in the linear density perturbation regime, and consistency with all direct dark matter search experiments, both negative (e.g. CDMS II) and positive (DAMA). In this paper we consider an important unsolved problem: Are there plausible reasons to explain why most of the mirror matter in spiral galaxies exists in the form of gaseous {\it spheroidal} galactic halos around ordinary matter {\it disks}? We compute an order-of-magnitude estimate that the mirror photon luminosity of a typical spiral galaxy today is around $10^{44}$ erg/s. Interestingly, this rate of energy loss is similar to the power supplied by ordinary supernova explosions. We discuss circumstances under which supernova power can be used to heat the gaseous part of the mirror matter halo and hence prevent its collapse to a disk. The {\it macro}scopic ordinary-mirror asymmetry plays a fundamental role in our analysis.