Synchrotron Emission from Dark Matter in Galactic Subhalos. A Look into the Smith Cloud

TL;DR

This paper investigates the potential radio signals from dark matter annihilation in the Smith Cloud, proposing that low-frequency radio observations could provide constraints on dark matter properties.

Contribution

It introduces a detailed phenomenological model of radio emission from dark matter annihilation in the Smith Cloud and derives constraints using existing radio data.

Findings

Radio fluxes are independent of propagation uncertainties.

Low-frequency observations are optimal for detecting dark matter signals.

Constraints on dark matter annihilation cross section are established from radio data.

Abstract

One of the key predictions of the WIMP paradigm for Dark Matter (DM) is that DM particles can annihilate into charged particles. These annihilations will proceed in e.g. Galactic subhalos such as dwarf Galaxies or, as recently pointed out, high velocity clouds such as the "Smith Cloud". In this note, we focus on the radio emission associated with DM annihilations into electrons and positrons occurring in the Smith Cloud. The phenomenology of this emission is discussed in quite some detail. We argue that the uncertainties in the propagation can be captured by the typical diffusion-loss length parameter (Syrovatskii variable) but that the angle-integrated radio fluxes are independent of the propagation. We conclude that if the Smith Cloud is indeed dominated by DM, radio signals from DM annihilation stand out amongst other messengers. Furthermore, low frequencies such as the ones observed by e.g. the Low Frequency Array (LOFAR) and the next-generation Square Kilometre Array (SKA) are optimal for searches for DM in the Smith Cloud. As a practical application, we set conservative constraints on dark matter annihilation cross section using data of continuum radio emission from the Galaxy at 22 MHz and at 1.4 GHz. Stronger constraints could be reached by background subtraction, exploiting the profile and frequency dependence of the putative DM signal. We set stronger but tentative limits using the median noise in brightness temperature from the Green Bank Telescope and the LOFAR sensitivities.