Testing the dark matter origin of the WMAP-Planck Haze with radio observations of Spiral Galaxies

TL;DR

This study tests the dark matter origin of the Milky Way's radio haze by examining 66 spiral galaxies, finding many are too faint in radio emission to support a universal dark matter explanation, thus challenging the haze's dark matter origin.

Contribution

It provides observational constraints on the dark matter hypothesis for galactic radio halos by analyzing radio emissions from a diverse sample of spiral galaxies.

Findings

Many galaxies are less than 5% as bright as expected from dark matter models.

At least 3 galaxies are less than 1% as bright as expected.

Results are marginally compatible with dark matter origin, suggesting possible suppression mechanisms.

Abstract

If the Galactic WMAP radio haze, as recently confirmed by Planck, is produced by dark matter annihilation or decay, similar diffuse radio halos should exist around other galaxies with physical properties comparable to the Milky Way. If instead the haze is due to an astrophysical mechanism peculiar to the Milky Way or to a transient event, a similar halo need not exist around all Milky Way "twins". We use radio observations of 66 spiral galaxies to test the dark matter origin of the haze. We select galaxies based on morphological type and maximal rotational velocity, and obtain their luminosities from a 1.49 GHz catalog and additional radio observations at other frequencies. We find many instances of galaxies with radio emission that is less than 5% as bright as naively expected from dark matter models that could produce the Milky Way haze, and at least 3 galaxies that are less than 1% as bright as expected, assuming dark matter distributions, magnetic fields, and cosmic ray propagation parameters equal to those of the Milky Way. For reasonable ranges for the variation of these parameters, we estimate the fraction of galaxies that should be expected to be significantly less bright in radio, and argue that this is marginally compatible with the observed distribution. While our findings therefore cannot rule out a dark matter origin for the radio haze at this time, we find numerous examples (including the Andromeda Galaxy) where, if dark matter is indeed the origin of the Milky Way haze, some mechanism must be in place to suppress the corresponding haze of the external galaxy. We point out that Planck data will offer opportunities to improve this type of constraint in a highly relevant frequency range and for a potentially larger set of candidate galaxies.