Comment on “Constraining the annihilating dark matter mass by the radio continuum spectral data of the NGC4214 galaxy”

In their recent paper, Chan and Lee discuss an interesting possibility: radio continuum emission from a dwarf irregular galaxy may be used to constrain upper limits on the cross section of annihilating dark matter. They claim that the contributions from nonthermal and thermal emission can be estimated with such accuracy that one can place new upper limits on the annihilation cross section. We argue that the observations presented can be explained entirely with a standard spectrum and no contribution from dark matter. As a result, the estimated upper limits of Chan and Lee are at least by a factor of 100 too low.

Figure 1

The 6-cm radio flux density in dwarf irregular galaxies as a function of the $\mathrm{H}\alpha$ flux. The data can be fitted by a power law with a slope of $0.99\pm 0.05$ in accordance with a linear relation. The standard deviation is 0.18 dex as indicated by the shaded region around the best-fitting line. As can be seen, NGC 4214 is entirely consistent with the relation and no additional radio-emitting component is needed. As we argue in the text, the radio flux density would have to be $8\times$ higher than what is observed to claim a $5\text{−}\sigma$ detection. A few brighter dwarf irregular galaxies are annotated. Data taken from Hindson et al. [4].