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Register a new userConstraining annihilating dark matter mass by the radio continuum spectral data of a high-redshift galaxy cluster
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In the past decade, the properties of annihilating dark matter models were examined by various kinds of data, including the data of gamma rays, radio waves, X-ray, positrons, electrons, antiprotons and neutrinos. In particular, most of the studies focus on the data of our Galaxy, nearby galaxies (e.g. M31 galaxy) or nearby galaxy clusters (e.g. Fornax cluster). In this article, we examine the archival radio continuum spectral data of a relatively high-redshift galaxy cluster (A697 cluster) to constrain the properties of annihilating dark matter. We find that leptophilic annihilation channels ($e^+e^-$, $mu^+mu^-$ and $tau^+tau^-$) can give very good fits to the radio continuum spectrum of the A697 cluster.
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Recent gamma-ray and radio observations provide stringent constraints for annihilating dark matter. The current $2sigma$ lower limits of dark matter mass can be constrained to $sim 100$ GeV for thermal relic annihilation cross section. In this article, we use the radio continuum spectral data of a nearby galaxy NGC4214 and differentiate the thermal contribution, dark matter annihilation contribution and cosmic-ray contribution. We can get more stringent constraints of dark matter mass and annihilation cross sections. The $5sigma$ lower limits of thermal relic annihilating dark matter mass obtained are 300 GeV, 220 GeV, 220 GeV, 500 GeV and 600 GeV for $e^+e^-$, $mu^+mu^-$, $tau^+tau^-$, $W^+W^-$ and $bbar{b}$ channels respectively. These limits challenge the dark matter interpretation of the gamma-ray, positron and antiproton excess in our Milky Way.
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 atleast by a factor of 100 too low.
Dark matter (DM) is the most abundant material in the Universe, but has so far been detected only via its gravitational effects. Several theories suggest that pairs of DM particles can annihilate into a flash of light at gamma-ray wavelengths. While gamma-ray emission has been observed from environments where DM is expected to accumulate, such as the centre of our Galaxy, other high energy sources can create a contaminating astrophysical gamma-ray background, thus making DM detection difficult. In principle, dwarf galaxies around the Milky Way are a better place to look -- they contain a greater fraction of DM with no astrophysical gamma-ray background -- but they are too distant for gamma-rays to have been seen. A range of observational evidence suggests that Omega Centauri (omega Cen or NGC 5139), usually classified as the Milky Ways largest globular cluster, is really the core of a captured and stripped dwarf galaxy. Importantly, Omega Cen is ten times closer to us than known dwarfs. Here we show that not only does Omega Cen contain DM with density as high as compact dwarf galaxies, but also that it emits gamma-rays with an energy spectrum matching that expected from the annihilation of DM particles with mass 31$pm$4 GeV (68% confidence limit). No astrophysical sources have been found that would otherwise explain Omega Cens gamma-ray emission, despite deep multi-wavelength searches. We anticipate our results to be the starting point for even deeper radio observations of Omega Cen. If multi-wavelength searches continue to find no astrophysical explanations, this pristine, nearby clump of DM will become the best place to study DM interactions through forces other than gravity.
In the past few years, some studies claimed that annihilating dark matter with mass $sim 10-100$ GeV can explain the GeV gamma-ray excess in our Galaxy. However, recent analyses of the Fermi-LAT and radio observational data rule out the possibility of the thermal relic annihilating dark matter with mass $m le 100$ GeV for some popular annihilation channels. By using the new observed radio data of the Andromeda galaxy, we rule out the existence of $sim 100-300$ GeV thermal relic annihilating dark matter for ten annihilation channels. The lower limits of annihilating dark matter mass are improved to larger than 330 GeV for the most conservative case, which is a few times larger than the current best constraints. Moreover, these limits strongly disfavor the benchmark model of weakly interacting massive particle (WIMP) produced through the thermal freeze-out mechanism.
In this Letter, we report the discovery of a radio halo in the high-redshift galaxy cluster PSZ2 G099.86+58.45 ($z=0.616$) with the LOw Frequency ARray (LOFAR) at 120-168 MHz. This is one of the most distant radio halos discovered so far. The diffuse emission extends over $sim$ 1 Mpc and has a morphology similar to that of the X-ray emission as revealed by XMM-Newton data. The halo is very faint at higher frequencies and is barely detected by follow-up 1-2 GHz Karl G.~Jansky Very Large Array (JVLA) observations, which enable us to constrain the radio spectral index to be $alphaleq 1.5-1.6$, i.e.; with properties between canonical and ultra-steep spectrum radio halos. Radio halos are currently explained as synchrotron radiation from relativistic electrons that are re-accelerated in the intra-cluster medium (ICM) by turbulence driven by energetic mergers. We show that in such a framework radio halos are expected to be relatively common at $sim150$ MHz ($sim30-60%$) in clusters with mass and redshift similar to PSZ2 G099.86+58.45; however, at least 2/3 of these radio halos should have steep spectrum and thus be very faint above $sim 1$ GHz frequencies. Furthermore, since the luminosity of radio halos at high redshift depends strongly on the magnetic field strength in the hosting clusters, future LOFAR observations will also provide vital information on the origin and amplification of magnetic fields in galaxy clusters.
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