A possible signature of annihilating dark matter

In this article, we report a new signature of dark matter annihilation based on the radio continuum data of NGC 1569 galaxy detected in the past few decades. After eliminating the thermal contribution of the radio signal, an abrupt change in the spectral index is shown in the radio spectrum. Previously, this signature was interpreted as an evidence of convective outflow of cosmic ray. However, we show that the cosmic ray contribution is not enough to account for the observed radio flux. We then discover that if dark matter annihilates via the 4-e channel with the thermal relic cross section, the electrons and positrons produced would emit a strong radio flux which can provide an excellent agreement with the observed signature. The best-fit dark matter mass is 25 GeV.Comment: Accepted for publication in MNRA

Constraining annihilating dark matter by x-ray data

In the past decade, gamma-ray observations and radio observations put strong constraints on the parameters of dark matter annihilation. In this article, we suggest another robust way to constrain the parameters of dark matter annihilation. We expect that the electrons and positrons produced from dark matter annihilation would scatter with the cosmic microwave background photons and boost the photon energy to $\sim$ keV order. Based on the x-ray data from the Draco dwarf galaxy, the new constraints for some of the annihilation channels are generally tighter than the constraints obtained from 6 years of Fermi Large Area Telescope (Fermi-LAT) gamma-ray observations of the Milky Way dwarf spheroidal satellite galaxies. The lower limits of dark matter mass annihilating via $e^+e^-$, $\mu^+\mu^-$, $\tau^+\tau^-$, $gg$, $u\bar{u}$ and $b\bar{b}$ channels are 40 GeV, 28 GeV, 30 GeV, 57 GeV, 58 GeV and 66 GeV respectively with the canonical thermal relic cross section. This method is particularly useful to constrain dark matter annihilating via $e^+e^-$, $\mu^+\mu^-$, $gg$, $u\bar{u}$ and $b\bar{b}$ channels.Comment: Accepted for publication in Astrophysics and Space Scienc

Sommerfeld enhancement of invisible dark matter annihilation in galaxies and galaxy clusters

Recent observations indicate that core-like dark matter structures exist in many galaxies, while numerical simulations reveal a singular dark matter density profile at the center. In this article, I show that if the annihilation of dark matter particles gives invisible sterile neutrinos, the Sommerfeld enhancement of the annihilation cross-section can give a sufficiently large annihilation rate to solve the core-cusp problem. The resultant core density, core radius, and their scaling relation generally agree with recent empirical fits from observations. Also, this model predicts that the resultant core-like structures in dwarf galaxies can be easily observed, but not for large normal galaxies and galaxy clusters.Comment: To appear in Astroparticle Physic

Does the gamma-ray signal from the central Milky Way indicate Sommerfeld enhancement of dark matter annihilation?

Recently, Daylan et al. (2014) show that the GeV gamma-ray excess signal from the central Milky Way can be explained by the annihilation of $\sim 40$ GeV dark matter through $b\bar{b}$ channel. Based on the morphology of the gamma-ray flux, the best-fit inner slope of the dark matter density profile is $\gamma=1.26$. However, recent analyses of Milky Way dark matter profile favor $\gamma=0.6-0.8$. In this article, we show that the GeV gamma-ray excess can also be explained by the Sommerfeld-enhanced dark matter annihilation through $b\bar{b}$ channel with $\gamma=0.85-1.05$. We constrain the parameters of the Sommerfeld-enhanced annihilation by using the data from Fermi-LAT. We also show that the predicted gamma-ray fluxes emitted from dwarf galaxies generally satisfy the recent upper limits of gamma-ray fluxes detected by Fermi-LAT.Comment: Accepted for publication in Research in Astronomy and Astrophysic

Indirect evidence of GeV Dark Matter

Recently, an excess of GeV gamma ray near the Galactic Centre has beenvreported. The spectrum obtained can be best fitted with the annihilationvof $30-40$ GeV dark matter particles through $b \bar{b}$ channel. In this letter, I show that this annihilation model can also solve the mysteries of heating source in x-ray plasma and the unexpected high gamma-ray luminosity. The cross section constrained by these observations give excellent agreements with both the predicted range by using Fermi-LAT data and the canonical thermal relic abundance cross section.Comment: Accepted by MNRAS Letter

Analytic expressions for the dark matter-baryon relations

Recently, some very strong correlations between the distribution of dark matter and baryons (the dark matter-baryon relations) in galaxies with very different morphologies, masses, sizes, and gas fractions have been obtained. Some models have been suggested to explain why the dark matter contribution is fully specified by that of the baryons. In this article, we derive two analytic expressions to explain the observed dark matter-baryon relations based on the cold dark matter (CDM) model. The resultant expressions give excellent agreement with the observational data. The parameters involved in the analytic expressions are closely related to the amount of the baryon content. This model can provide a theoretical understanding of the strong correlations observed. We suggest that the observed relation represents the end product of galaxy formation.Comment: 9 pages, appear in International Journal of Modern Physics

Electron-positron pair production near the Galactic Centre and the 511 keV emission line

Recent observations indicate that a high production rate of positrons (strong 511 keV line) and a significant amount of excess GeV gamma-ray exist in our Galactic bulge. The latter issue can be explained by $\sim 40$ GeV dark matter annihilation through $b \bar{b}$ channel while the former one remains a mystery. On the other hand, recent studies reveal that a large amount of high density gas might exist near the Galactic Centre million years ago to account for the young, massive stars extending from 0.04 pc - 7 pc. In this article, I propose a new scenario and show that the 40 GeV dark matter annihilation model can also explain the required positron production rate (511 keV line) in the bulge due to the existence of the high density gas cloud near the supermassive black hole long time ago.Comment: Accepted for publication in MNRAS Letter

Reconciliation of MOND and Dark Matter theory

I show that Modified Newtonian Dynamics (MOND) is equivalent to assuming an isothermal dark matter density profile, with its density related to the enclosed total baryonic mass. This density profile can be deduced by physical laws if a dark matter core exists and if the baryonic component is spherically-symmetric, isotropic and isothermal. All the usual predictions of MOND, as well as the universal constant $a_0$, can be derived in this model. Since the effects of baryonic matter are larger in galaxies than in galaxy clusters, this result may explain why MOND appears to work well for galaxies but poorly for clusters. As a consequence of the results presented here, MOND can be regarded as a misinterpretation of a particular dark matter density profile.Comment: Accepted by Physical Review

A tight scaling relation of dark matter in galaxy clusters

Recent studies in different types of galaxies reveal that the product of the central density and the core radius ($\rho_cr_c$) is a constant. However, some empirical studies involving galaxy clusters suggest that the product $\rho_cr_c$ depends weakly on the total dark halo mass. In this article, we re-analyse the hot gas data from 106 clusters and obtain a surprisingly tight scaling relation: $\rho_c \propto r_c^{-1.46 \pm 0.16}$. This result generally agrees with the claims that $\rho_cr_c$ is not a constant for all scales of structure. Moreover, this relation does not support the velocity-dependent cross section of dark matter if the core formation is due to the self-interaction of dark matter.Comment: Accepted by MNRAS Letter

Can decaying sterile neutrinos account for all dark matter?

The recent discovery of unexplained X-ray line of $3.5-3.6$ keV emitted from the Perseus cluster of galaxies and M31 and the excess X-ray line of $8.7$ keV emitted from the Milky Way center may indicate that dark matter would decay. In this article, I show that approximately 80 \% of dark matter being 7.1 keV sterile neutrinos and 20 \% of dark matter being 17.4 keV sterile neutrinos can satisfactorily explain the observed X-ray lines and account for all missing mass. No free parameter is needed in this model. This scenario is also compatible with current robust observational constraints from the matter power spectrum in large-scale structures and would alleviate the challenges faced by the existing dark matter models.Comment: Accepted in Astrophysics and Space Scienc