MeV Dark Matter and Small Scale Structure

Abstract

WIMPs with electroweak scale masses (neutralinos, etc.) remain in kinetic equilibrium with other particle species until temperatures approximately in the range of 10 MeV to 1 GeV, leading to the formation of dark matter substructure with masses as small as $10^{-4} M_{\odot}$ to $10^{-12} M_{\odot}$. However, if dark matter consists of particles with MeV scale masses, as motivated by the observation of 511 keV emission from the Galactic Bulge, such particles are naturally expected to remain in kinetic equilibrium with the cosmic neutrino background until considerably later times. This would lead to a strong suppression of small scale structure with masses below about $10^7 M_{\odot}$ to $10^4 M_{\odot}$. This cutoff scale has important implications for present and future searches for faint Local Group satellite galaxies and for the missing satellites problem.

Figures (3)

• Figure 1: The effect of keV scale kinetic decoupling on the matter power spectrum, as predicted in MeV dark matter. Shown are results for a 1 MeV dark matter particle with a 10 keV (solid) and 1.0 keV (dashed) kinetic decoupling temperature. The dotted line denotes the limit relevant for warm dark matter, as inferred from observations of the lyman-alpha forest lymanalpha.
• Figure 2: The thermal relic abundance of dark matter as a function of its mass for $m_U=1$ MeV (solid), 3 MeV (dashed) and 10 MeV (dotted). In each case, the product $g_{U\phi \phi} g_{U f f}$ was set to $10^{-6}$ for each of $f=e$, $\nu_e$, $\nu_{\mu}$ and $\nu_{\tau}$. The dashed horizontal lines denote the measured density of dark matter wmap. For illustration, we have shown the complete range for the dark matter mass, although only $m_{\phi} < m_U$ is viable Jacoby:2007vs.
• Figure 3: Regions in the $m_U$ versus $g_{U\phi \phi} g_{U f f}$ plane in which the measured dark matter density matches the thermal relic abundance for some value of $m_{\phi}$ in the range of $m_e$ to 3 MeV. We have adopted a common $U$-fermion-fermion coupling for electrons and neutrinos. The dashed line denotes the constraint from $\nu e$ scattering experiments (for the optimal case of $g_{U \phi \phi}\approx 1)$scalarfayet. The dotted line denotes the (weaker) constraint from measurements of the electron's magnetic moment scalar. The light blue lines are contours of constant $M_c$ from $10^4$ to $10^7$ solar masses. Here, we have used $m_{\phi}=1$ MeV. From this figure, we see that once all of the constraints are considered, $M_c$ in the range of $10^4 \, M_{\odot}$ to $10^7$$M_{\odot}$ are generally expected.