Using the Milky Way satellites to study interactions between cold dark matter and radiation

Abstract

The cold dark matter (CDM) model faces persistent challenges on small scales. In particular, taken at face value, the model significantly overestimates the number of satellite galaxies around the Milky Way. Attempts to solve this problem remain open to debate and have even led some to abandon CDM altogether. However, current simulations are limited by the assumption that dark matter feels only gravity. Here, we show that including interactions between CDM and radiation (photons or neutrinos) leads to a dramatic reduction in the number of satellite galaxies, alleviating the Milky Way satellite problem and indicating that physics beyond gravity may be essential to make accurate predictions of structure formation on small scales. The methodology introduced here gives constraints on dark matter interactions that are significantly improved over those from the cosmic microwave background.

Figures (4)

• Figure 1: The linear matter power spectra for CDM, $\gamma$CDM with $\sigma_{\rm{DM}-\gamma} = 2 \times 10^{-9}~\sigma_{\rm Th}\left(m_{\rm{DM}}/\rm{GeV}\right)$, WDM with $m_{\rm DM} = 1.24~\mathrm{keV}$ and $\gamma$CDM' with $\sigma_{\rm{DM}-\gamma} = 10^{-7}~\sigma_{\rm Th}\left(m_{\rm{DM}}/\rm{GeV}\right)$. We take $\sigma_{\rm{DM}-\gamma}$ to be constant and use the best-fitting cosmological parameters from PlanckAde:2013zuv.
• Figure 2: The simulated distribution of DM in a MW-like halo. The shading represents the DM density, with brighter colours indicating higher densities. The panels show the halo in simulations of different cosmological models: CDM (top left), $\gamma$CDM with $\sigma_{\rm{DM}-\gamma} = 2 \times 10^{-9}~\sigma_{\rm Th}\left(m_{\rm{DM}}/\rm{GeV}\right)$ (bottom left), the equivalent model of WDM with $m_{\rm DM} = 1.24~\mathrm{keV}$ (top right) and $\gamma$CDM' with $\sigma_{\rm{DM}-\gamma} = 10^{-7}~\sigma_{\rm Th}\left(m_{\rm{DM}}/\rm{GeV}\right)$ (bottom right). The large number of subhaloes observed in the top-left panel illustrates the MW satellite problem. By replacing CDM with WDM (top right), the number of subhaloes is reduced dramatically. A similar paucity of subhaloes is seen in the bottom-right panel, in which the DM--photon interaction strength is just allowed by CMB constraints wilkinson_using_2013. This model underestimates the number of MW satellites. The model in the bottom-left panel has an interaction strength that is 1000 times smaller than the CMB limit, in which the number of subhaloes is a much better match to the observed number of satellites.
• Figure 3: The number of satellite galaxies in a MW-like DM halo as a function of their maximal circular velocity: CDM (left), $\gamma$CDM with $\sigma_{\rm{DM}-\gamma} = 2 \times 10^{-9}~\sigma_{\rm Th}\left(m_{\rm{DM}}/\rm{GeV}\right)$ (middle) and $\gamma$CDM' with $\sigma_{\rm{DM}-\gamma} = 10^{-7}~\sigma_{\rm Th}\left(m_{\rm{DM}}/\rm{GeV}\right)$ (right). The lines and shading show the mean cumulative number counts of MW satellites for a simulated DM halo in the mass bin $(2.3-2.7) \times 10^{12} {\rm M}_\odot$ and the 1$\sigma$ uncertainty. Also plotted are the observational results (Willman:2009dv, solid black lines), which are then corrected for the completeness of the Sloan Digital Sky Survey coverage (dashed lines). The maximal circular velocity, $V_{\rm max}$, is selected as a measure for the mass and is determined directly from the simulations (it is derived from the observed stellar line-of-sight velocity dispersions using the assumption that $V_{\rm max} = \sqrt{3} \sigma_{\star}$; Klypin:1999uc). The number of selected MW-like haloes are 11, 13 and 3 for CDM, $\gamma$CDM and $\gamma$CDM', respectively (the reduced scatter for $\gamma$CDM' is simply a result of the small-number statistics in this extreme model).
• Figure 4: Constraints on the $\gamma$CDM cross section. Top panel: the overabundance of satellites versus the cross section for the MW halo mass bin $(2.3-2.7) \times 10^{12} {\rm M}_\odot$, where the shaded bands represent the 1$\sigma$ and 2$\sigma$ uncertainties. Bottom panel: constraints on the cross section are plotted with respect to the MW halo mass. The most recent CMB constraint wilkinson_using_2013 and selected upper mass bounds for the MW halo are shown for comparison.