Constraints on Dark Matter Properties from Observations of Milky Way Satellite Galaxies

We perform a comprehensive study of Milky Way (MW) satellite galaxies to constrain the fundamental properties of dark matter (DM). This analysis fully incorporates inhomogeneities in the spatial distribution and detectability of MW satellites and marginalizes over uncertainties in the mapping between galaxies and DM halos, the properties of the MW system, and the disruption of subhalos by the MW disk. Our results are consistent with the cold, collisionless DM paradigm and yield the strongest cosmological constraints to date on particle models of warm, interacting, and fuzzy dark matter. At 95% confidence, we report limits on (i) the mass of thermal relic warm DM, $m_{\mathrm{WDM}}>6.5\mathrm{keV}$ (free-streaming length, ${\lambda }_{\mathrm{fs}}\lesssim 10h^{-1}\mathrm{kpc}$), (ii) the velocity-independent DM-proton scattering cross section, ${\sigma }_0<8.8\times {10}^{-29}{\mathrm{cm}}^2$ for a 100 MeV DM particle mass [DM-proton coupling, $c_p\lesssim (0.3\mathrm{GeV}{)}^{-2}$], and (iii) the mass of fuzzy DM, $m_{\varphi }>2.9\times {10}^{-21}\mathrm{eV}$ (de Broglie wavelength, ${\lambda }_{\mathrm{dB}}\lesssim 0.5\mathrm{kpc}$). These constraints are complementary to other observational and laboratory constraints on DM properties.

Figure 1

Left panel: Transfer functions for the WDM (orange line), IDM (blue line), and FDM (magenta line) models that are ruled out by our analysis at 95% confidence, corresponding to $m_{\mathrm{WDM}}=6.5\mathrm{keV}$, ${\sigma }_0=8.8\times {10}^{-29}{\mathrm{cm}}^2$ (for DM particle mass $m_{\chi }=100\mathrm{MeV}$), and $m_{\varphi }=2.9\times {10}^{-21}\mathrm{eV}$, respectively. These constraints are marginalized over our MW satellite model and the properties of the MW system. Middle panel: SHMF suppression relative to CDM for each ruled-out non-CDM model. The vertical dashed line indicates the 95% confidence upper limit on the lowest-mass halo inferred to host MW satellite galaxies [36]. Note that the IDM SHMF is assumed to be identical to the WDM SHMF in our analysis and is offset slightly for visual clarity. Right panel: Predicted MW satellite galaxy luminosity functions for each ruled-out non-CDM model compared to DES and PS1 observations, evaluated at the best-fit MW satellite model parameters from Ref. [36]. The shaded band illustrates the uncertainty of our WDM prediction due to the stochasticity of our galaxy-halo connection model and the limited number of simulations used in our analysis; the size of this uncertainty is very similar to that in CDM and the other alternative DM models shown. This panel is a simple one-dimensional representation of our MW satellite and DM model fit to the luminosity, size, and spatial distribution of satellites in the DES and PS1 survey footprints. The comparison of our CDM model to data is described in Ref. [36], and full posterior distributions for our non-CDM analyses are provided in Supplemental Material [46].

Figure 2

Exclusion regions for WDM and IDM models from our analysis of MW satellites observed with DES and PS1 (red) compared to previous constraints from classical and SDSS satellites [22] (blue) and other experimental results. Left panel: Constraints on the mass and mixing angle of resonantly produced sterile neutrino DM. These constraints are derived by finding mass and mixing angle combinations that suppress the linear matter power spectrum more strongly than the $m_{\mathrm{WDM}}=6.5\mathrm{keV}$ thermal relic ruled out at 95% confidence by our analysis. The black point with error bars shows the sterile neutrino interpretation of the 3.5 keV x-ray line [74]. The dark gray region is ruled out by dwarf galaxy internal dynamics [82], and the gray contour shows x-ray constraints [83, 84, 85]. Solid black lines indicate regions of parameter space in which resonantly produced sterile neutrinos cannot constitute all of the DM in the neutrino minimal standard model [66, 73]. Right panel: Constraints on the interaction cross section and DM mass for velocity-independent DM-proton scattering. Green contours show cosmological limits from the CMB [23, 25] and the Lyman-$\alpha$ forest [86]. Light gray contours show experimental limits from the x-ray quantum calorimeter [87] and direct detection results as interpreted by Ref. [88].

Figure 3

Constraints on ultralight axion particle mass versus axion-photon coupling from our analysis of the MW satellite population (red). Limits from CMB polarization washout [89] and the Lyman-$\alpha$ forest [39] are shown in green, and haloscope limits are shown as gray vertical bands. Experimental constraints from the CAST experiment [90], the lack of a $\gamma$-ray signal from SN1987A [91], and the x-ray transparency of the intracluster medium [92] are shown in gray and do not require that the ultralight axion makes up all of the DM. The dashed lines [93] span canonical QCD axion models [94, 95].