Diverse dark matter density at sub-kiloparsec scales in Milky Way satellites: Implications for the nature of dark matter

Milky Way (MW) satellites reside within dark matter (DM) subhalos with a broad distribution of circular velocity profiles. This diversity is enhanced with the inclusion of ultrafaint satellites, which seemingly have very high DM densities, albeit with large systematic uncertainties. We argue that if confirmed, this large diversity in the MW satellite population poses a serious test for the structure formation theory with possible implications for the DM nature. For the cold dark matter model, the diversity might be a signature of the combined effects of subhalo tidal disruption by the MW disk and strong supernova feedback. For models with a dwarf-scale cutoff in the power spectrum, the diversity is a consequence of the lower abundance of dwarf-scale halos. This diversity is most challenging for self-interacting dark matter (SIDM) models with cross sections $\sigma /m_{\chi }\gtrsim 1{\mathrm{cm}}^2{\mathrm{g}}^{-1}$ where subhalos have too low densities to explain the ultrafaint galaxies. We propose a novel solution to explain the diversity of MW satellites based on the gravothermal collapse of SIDM haloes. This solution requires a velocity-dependent cross section that predicts a bimodal distribution of cuspy dense (collapsed) subhaloes consistent with the ultrafaint satellites, and cored lower density subhaloes consistent with the brighter satellites.

Figure 1

Left: Dimensionless linear DM power spectra (${\mathrm{\Delta }}^2(k)=k^3{P}_{\text{linear}}(k)/2{\pi }^2$) for the simulations used in this work. The CDM (black), SIDM (gray), and vdSIDM (orange) cases have a CDM-like power spectrum while the WDM (red) and ETHOS (blue) cases have galactic-scale cutoffs produced by free streaming and DM-dark radiation interactions in the early Universe, respectively. In the black hashed area, DM is constrained to behave like CDM (e.g., Ly-$\alpha$ constraints on WDM [36]). Notice that not all simulations have the same cosmological parameters, and thus there is a mismatch at large scales. Right: The transfer cross section as a function of relative velocity for the simulations with self-interactions. The black hashed area with ${\sigma }_T/m_{\chi }<0.1{\mathrm{cm}}^2{\mathrm{g}}^{-1}$ marks the region where DM is effectively collisionless. The green hashed area on the left is the relevant region for MW satellites. Above the dashed line, self-interactions are frequent enough for the onset of gravothermal collapse within a Hubble time in SIDM halos. The magenta arrow is the limit to the cross section from shape measurements of the elliptical galaxy NGC720 [37].

Figure 2

The circular velocity profiles for the 24 subhalos with the largest values of $V_{\max }(z=0)$ within 300 kpc of the center of the MW-size halo, after excluding Magellanic Cloud analogs. We show four different cosmologies (see Fig. 1): CDM, SIDM with ${\sigma }_T/m_{\chi }=1{\mathrm{cm}}^2{\mathrm{g}}^{-1}$, WDM with $m_{\chi }=2.3\mathrm{keV}$, and a benchmark model within the ETHOS framework, which has self-interactions and a primordial power spectrum cutoff [26, 34]. The solid lines show the profiles beyond the convergence radius. Below this radius (thin lines), most subhalos have reached the expected asymptotic values: $V_{\mathrm{circ}}\propto r^{\gamma }$ for cuspy (left panels) and cored profiles (right panels). Open symbols with error bars show $V_{\mathrm{circ}}$ values at the half light radius for 24 MW satellites as given in [32, 58]. Lines and symbols in gray are examples of consistent matches of simulated subhalos and data points (the largest possible number of matching pairs is shown in the lower right). The mismatches are shown in green.

Figure 3

As Fig. 2 but for the vdSIDM simulation (see Fig. 1). The velocity dependence of this SIDM model has cross sections near and above the onset of gravothermal collapse for MW-like subhalos. This produces a bimodal subhalo distribution, with some of the systems developing a central cusp, $V_{\mathrm{circ}}\propto r^{0.6}$, while the others still retain a core, $V_{\mathrm{circ}}\propto r^{0.9}$.