http://arxiv.org/abs/2106.09050
Twenty years ago, the mismatch between the observed number of Milky Way satellite galaxies and the predicted number of cold dark matter subhalos was dubbed the “missing satellites problem”. Although mostly framed since in terms of satellite counts in luminosity space, the missing satellites problem was originally posed in velocity space. The stellar velocity dispersion function encodes information about the density profile of satellites as well as their abundance. We compare the completeness-corrected MW satellite velocity function down to its ultrafaint dwarfs (L > 340 L$\odot$) against well-motivated, semi-empirical predictions based on galaxy-halo scaling relations. For our most conservative completeness correction, we find good agreement with a simple CDM model in which massive, classical satellites (M${\rm vir} \gtrsim 10^9~$M$\odot$) have baryon-driven cores, while low-mass, ultrafaint satellites (M${\rm vir} \lesssim 10^9~$M$\odot$) inhabit cuspy halos that are not strongly tidally stripped. This bifurication is required to explain a non-power-law feature in the velocity function at $\sigma{\rm los}^* \approx 10$ km/s. Intriguingly, this feature could point to a flattening of the stellar-mass–halo-mass relation. Tidal destruction of satellites by the Milky Way’s disk must be minimal, or the corrected velocity function exceeds any plausible prediction — a “too many satellites” problem. We rule out non-core-collapsing self-interacting dark matter models with a constant cross section $\gtrsim$ 0.3 cm$^2$/g. Constraints on warm dark matter are stronger than those based on the luminosity function on account of the velocity function’s additional sensitivity to the central densities of subhalos. Reducing uncertainties on stellar kinematics and the amount of tidal stripping experienced by the faintest dwarfs is key to determining the severity of the too many satellites problem.
S. Kim and A. Peter
Fri, 18 Jun 21
60/62
Comments: 19 pages, 13 figures. Key results are summarized in Figure 6. To be submitted to MNRAS. Comments welcome!
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