Not so lumpy after all: modelling the depletion of dark matter subhaloes by Milky Way-like galaxies 

Abstract

Among the most important goals in cosmology is detecting and quantifying small (M halo 10 6−9 M ) dark matter (DM) subhalos. Current probes around the Milky Way (MW) are most sensitive to such substructure within ∼ 20 kpc of the halo center, where the galaxy contributes significantly to the potential. We explore the effects of baryons on subhalo populations in ΛCDM using cosmological zoom-in baryonic simulations of MW-mass halos from the Latte simulation suite, part of the Feedback In Realistic Environments (FIRE) project. Specifically, we compare simulations of the same two halos run using (1) DM-only (DMO), (2) full baryonic physics, and (3) DM with an embedded disk potential grown to match the FIRE simulation. Relative to baryonic simulations, DMO simulations contain ∼ 2× as many subhalos within 100 kpc of the halo center; this excess is 5× within 25 kpc. At z = 0, the baryonic simulations are completely devoid of subhalos down to 3 × 10 6 M within 15 kpc of the MW-mass galaxy, and fewer than 20 surviving subhalos have orbital pericenters < 20 kpc. Despite the complexities of baryonic physics, the simple addition of an embedded central disk potential to DMO simulations reproduces this subhalo depletion, including trends with radius, remarkably well. Thus, the additional tidal field from the central galaxy is the primary cause of subhalo depletion. Subhalos on radial orbits that pass close to the central galaxy are preferentially destroyed, causing the surviving population to have tangentially biased orbits compared to DMO predictions. Our method of embedding a potential in DMO simulations provides a fast and accurate alternative to full baryonic simulations, thus enabling suites of cosmological simulations that can provide accurate and statistical predictions of substructure populations.

Constraints to Dark Matter Annihilation from High-Latitude HAWC Unidentified Sources

“…Finally, it is important to note that our DM limits are subject to potentially large uncertainties. In addition to the already mentioned caveats in the computation of the minimum detection flux (see Section 3.1), 10 there exist important theoretical uncertainties in the N-body simulation predictions, such as the survival probability of the lightest subhalos near the Galactic center [67][68][69], 11 the precise subhalo structural properties, the impact of baryons on the subhalo population [22,70] or the value of the minimum subhalo mass that is adopted to separate between visible and dark satellites [71]. These and other issues that affect the Galactic subhalo population will be addressed elsewhere.…”

Small-Scale Challenges to the ΛCDM Paradigm

“…The existence of these structures is an unambiguous prediction of all WIMP-based dark matter models (though it is not unique to WIMP models), and confirmation of the existence of dark matter halos with M ∼ 10 6 M or less would strongly constrain particle physics of dark matter and effectively rule out any role of dark matter free-streaming in galaxy formation. Here, too, accurate predictions for the number of expected dark subhalos will require an honest accounting of baryon physics -specifically the destructive effects of central galaxies themselves (e.g., Garrison-Kimmel et al 2017b). …”

The biggest splash

“…We have extracted a solar neighbourhood sample of 100, 000 stars from the ananke mock Gaia catalogue for the m12f simulation (using the Sun location lsr0) subject to the selection: parallax> 0.5 mas, |b| > 15 deg and G < 17, which approximately resembles the selection from a typical spectroscopic survey. Of the three simulations, m12f (first presented in Garrison-Kimmel et al 2017) was selected due to its resemblance to the Milky Way. ), retrograde (v φ < 0) and split by tracks in age-metallicity space.…”