Major mergers between dark matter haloes – II. Profile and concentration changes

Abstract

Several lines of evidence suggest that as dark matter haloes grow their scale radius increases, and that the density in their central region drops. Major mergers seem an obvious mechanism to explain both these phenomena, and the resulting patterns in the concentration-mass-redshift relation. To test this possibility, we have simulated equal-mass mergers between haloes with a variety of cosmological density profiles, placed on various different orbits. The remnants typically have higher densities than the initial conditions, but differ only slightly from self-similar scaling predictions. They are reasonably well fit by Einasto profiles, but have parameters distinct from those of the initial conditions. The net internal energy available to the merger remnant, relative to the internal energy of the initial conditions, κ, has the greatest influence on the properties of the final mass distribution. As expected, energetic encounters produce more extended remnants while mergers of strongly bound systems produce compact remnants. Surprisingly, however, the scale radius of the density profile shows the opposite trend, increasing in the remnants of low-energy encounters relative to energetic ones. Also even in the most energetic encounters, the density within the scale radius decreases only slightly (by 10-20%), while for very low-energy systems it increases significantly after the merger. We conclude that while major mergers can produce remnants that are more diffuse at large radii, they are relatively ineffective at changing the central densities of haloes, and seem unlikely to explain the mean trends in the concentration-mass-redshift relation.

Mass-loss in tidally stripped systems: the energy-based truncation method

“…Not only is the central density of haloes important to halo disruption predictions, it also has implications for the dark matter annihilation boost factor (e.g. Okoli et al 2018;Drakos et al 2019b), as we will explore in Paper III. We also note that our model may not capture decreased central density due to heating, though we suspect that neglecting this is valid, as it has been found that the central regions of haloes are adiabatically shielded from heating (Weinberg 1994a,b).…”

Breaking a dark degeneracy: The gamma-ray signature of early matter domination

“…We employed an approximate model based on the results of Paper I and Ref. [104] to treat these mergers, and our results are subject to this approximation's accuracy. Further study is needed to precisely understand how mergers influence a microhalo population.…”

“…Indeed, Ref. [104] found that in mergers between identical halos, ρ s tends to either be preserved or grow slightly, and r s grows roughly as would be expected from the doubling 9 The annihilation rate from a ρ ∝ r −3/2 cusp diverges, which implies that the profile would shallow at some minimum radius due to these annihilations. 10 We integrate the NFW profile to radius ∞, but the result is only marginally different if the profile is cut off at any radius r > ∼ rs.…”

Quantifying the impact of the Large Magellanic Cloud on the structure of the Milky Way's dark matter halo using Basis Function Expansions

“…These studies of Sgr. Stream indicate that stellar streams probe asymmetries in the MW's DM halo owing to the LMC in addition to structure arising from the cosmological assembly history of the MW (e.g: Wechsler et al 2002;Ludlow et al 2013;Correa et al 2015;Wang et al 2020;Vera-Ciro et al 2011;Prada et al 2019;Drakos et al 2019) or changes in halo shape expected from different DM particle models (e.g: Yoshida et al 2000;Peter et al 2013;Bose et al 2016). Without a framework to quantify the LMC's impact on the halo, we cannot disentangle these processes.…”