Subhaloes in Self-Interacting Galactic Dark Matter Haloes

TL;DR

This work tests velocity-dependent self-interacting dark matter (vdSIDM) models, motivated by a Yukawa-mediated dark force, against a Milky Way–sized halo from the Aquarius simulations. Using a Monte Carlo SIDM implementation in GADGET-3, the authors compare three reference scenarios (RefP1: large constant cross section, RefP2 and RefP3: allowed velocity-dependent cross sections) and find that vdSIDM leaves the main halo profile largely CDM-like outside ~1 kpc, while inducing ~600 pc cores in subhaloes. The subhalo abundance and radial distribution remain essentially unchanged, but the inner density structure of the top subhaloes becomes cored, yielding circular-velocity profiles more consistent with the brightest MW dSphs and removing the CDM excess of overly-concentrated subhaloes. These results position vdSIDM as a viable alternative to CDM on small scales, with further work needed on inelastic scattering channels and broader halo samples to fully test the framework.

Abstract

We present N-body simulations of a new class of self-interacting dark matter models, which do not violate any astrophysical constraints due to a non-power-law velocity dependence of the transfer cross section which is motivated by a Yukawa-like new gauge boson interaction. Specifically, we focus on the formation of a Milky Way-like dark matter halo taken from the Aquarius project and re-simulate it for a couple of representative cases in the allowed parameter space of this new model. We find that for these cases, the main halo only develops a small core (~1 kpc) followed by a density profile identical to that of the standard cold dark matter scenario outside of that radius. Neither the subhalo mass function nor the radial number density of subhaloes are altered in these models but there is a significant change in the inner density structure of subhaloes resulting in the formation of a large density core. As a consequence, the inner circular velocity profiles of the most massive subhaloes differ significantly from the cold dark matter predictions and we demonstrate that they are compatible with the observational data of the brightest Milky Way dSphs in such a velocity-dependent self-interacting dark matter scenario. Specifically, and contrary to the cold dark matter case, there are no subhaloes that are more concentrated than what is inferred from the kinematics of the Milky Way dSphs. We conclude that these models offer an interesting alternative to the cold dark matter model that can reduce the recently reported tension between the brightest Milky Way satellites and the dense subhaloes found in cold dark matter simulations.

Figures (9)

• Figure 1: Left panel: Analytic scatter rate profiles for Hernquist haloes of different masses following the average cosmological concentration-mass relation. Dashed lines show the result for a velocity-dependent cross section ($v_{\rm max}=30~{\rm km}\,{\rm s}^{-1}$, $\sigma_T^{\rm max}/m_\chi=10~{\rm cm}^2\,{\rm g}^{-1}$), whereas solid lines show the rates for a constant cross section ($\sigma_T^{\rm max}/m_\chi=10~{\rm cm}^2\,{\rm g}^{-1}$). The remaining two panels show a comparison of these scatter rates with those obtained from a frozen N-body representation. The simulations were run for $1~{\rm Gyr}$, and the different halo profiles are all offset by one dex for clarity. Solid lines show the N-body result, whereas black dashed lines represent the analytic result repeated from the leftmost panel. Middle panel: constant cross section. Right panel: velocity-dependent cross section.
• Figure 2: Density (left panels) and velocity dispersion profiles (right panels) of haloes of different masses. The top panels are for the case of a constant cross section ($\sigma_T^{\rm max}/m_\chi=10~{\rm cm}^2\,{\rm g}^{-1}$) showing the profiles after $25~t_0$. Bottom panels are for the case of a velocity-dependent cross section ($v_{\rm max}=30~{\rm km}\,{\rm s}^{-1}$, $\sigma_T^{\rm max}/m_\chi=10~{\rm cm}^2\,{\rm g}^{-1}$) after $1~{\rm Gyr}$. In scaled units, the constant cross section curves for all masses collapse to a single one. For the velocity-dependent case, evolution progresses faster for lower mass systems, because $(\sigma_T v)$ peaks at a velocity of $30~{\rm km/s}$.
• Figure 3: Density projections of the Aq-A halo for the different DM models of Table \ref{['table:ref_points']} (RefP0-3). The projection cube has a side length of $270$ kpc. Clearly, the disfavoured RefP1 model with a large constant cross section produces a very different density distribution with a spherical core in the centre, contrary to the elliptical and cuspy CDM halo. Also, substructures are less dense and more spherical in this simulation. The vdSIDM models RefP2 and RefP3 on the other hand can hardly be distinguished from the CDM case (RefP0).
• Figure 4: Left panel: Density profile of the Aquarius Aq-A main halo at resolution levels 5 (dotted), 4 (dashed) and 3 (solid) for the different SIDM reference points we consider (see Table \ref{['table:ref_points']}) as shown in the legend. Right panel: Mean free path as a function of radius for the SIDM models we used. The softening length ($2.8$ times Plummer equivalent) of each resolution is marked by a vertical black line. The standard CDM profiles (RefP0) are shown in black. We achieve good convergence in all our runs, with the inner profiles changing significantly depending on the DM model employed. Clearly, RefP1 produces the largest difference having a large core due to the constant scattering cross section. We note that this model is ruled out by current astrophysical constraints and is shown here just as reference.
• Figure 5: Radial profiles of the mean number of scatter events for the Aq-A-3 simulations RefP1, RefP2 and RefP3. The large and constant cross-section of RefP1 produces a significantly higher number of scatter events at a given radius compared to RefP2 and RefP3. We note that the vdSIDM points RefP2 and RefP3 lead to slightly different scatter profiles.