A test of invariance of halo surface density for FIRE-2 simulations with cold dark matter and self-interacting dark matter

TL;DR

This work tests the invariance of the dark matter halo surface density, defined as $S = \rho_c r_c$, for FIRE-2 dwarf halos under CDM and SIDM with and without baryons. By fitting multiple DM density profiles (Burkert, core-Einasto, $\alpha\beta\gamma$) to simulated halos and computing both surface and column densities, the authors find near-constant values across the mass range and models, with Burkert-based results consistent with the observed $\rho_c r_c$ benchmark within $1\sigma$. They also reconstruct the $\bar{\Sigma}(<r_{max})$–$V_{max}$ relation and show agreement with Milky Way and M31 dwarf data, favoring cusp-to-core transformation scenarios over pure cuspy NFW predictions. The findings imply that halo core formation processes imprint robust, model-insensitive surface-density behavior at dwarf-galaxy scales, though the invariance does not extend to larger systems like groups or clusters.

Abstract

Numerous observations have shown that the dark matter halo surface density, defined as the product of core radius and halo central density of cored dark matter haloes is nearly constant and independent of galaxy mass over a whole slew of galaxy types. Here we calculate the surface density in cold dark matter(CDM) and self-interacting dark matter (SIDM) models including baryons, as well as SIDM without baryons, for dwarf galaxies of masses $\approx 10^{10} M_{\odot}$ using mock catalogs obtained from simulations as part of the Feedback In Realistic Environments project. We find that the dark matter surface density and column density are nearly constant for CDM and SIDM for this mass range. The halo surface density obtained from the Burkert profile fit is consistent with galactic-scale observations within $1σ$. We also computed the empirical scaling relations between the central surface density and maximum velocity using the best-fit dark matter profiles, and found that they agree with observations of Milky Way and M31 dwarfs.

Figures (6)

• Figure 1: This figures shows the density profiles along with the fitted Burkert profile for the CDM+baryons, SIDM+baryons, and SIDM DMO of halo m10m, simulated with full galaxy formation physics. Here, the solid lines are the fitted Burkert profiles and dotted lines indicate the density distribution.
• Figure 2: The column density for $\alpha\beta\gamma$ profile as a function of halo mass for all the three sets of simulations (CDM+baryons, SIDM+baryons, SIDM DMO only) evaluated using Eq. \ref{['eq:gnfw']} at $r_{200}$. We find that the column density is roughly constant with over this mass range for all the three simulations.
• Figure 3: The column density ($S^*_{\mathrm{cEin}}$) for core-Einasto profile evaluated using Eq.\ref{['eq:cEin']} at $R =\tilde{r}_s$ for the same set of simulations as in Fig. \ref{['fig:g_column']}, as a function of $M_{halo}$. Once again, we find that the column density is roughly constant with for over this mass range for all the three simulations.
• Figure 4: The column density ($S^*_{Bur}$) evaluated using Eq. \ref{['eq:SBUR1']} at $R = 1.66r_c$ for the same set of simulations as in Fig. \ref{['fig:g_column']}, as a function of $M_{halo}$. Once again, it is roughly constant. We note that density profiles with $Q>0.25$ have been removed from this analysis. The haloes which have been included in the figure include m10k and m10m for CDM+hydro, m10f, m10h, m10k and m10m for SIDM+hydro and m10d, m10f, m10h, m10k and m10m.
• Figure 5: Here surface density($S_{Bur}$) is calculated using Eq. \ref{['eq:halosurfacedensity']} where $\rho_c$ and $r_c$ are scale parameter of fitted Burkert profile with Q values less than 0.25. $S_{Bur}$ gives a constant surface density with $M_{halo}$. We have also overlayed $S_{Bur}$ with $\rho_c r_c = (141 \pm 65) M_{\odot} pc^{-2}$Salucci19.