Central densities of dark matter halos in FIRE-2 simulations of low-mass galaxies with cold dark matter and self-interacting dark matter

TL;DR

This work tests whether baryonic feedback or self-interacting dark matter can explain the central density structure of dwarf halos using FIRE-2 simulations. It compares CDM and SIDM runs with and without full baryonic physics across eight dwarf halos, analyzing density slopes and core transitions via radius markers and analytic profile fits. It finds that SIDM produces cores with sharper transitions than CDM, and that the three-parameter alpha-beta-gamma profile best describes SIDM halos, while core-Einasto underfits SIDM. It further shows that increasing the SIDM cross section enlarges cores and steepens the turnover, suggesting that observations of dwarf galaxy density profiles can help discriminate between CDM and SIDM.

Abstract

We investigate the central density structure of dark matter halos in cold dark matter (CDM) and self-interacting dark matter (SIDM) models using simulations that are part of the Feedback In Realistic Environments (FIRE) project. For simulated halos of dwarf galaxy scale ($M_{\rm halo}(z=0)\approx 10^{10}\,M_\odot$), we study the central structure in both dissipationless simulations and simulations with full FIRE-2 galaxy formation physics. As has been demonstrated extensively in recent years, both baryonic feedback and self-interactions can convert central cusps into cores, with the former process doing so in a manner that depends sensitively on stellar mass at fixed $M_{\rm halo}$. Whether the two processes (baryonic feedback and self-interactions) are distinguishable, however, remains an open question. Here we demonstrate that, compared to feedback-induced cores, SIDM-induced cores transition more quickly from the central region of constant density to the falling density at larger radial scales. This result holds true even when including identical galaxy formation modeling in SIDM simulations as is used in CDM simulations, since self-interactions dominate over galaxy formation physics in establishing the central structure of SIDM halos in this mass regime. The change in density profile slope as a function of radius therefore holds the potential to discriminate between self-interactions and galaxy formation physics as the driver of core formation in dwarf galaxies.

Figures (16)

• Figure 1: Dark matter halo density profiles from FIRE-2 simulations with full baryonic physics for eight galaxies of increasing stellar mass (galaxy properties in Table \ref{['tab:CDM-SIDM-parameters']}). The solid lines show cold dark matter profiles (CDM+hydro) and the dashed lines show the analogous self-interacting dark matter profiles (SIDM+hydro). The profiles begin at each halo's convergence radius ($r_{\rm conv}\sim 0.2\ {\rm kpc}$) calculated using the method in power_inner_2003. The central regions of CDM halo density profiles have shallower slopes (more cored) in galaxies with greater stellar masses due to the increased stellar feedback. At fixed stellar mass, the SIDM halos have more cored inner densities than CDM halos.
• Figure 2: Individual and averaged logarithmic slopes of the density profiles for CDM (blue) and SIDM (orange) halos. Markers indicate where the logarithmic slopes of the density profiles are equal to -1 and -2. The distance between these two radii, $r_{-1}$ and $r_{-2}$, provides a measure of how quickly the profiles transition from a steep outer profile to a shallow inner core. Comparing CDM and SIDM with baryonic physics (+hydro; solid lines) and with dark matter only (DMO; dashed lines) shows that dark matter self-interactions shorten the distance between $r_{-1}$ and $r_{-2}$ more significantly than the inclusion of baryonic physics in CDM simulations.
• Figure 3: Left: The radius at which the logslope of the density profile is -1, $r_{-1}$, for eight FIRE-2 classical dwarf galaxy dark matter halos simulated in CDM and SIDM with and without hydrodynamics. Both the self-interacting dark matter with baryons (SIDM+hydro; filled squares) and the dark-matter-only simulation of self-interacting dark matter (SIDM DMO; open squares, plotted at the stellar mass of the analogous SIDM+hydro runs) have higher values of $r_{-1}$ than cold dark matter (CDM+hydro; filled circles). Triangular markers indicate halos in which the density profile logslope is steeper than -1 at all radii larger than the convergence radius, characteristic of a cuspy profile. With or without the inclusion of baryons, halos of self-interacting dark matter form larger cores than halos of cold dark matter. Right: The ratio between $r_{-2}$ and $r_{-1}$ for each halo. The smaller ratios for the SIDM profiles correspond to a shorter distance between the two radii, indicating a sharper transition in the density profile between the central core and outer envelope.
• Figure 4: Density profiles and fitted analytical profiles (upper panel) and profile residuals (lower panel) for the CDM (blue) and SIDM (orange) versions of halo m10h simulated with full galaxy formation physics. The core-Einasto fit (dashed lines) does not change rapidly enough between the flatter inner region and the outer steep envelope to fit either the CDM or SIDM halos, though it performs better with the cuspier CDM halo. The residuals show that the fit is worst at $r \sim 1~{\rm kpc}$ in both cases, comparable to $r_{-1}$ (marked with a diamond and vertical line). The $\alpha\beta\gamma$ profile fit (dotted line) shows an improved fit compared to the core-Einasto fit to the SIDM halo's density profile. The more rapid transition enabled by the $\alpha_{\rm s}$ parameter allows a good fit of the $\alpha\beta\gamma$ profile to the SIDM profile at all radii.
• Figure 5: The core-Einasto residuals for the SIDM density profiles as a function of the radius scaled by $r_{-1}$. The residual averaged over all eight simulations is shown by the thick black line, while residuals of individual halo density profile fits are shown by lines with colors corresponding to stellar mass, as indicated by the colorbar to the right. The analytical core-Einasto profile, designed for feedback-affected halos, fails to capture the sharp transition to a core found in the density profiles of SIDM halos, particularly around $r_{-1}$, the radius at which the logslope is $-1$.