Cusp-to-Core Transition of Dark Matter Halos across Galaxy Mass Scales

TL;DR

This work analyzes a large SPARC galaxy sample with flexible, axisymmetric DM density profiles to probe the cusp–core transition across galaxy mass scales. By performing Bayesian fits to high-quality rotation curves and introducing the central surface density Σ_DM(<0.01 r_Vmax) as a diagnostic, the study finds substantial diversity in inner DM slopes, with many galaxies favoring core-like structures while dwarfs and clusters remain more cuspy. The results broadly align with baryonic feedback–driven core formation predicted by hydrodynamical simulations, though observational uncertainties and modeling systematics prevent definitive conclusions. The analysis highlights a mass-dependent cusp–core transition and underscores the need for improved simulations and deeper data to fully unravel the interplay between baryons and dark matter.

Abstract

We investigate the diversity of dark matter (DM) density profiles in a large sample of late-type galaxies from the SPARC database, with the goal of testing whether a cusp-to-core transition occurs across galaxy mass scales. We perform Bayesian fits to high-quality rotation curves using flexible halo models that allow for variations in the inner slopes of DM density profiles. We quantify the central dark matter structure using the surface density within the inner region of the halo, defined as $Σ_{\rm DM}(<0.01r_{V_{\rm max}})$, and compare the SPARC galaxies with Milky Way dwarf satellites as well as galaxy groups and clusters. Our results reveal significant diversity in the inner density slopes of SPARC galaxies, ranging from steep cusps to shallow cores, and show that many of them lie below the cuspy profiles predicted by the cold dark matter model, consistent with core-like structures. In contrast, both lower-mass dwarf galaxies and higher-mass galaxy clusters tend to follow the cuspy DM halos. These findings suggest that baryonic feedback may induce a cusp-to-core transition in Milky Way-mass galaxies, as predicted by hydrodynamical simulations. However, observational limitations and modeling uncertainties still prevent a definitive conclusion. This study provides new empirical insights into the halo mass-dependent nature of DM inner structures and the role of baryonic processes in shaping them.

Figures (3)

• Figure 1: Rotation curve fits and the 1D and 2D marginalized posterior distributions of the fitting parameters for the UGC12506 (left panels) and NGC0055 (right panels). In the top panels, Green, blue, and red lines denote the contributions of gas, disc, and dark matter respectively. Orange lines represent the total fitted rotation curves. In the bottom panels, the solid, dashed, and dotted lines denote 68%, 95%, and 99% probabilities, respectively. The complete figures of the posterior maps for 115 SPARC galaxies are available in the online journal.
• Figure 2: The central surface density of dark matter halo within 1% of the radius of the maximum circular velocity, $\Sigma_\textrm{DM}(<0.01r_{V_\textrm{max}})$, as a function of the maximum circular velocity, $V_\textrm{max}$. The orange points represent classical dSphs associated with the Milky Way, derived from 2020ApJ...904...45H, while the green points correspond to ultra-faint dSphs and ultra-diffuse galaxies, based on 2023ApJ...953..185H. The blue squares indicate the SPARC galaxies analyzed in this work. The black symbols denote galaxy groups and clusters, taken from 2007ApJ...669..158G, 2015ApJ...806....4M, and 2016ApJ...821..116U. The red solid line and the shaded regions are the median and 1, 2, and 3-$\sigma$ halo-to-halo scatter predicted from FIRE-2 2020MNRAS.497.2393L hydrodynamical plus dark matter simulations (see text for details). In contrast, the two purple dashed lines represent the predictions from 2024PASJ...76.1026K for the NFW (cuspy) and Burkert (cored) profiles, respectively.
• Figure 3: The inner dark matter density slope at 1.5% of $r_\textrm{vir}$ is shown as a function of the stellar-to-halo mass ratio. The filled orange and green circles with error bars are taken from 2020ApJ...904...45H and 2023ApJ...953..185H, respectively, while the filled blue squares represent the results from this work. The shaded gray band indicates the expected range of inner slopes for NFW profiles, as derived from dark-matter-only simulations 2016MNRAS.456.3542T. The pink and magenta shaded bands show the predicted ranges from the NIHAO 2016MNRAS.456.3542T and FIRE-2 2020MNRAS.497.2393L simulations, respectively (shown for visual reference).