The Inner Dark-Matter Structure of Galaxies

Abstract

In the framework of the $Λ$CDM model, galaxies evolve within dark matter (DM) haloes, where baryonic processes modify the inner structure of the DM distribution. In particular, baryon condensation and feedback can alter the inner density profiles of haloes, motivating studies of their central regions. The aim of this work is to investigate the inner slope of the DM density profiles of galaxies in the TNG50 simulation, its relation to galaxy properties, its evolution with redshift, and the impact of baryonic processes by comparing galaxies to a corresponding dark matter-only (DMO) realisation. Spherically averaged DM density profiles are constructed for galaxies in TNG50 and the DMO run. The inner slope is quantified using an Inner Linear Fit (ILF), defined as a power-law fit to the central region of the density profiles and motivated by the asymptotic behaviour of generalized NFW models. Subhaloes are matched between simulations and tracked across $z=0$, $0.2$, $0.7$, and $1$. The inner DM structure of galaxies in TNG50 shows that high-stellar-mass systems ($M_\star \gtrsim 10^{11}$ M$_\odot$) exhibit shallow inner slopes irrespective of being centrals or satellites, while lower-mass galaxies ($M_\star \lesssim 10^{9}$ M$_\odot$) show a broader diversity of profiles. At fixed stellar mass, low-mass satellites tend to be more cuspy, with the steepest slopes found in redder systems with lower $V_{\max}$ in more massive host haloes. We find a clear cosmic evolution, from shallower slopes at $z \sim 1$ to steeper profiles towards low redshift in both hydrodynamical and DMO runs, with hydrodynamical galaxies steeper. Finally, we verify that the population exhibiting the steepest slopes remains qualitatively robust to variations in the adopted fitting range, as extending the fit to larger radii$-$thereby excluding the innermost regions$-$generally leads to even steeper inferred slopes.

Figures (12)

• Figure 1: Examples of DM density profiles from our catalogue for galaxies spanning a range of DM masses, together with the six fitting models adopted in this work. Each panel shows the full radial profile, with an inset highlighting the inner region above the resolution limit. The shaded vertical region indicates radii below the adopted Plummer-equivalent gravitational softening length. Grey points with error bars represent the measured density profile, with uncertainties reflecting Poisson noise in the particle counts, while the coloured curves show the corresponding best-fitting models. The top panels correspond to satellite galaxies, and the bottom panels to central galaxies. The ID reported in each panel corresponds to the SubhaloID of the galaxy in TNG50, and the reported $\chi^2_\nu$ corresponds to the ILF.
• Figure 2: Reduced chi-square, $\chi^2_\nu$, of the linear fits as a function of the DM mass of the galaxies. Black and gray curves show the mean and median values in logarithmic mass bins, while the shaded histogram indicates the underlying mass distribution of the sample. Error bars are obtained through bootstrap resampling.
• Figure 3: Posterior distributions of the slope and intercept of the ILF obtained from the MCMC analysis. The top and right panels show the one-dimensional posteriors, while the lower-left panel displays the joint posterior with credibility contours. Vertical lines mark the median and the 16th and 84th percentiles of the corresponding posterior distributions.
• Figure 4: Two-dimensional distribution of the inner slope $\gamma$ as a function of galaxy DM mass. The colour map shows the reduced chi-square $\chi^2_\nu$ of the linear fits, and contours indicate regions of increasing point density.
• Figure 5: Galaxy properties colour-coded by the inner slope $\gamma$. Top-left: specific star formation rate versus stellar mass. Top-right:$(g-r)$ colour versus stellar mass. Bottom-left: stellar mass versus maximum circular velocity $V_{\rm max}$. Bottom-right: stellar mass versus host-halo mass $M_{\rm host}$.