A Universal Density Profile for Dark and Luminous Matter?

TL;DR

The paper investigates whether the same three-parameter Sérsic density law that describes luminous spheroids also accurately describes dark matter halos in LCDM simulations. By constructing nonparametric estimates of both space and projected densities for 19 halos and fitting Sérsic, deprojected Sérsic, and generalized NFW models, the authors show that a Sérsic form provides an excellent description of dark halos across dwarf, galaxy, and cluster masses, with mean indices $n$ around $3.0$ for galaxies and dwarfs and $2.38$ for clusters. The space-density fits indicate Sérsic-based models perform as well as or better than alternative three-parameter forms, especially for dwarfs, and the slope analysis confirms a scale-free behavior rather than a single power law. The results imply a possible common underlying density law for dark and luminous systems, likely tied to gravitational clustering and mergers, and position the Sérsic law as a concise description spanning seven orders of magnitude in mass. Future work may explore other three-parameter fits and the physical origin of the observed $n$–mass trend.

Abstract

We explore similarities in the luminosity distribution of early type galaxies and the mass profiles of LCDM halos. The spatial structure of these systems may be accurately described by a simple law where the logarithmic slope of the projected density is a power law of radius; the Sersic law. We show that this law provides a significantly better fit than a three-parameter generalization of the NFW profile and derive the best-fitting Sersic parameters for a set of high-resolution LCDM halos spanning a wide range in mass. The mean Sersic n values are 3.0 for dwarf- and galaxy-sized halos and 2.4 for cluster-sized halos, similar to the values that characterize luminous elliptical galaxies. We discuss possible reasons why the same law should describe dark and luminous systems that span a range of over seven decades in mass.

Figures (4)

• Figure 1: (a) Nonparametric estimates of the surface density profiles of the 19 halo models. Profiles of the $D$ ($C$) models have been shifted downward (upward) by $0.75$ in the logarithm. (b) Deviations of the best-fitting Sérsic model from $\hat{\Sigma}(R)$. Fitting parameters are given in Table 1.
• Figure 2: (a) Nonparametric estimates of the space density of the 19 dark halos. Vertical normalization is arbitrary. (b-d) Deviations in magnitudes of three parametric models from $\hat{\rho}(r)$: (b) deprojected Sérsic model; (c) equation (\ref{['eq:alpha']}); (d) generalized NFW model, equation (\ref{['eq:nfw']}). Best-fit parameters are given in Table 1.
• Figure 3: Nonparametric estimates of the logarithmic derivative of the space density for the 19 halo models.
• Figure 4: Sérsic index (derived from fits to the surface density) versus mass for galaxies (open circles) and dark halos. Galaxy points are taken from bj98stiavelli01gguzman03caon93dono94. Halo masses are $M_{200}$ from paper3. Galaxy masses were computed from total luminosities assuming the Magorrian et al. (1998) mass-to-light ratio, with $H_0=70$ km s$^{-1}$ Mpc$^{-1}$.