Halo density profiles

There is a pervasive, yet inaccurate, belief that the density structure of dark matter halos is a solved problem. According to the popular Navarro-Frenk-White or Einasto functional forms, the profiles steepen with radius from an inner logarithmic slope between 0 and −1 to an outer slope of about −3 (Einasto 1965; Navarro et al. 1997).

We show that this picture is incomplete: at a radius somewhat larger than the virial radius, simulated profiles of halos tend to steepen well beyond the expected slope. The figure below shows the median profiles of low-mass (left) and high-mass (right) halos, and the slope of those profiles (bottom). As expected, the NFW and Einasto profiles describe the low-mass profiles out to about the virial radius where the profiles flatten due to newly infalling material. The profiles of high-mass halos, however, steepen to a slope of -4.

We found that the significance of the steepening and its location depend on the mass accretion rate of the halo which led to the concept of the splashback radius, the radius where particles on a first orbit reach their apocenter. The steepening in the profiles has since been detected observationally (see the splashback radius page for more information).

Additionally, we find that the outermost density profiles (r > R200m) are remarkably self-similar when radii are rescaled by R200m whereas the inner density profiles are most self-similar when radii are rescaled by R200c. We also propose a new fitting formula that describes the median and mean profiles of halo samples selected by their peak height or mass accretion rate with accuracy < 10% at all radii, redshifts and masses we studied.

Related publications:

  • Brown, McCarthy, Diemer, Font, Stafford & Pfiefer
    Connecting the structure of dark matter haloes to the primordial power spectrum
    [ads] [arXiv] [2020 MNRAS 495, 4994]
  • Diemer & Kravtsov
    Dependence of the outer density profile of halos on their mass accretion rate
    [ads] [arXiv] [2014 ApJ 789, 1]
  • Diemer & Kravtsov
    Poster: Non-universality of halo profiles and their scaling with the “virial” radius
  • Diemer & Kravtsov
    Poster: Are halo profiles and concentrations universal?