Halo density profiles

Recent work has shown that the density profile of dark matter halos is better understood as a superposition of the profiles of orbiting and newly infalling material (Diemer 2022). Modeling these components separately provides an excellent description of the outer halo profile, a region that has received less attention due to its apparent complexity.

A long-standing open question in halo formation is the origin of the near-universal shape of the halo density profile. Could this orbiting–infalling framework provide insight into this puzzle? To explore this, I decomposed the orbiting population according to the number of pericentric passages completed by each particle. The resulting density profiles are surprisingly simple: unlike the total orbiting profile, these “p-profiles” are characterized by a single radial and density scale. This simplicity reflects the short timescale over which these populations are accreted onto the halo, whereas the two radial scales of the total orbiting profile correspond to long-term and recent accretion epochs.

The left panel of the figure below shows the mean p-profiles for p=1 to p=10, with their characteristic radial and density scales indicated. The right panel shows the same profiles rescaled by these characteristic values, revealing deviations from strict universality. Interestingly, the characteristic radii and densities follow a power-law relation that persists across at least three orders of magnitude in halo mass, though the underlying reason for this scaling remains unclear.

Though there is still much work to be done, these results already give us a hint that there is something deeper at play. For example, there may be a strong connection between the characteristic density of each p-profile and the cosmic density at the time that population was accreted—echoing findings from Ludlow et al. 2013.