Cusped Mass Density Profiles and Magnification Ratios of Double Image Gravitational Lenses

TL;DR

This paper develops an analytical model linking dark matter halo density profiles to observed gravitational lens magnification ratios, revealing two distinct halo populations with different cusp slopes.

Contribution

It introduces a cosmology-independent relation between cusp slope and magnification ratio, and statistically constrains dark matter halo profiles using double image lens data.

Findings

Most haloes have cusp slope alpha ≈ -1.95

Evidence for a second halo population with alpha ≈ -1.49

Approximately 99% probability that the second population explains observed features

Abstract

We have been able to connect the statistics of the observed double image gravitational lenses to the general properties of the internal structure of dark matter haloes. Our analytical theory for the GNFW lenses with parametrized cusp slope (alpha) gives us a relation connecting the cusp slope of the lensing profile to the observed magnification ratio of the produced images and location of the optical axis. The relation does not depend on cosmology, total lens mass, concentration or redshifts of the the lens and the lensed object. Simple geometry of axially symmetric lensing and aforementioned relation enables us to define a threshold value alpha_CSL for the cusp slope, independent from location of the optical axis. The threshold cusp slope value alpha=alpha_CSL is the shallowest slope for the inner part of the GNFW profile that can produce the observed magnification ratio with any lensing configuration. We use distribution of these threshold values in a statistical study of the double image lenses in order to limit the possible cusp slope values, and identify whether there exists a population of haloes with similar profiles. Our theoretical fit indicates that within our sample of double image gravitational lenses, most of the haloes have cusp slope alpha=-1.95 +/- 0.02. We have also found an indication of a second population of lenses with a cusp slope value alpha=-1.49 +/- 0.09. We estimate that there is about 99 per cent probability that the observed feature in the threshold value limit distribution is produced by the second population of lenses, with their own characteristic density profile.