The Effect of Adiabatic Compression on Dark Matter Halos and the Radial Acceleration Relation

TL;DR

This study uses a semi-empirical CDM model to analyze how adiabatic compression affects dark matter halos and the radial acceleration relation, revealing systematic deviations from observations that challenge the model's sufficiency.

Contribution

It demonstrates that adiabatic compression in CDM halos causes deviations from the observed RAR, indicating the need for additional effects like feedback to match observations.

Findings

High-mass galaxies initially align with observed RAR.

Low-mass galaxies show deviations due to DM cusps.

Halo compression increases accelerations, moving models away from observed RAR.

Abstract

We use a semi-empirical model to investigate the radial acceleration relation (RAR) in a cold dark matter (CDM) framework. Specifically, we build 80 model galaxies covering the same parameter space as the observed galaxies in the SPARC database, assigning them to dark matter halos using abundance matching and halo mass-concentration relations. We consider several abundance matching relations, finding some to be a better match to the kinematic data than others. We compute the unavoidable gravitational interactions between baryons and their dark matter halos, leading to an overall compression of the original NFW halos. Before halo compression, high-mass galaxies approximately lie on the observed RAR whereas low-mass galaxies display up-bending "hooks" at small radii due to DM cusps, making them deviate systematically from the observed relation. After halo compression, the initial NFW halos become more concentrated at small radii, making larger contributions to rotation curves. This increases the total accelerations, moving all model galaxies away from the observed relation. These systematic deviations suggest that the CDM model with abundance matching alone cannot explain the observed RAR. Further effects (e.g., feedback) would need to counteract the compression with precisely the right amount of halo expansion, even in high mass galaxies with deep potential wells where such effects are generally predicted to be negligible.