Incorporating baryon-driven contraction of dark matter halos in rotation curve fits

TL;DR

This paper investigates how baryon-driven contraction affects dark matter halos in galaxy rotation curves, revealing that high surface brightness galaxies experience significant halo contraction, impacting the inferred stellar masses and the cusp-core problem.

Contribution

It quantifies baryon-driven contraction effects on dark matter halos in observed galaxies and explores implications for galaxy mass modeling and the cusp-core problem.

Findings

High surface brightness galaxies show strong halo contraction.

Halo contraction increases the inner density slope with baryonic surface density.

Lower stellar mass-to-light ratios are needed to fit rotation curves considering contraction.

Abstract

The condensation of baryons within a dark matter (DM) halo during galaxy formation should result in some contraction of the halo as the combined system settles into equilibrium. We quantify this effect on the cuspy primordial halos predicted by DM-only simulations for the baryon distributions observed in the galaxies of the SPARC database. We find that the DM halos of high surface brightness galaxies (with $\Sigma_{\rm eff}\gtrsim100$ $L_\odot$ pc$^{-2}$ at 3.6 $\mu$m) experience strong contraction. Halos become more cuspy as a result of compression: the inner DM density slope increases with the baryonic surface mass density. We iteratively fit rotation curves to find the balance between initial halo parameters (constrained by abundance matching), compression, and stellar mass-to-light ratio. The resulting fits often require lower stellar masses than expected for stellar populations, particularly in galaxies with bulges: stellar mass must be reduced to make room for the DM it compresses. This trade off between dark and luminous mass is reminiscent of the cusp-core problem in dwarf galaxies, but occurs in more massive systems: the present-epoch DM halos cannot follow from cuspy primordial halos unless (1) the stellar mass-to-light ratios are systematically smaller than expected from standard stellar population synthesis models, and/or (2) there is a net outward mass redistribution from the initial cusp, even in massive galaxies widely considered to be immune from such effects.