The inner regions of disk galaxies: a constant baryonic fraction?

TL;DR

This paper investigates the inner regions of disk galaxies, showing that their baryonic and dynamical mass densities are closely linked, and proposes a simple model assuming a fixed baryonic fraction to explain observed relations.

Contribution

It introduces a model assuming a constant baryonic fraction in galaxy centers, explaining the correlation between rotation curve steepness and surface brightness.

Findings

Inner velocity gradient correlates with surface brightness (~0.5 slope)

Central baryonic fraction likely close to 1, with scatter due to M/L and thickness variations

Models with lower baryonic fractions are possible but require fine-tuning

Abstract

For disk galaxies (spirals and irregulars), the inner circular-velocity gradient (inner steepness of the rotation curve) correlates with the central surface brightness with a slope of ~0.5. This implies that the central dynamical mass density scales almost linearly with the central baryonic density. Here I show that this empirical relation is consistent with a simple model where the central baryonic fraction f_bar(0) is fixed to 1 (no dark matter) and the observed scatter is due to differences in the baryonic mass-to-light ratio M_bar/L (ranging from 1 to 3 in the R-band) and in the characteristic thickness of the central stellar component dz (ranging from 100 to 500 pc). Models with lower baryonic fractions are possible, although they require some fine-tuning in the values of M_bar/L and dz. Regardless of the actual value of f_bar(0), the fact that different types of galaxies do not show strong variations in f_bar(0) is surprising, and may represent a challenge for models of galaxy formation in a LCDM cosmology.