Abstract
For disk galaxies (spirals and irregulars), the inner circular-velocity gradient dRV0 (inner steepness of the rotation curve) correlates with the central surface brightness ∑*,0 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 ƒbar,0 is fixed to 1 (no dark matter) and the observed scatter is due to differences in the baryonic mass-to-light ratio Mbar / LR (ranging from 1 to 3 in the R-band) and in the characteristic thickness of the central stellar component Δz (ranging from 100 to 500 pc). Models with lower baryonic fractions are possible, although they require some fine-tuning in the values of Mbar/LR and Δz. Regardless of the actual value of ƒbar,0, the fact that different types of galaxies do not show strong variations in ƒbar,0 is surprising, and may represent a challenge for models of galaxy formation in a Λ Cold Dark Matter (ΛCDM) cosmology.
Highlights
Galaxies are known to follow tight scaling relations linking their observed baryonic content to their dynamical properties
While Vflat is related to the total dynamical mass of the galaxy, the inner steepness of the rotation curve provides information on the central dynamical mass density, including both baryons and dark matter (DM)
We found that dR V0 correlates with the central surface brightness Σ∗,0 over more than two orders of magnitude in dR V0 and four orders of magnitude in Σ∗,0
Summary
Galaxies are known to follow tight scaling relations linking their observed baryonic content to their dynamical properties. While Vflat is related to the total dynamical mass of the galaxy, the inner steepness of the rotation curve provides information on the central dynamical mass density, including both baryons and dark matter (DM).
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