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Galaxies 2014, 2(3), 292-299; doi:10.3390/galaxies2030292

The Inner Regions of Disk Galaxies: A Constant Baryonic Fraction?

Department of Astronomy, Case Western Reserve University, 10090 Euclid Ave, Cleveland, OH 44106, USA
Received: 10 April 2014 / Revised: 18 June 2014 / Accepted: 19 June 2014 / Published: 10 July 2014
(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)
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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.
Keywords: dark matter; galaxies: formation; galaxies; evolution dark matter; galaxies: formation; galaxies; evolution

This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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Lelli, F. The Inner Regions of Disk Galaxies: A Constant Baryonic Fraction? Galaxies 2014, 2, 292-299.

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