Perfect fluid dark matter: a viability test with galaxy rotation curves

Abstract: The anomalous rotation curves of galaxies provide compelling evidence for dark matter, yet its fundamental nature and distribution remain key unresolved issues in astrophysics. In this work, we investigate a dark matter model derived from first principles within General Relativity, treating the halo as a perfect fluid with a specific anisotropic equation of state characterized by a single parameter. This framework yields two families of static, spherically symmetric solutions: a Power-Law metric and a Logarithmic metric. As an initial viability test, we fit the model's derived circular velocity profiles to the dark matter contributions of representative galaxies from the SPARC database. Our analysis reveals that the two solutions effectively describe different regions of the halo: the Logarithmic form accurately models the large-radius behavior, while the Power-Law form successfully reproduces the inner rotation curve. Notably, the model consistently favors a shallow central density profile, aligning with cored halo models and providing a fit for galaxies with a gradual rise in velocity. We conclude that this simple, analytically-derived fluid model provides a compelling and physically-motivated framework for describing galactic rotation curves, warranting a more exhaustive study across a larger sample of galaxies.