  We use the SPARC (Spitzer Photometry & Accurate Rotation Curves) database to
study the relation between the central surface density of stars Sstar and
dynamical mass Sdyn in 135 disk galaxies (S0 to dIrr). We find that Sdyn
correlates tightly with Sstar over 4 dex. This central density relation can be
described by a double power law. High surface brightness galaxies are
consistent with a 1:1 relation, suggesting that they are self-gravitating and
baryon dominated in the inner parts. Low surface brightness galaxies
systematically deviate from the 1:1 line, indicating that the dark matter
contribution progressively increases but remains tightly coupled to the stellar
one. The observed scatter is small (~0.2 dex) and largely driven by
observational uncertainties. The residuals show no correlations with other
galaxy properties like stellar mass, size, or gas fraction.
