The Fundamental Plane of Spiral Galaxies: Search from Observational Data

The fundamental plane of spiral galaxies was searched from observational data, which can be approximately represented in observational parameters by $ L \propto V^2 R $, where $L$, $R$, and $V$ are the luminosity, the linear size of galactic disk, and the rotation velocity. This plane exists at all optical bands in our samples of more than 500 spiral galaxies in total. It is more fundamental than the relationship between any two of the parameters, and can reduce the residual of the Tully--Fisher $L$--$V$ relations by about 50\%. Noticed that the power index of $V$ is doubled from that of $R$, which implies that the total mass and the mass distribution of galaxies plays an important role in forming the fundamental plane. Involving the third parameter of galactic size has a strong physical implication on galaxy formation and the dark--mass distribution.