A dimensional study of disc galaxies

We present a highly simplified model of the dynamical structure of a disc galaxy where only two parameters fully determine the solution, mass and angular momentum. We show through simple physical scalings that once the mass has been fixed, the angular momentum parameter lambda is expected to regulate such critical galactic disc properties as colour, thickness of the disc and bulge-to-disc ratio. It is, hence, expected to be the determinant physical ingredient resulting in a given Hubble type. A simple analytic estimate of lambda for an observed system is provided. An explicit comparison of the distribution of several galactic parameters against both Hubble type and lambda is performed using observed galaxies. Both such distributions exhibit highly similar characteristics for all galactic properties studied, suggesting lambda as a physically motivated classification parameter for disc galaxies.