The Tully-Fisher relation for low surface brightness galaxies: implications for galaxy evolution

We present the B-band Tully-Fisher relation for low surface brightness (LSB) galaxies. These LSB galaxies follow the same Tully-Fisher relation as normal spiral galaxies. This implies that the mass-to-light ratio (M/L) of LSB galaxies is typically a factor of 2 larger than that of normal galaxies of the same total luminosity and morphological type. Since the dynamical mass of a galaxy is related to the rotational velocity and scalelength via M~V^2h, at fixed linewidth LSB galaxies must be twice as large as normal galaxies. This is confirmed by examining the relation between scalelength and linewidth for LSB and normal galaxies. The universal nature of the Tully-Fisher relation can be understood if LSB galaxies are galaxies with low mass surface density, sigma. The mass surface density apparently controls the luminosity evolution of a galaxy in such a way as to keep the product sigma M/L constant.