Observational constraints on disc galaxy formation

We use data from the literature to constrain theoretical models of galaxy f ormation. We show how to calculate the dimensionless spin parameter lambda of the haloes of disc galaxies, and we compare the distribution of lambda w ith that observed in cosmological N-body simulations. The agreement is exce llent, which provides strong support for the hierarchical picture of galaxy formation. Assuming only that the radial surface density distribution of d iscs is exponential, we estimate crudely the maximum-disc mass-to-light rat io in the I band, and obtain [Y-I] less than or similar to 3.56 h, for a Hu bble constant of 100 h km s(-1) Mpc(-1). We discuss this result and its lim itations in relation to other independent determinations of Y-I. We also de fine a dimensionless form of the Tully-Fisher relation, and use it to deriv e a value of the baryon fraction in disc galaxies; the median value is m(d) = 0.084(Y-I/3.56 h). Assuming that the gas fraction in galactic haloes is at most as large as that in clusters, we also conclude that [Y-I] less than or similar to 2.56 h(-1/2).