Observational constraints on disk galaxy formation

We use data from the literature to constrain theoretical models of galaxy formation. We show how to calculate the dimensionless spin parameter $\lambda$ of the halos of disk galaxies and we compare the distribution of $\lambda$ with that observed in cosmological $N$-body simulations. The agreement is excellent, which provides strong support for the hierarchical picture of galaxy formation. Assuming only that the radial surface density distribution of disks is exponential, we estimate crudely the maximum-disk mass-to-light ratio in the $I$-band and obtain $<M/L> \la 3.56h$, for a Hubble constant of $100h~\kms \mpc^{-1}$. We discuss this result and its limitations in relation to other independent determinations of $M/L$. We also define a dimensionless form of the Tully-Fisher relation, and use it to derive a value of the baryon fraction in disk galaxies. For galaxies with circular velocity $\vmax>100\kms$, the median value is $md = 0.086 (M/L/3.56h)$. Assuming that the gas fraction in galactic halos is at most as large as that in clusters, we also conclude that $<M/L> \la 2.48h^{-1/2}$.