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 \langle \Upsilon _ { I } \rangle \la 3.56 h , for a Hubble constant of 100 h~ { } \mbox { km s$ { } ^ { -1 } $ } \mbox { Mpc } ^ { -1 } .
We discuss this result and its limitations in relation to other independent determinations of \Upsilon _ { I } .
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 v _ { m } > 100 \mbox { km s$ { } ^ { -1 } $ } , the median value is m _ { d } = 0.086 ( { \Upsilon _ { I } / 3.56 h } ) .
Assuming that the gas fraction in galactic halos is at most as large as that in clusters , we also conclude that \langle \Upsilon _ { I } \rangle \la 2.48 h ^ { -1 / 2 } .