Collapse redshift for cold dark matter (dashed lines) and baryons (solid lines) in spheres of various masses and overdensities [the latter in units of σ(M) for a BBKS power spectrum]. The collapse of the baryons is delayed relative to the dark matter because of gas pressure. The curves were obtained by following the motion of the baryonic and dark matter shells with a spherically symmetric, Lagrangian hydrodynamics code (HTL96). The dashed lines agree with the collapse redshift for the dark matter based on the linear extrapolation of the spherical collapse model for pressureless matter, z_coll = νσ/1.69 − 1.