The initial fragmentation of collapsing gaseous protoclusters

The initial fragmentation of a 'pancaking' protocluster of galaxies is considered. The fragment mass Msub(c)sup(*) is determined by the Jeans mass at the time when the collapse time-scale of the fragment is equal to the growth time-scale for the cold compressed layer, and is on the order of 3.5 x 108nsub(-3)sup(-1/2)T4sup(-1/4) solar mass, nsub(-3) being the immediate pre-shock particle density (units of 10-3 cm-3) and T4 being the immediate post-shock temperature (units of 104 K). Other aspects of this picture of galaxy formation are also discussed. (author)