We use 1D radiative transfer simulations to study the evolution of H _ { 2 } gas-phase ( H ^ { - } catalyzed ) formation and photo-dissociation regions in the primordial universe .
We find a new positive feedback mechanism capable of producing shells of H _ { 2 } in the intergalactic medium , which are optically thick in some Lyman-Werner bands .
While these shells exist , this feedback effect is important in reducing the H _ { 2 } dissociating background flux and the size of photo-dissociation spheres around each luminous object .
The maximum background opacity of the IGM in the H _ { 2 } Lyman-Werner bands is \tau _ { H _ { 2 } } \approx 1 - 2 for a relic molecular fraction x _ { H _ { 2 } } = 2 \times 10 ^ { -6 } , about 6 times greater than found by ( ) .
Therefore , the relic molecular hydrogen can decrease the photo-dissociation rate by about an order of magnitude .
The problem is relevant to the formation of small primordial galaxies with masses M _ { DM } \lesssim 10 ^ { 8 } M _ { \odot } , that rely on molecular hydrogen cooling to collapse .
Alternatively , the universe may have remained dark for several hundred million years after the birth of the first stars , until galaxies with virial temperature T _ { vir } \gtrsim 10 ^ { 4 } K formed .