Supermassive black hole formation by the cold accretion shocks in the first galaxies

We propose a new scenario for supermassive star (SMS; greater than or similar to 10(5) M-circle dot) formation in shocked regions of colliding cold accretion flows near the centers of first galaxies. When the post-shock density is high enough for collisionally exciting H-2 rovibrational levels (greater than or similar to 10(4) cm(-3)), enhanced H-2 collisional dissociation suppresses the gas to cool below 8000 K. In this case, the layer fragments into massive clouds (greater than or similar to 105 M-circle dot), which collapse isothermally (similar to 8000 K) by the Ly alpha cooling without subsequent fragmentation. As an outcome, SMSs are expected to form and evolve eventually to seeds of supermassive black holes. By calculating thermal evolution of the shocked gas, we delimit the range of post-shock conditions for the SMS formation. We also find that metal enrichment does not affect the SMS forming condition for less than or similar to 10(-3) Z(circle dot) if metals are in the gas phase.