Triggering the Formation of Direct Collapse Black Holes by Their Congeners

Direct collapse black holes (DCBHs) are excellent candidates for seeds of supermassive black holes observed at z greater than or similar to 6. The formation of a DCBH requires a strong external radiation field to suppress H-2 formation and cooling in a collapsing gas cloud. Such a strong field is not easily achieved by first stars or normal star-forming galaxies. Here we investigate a scenario in which a previously formed DCBH can provide the necessary radiation field for the formation of additional ones. Using a one-zone model and simulated DCBH Spectral Energy Distributions (SEDs) filtered through absorbing gas initially having column density N-H, we derive the critical field intensity, J(LW)(crit), to suppress H-2 formation and cooling. For the SED model with N-H= 1.3 x 10(25) cm(-2), 8.0. x. 10(24) cm(-2), and 5.0. x. 10(24) cm(-2), we obtain J(LW)(crit) approximate to 22, 35, and 54, all much smaller than the critical field intensity for normal starforming galaxies J(LW)(crit) greater than or similar to 1000 X-ray photons from previously formed DCBHs build up a high-z X-ray background (XRB) that may boost the J(LW)(crit). However, we find that in the three SED models, J(LW)(crit) only increases to approximate to 80, 170, and 390, even when rho center dot reaches the maximum value allowed by the present-day XRB level (0.22, 0.034, 0.006M(circle dot) yr(-1) Mpc(-3)), which is still much smaller than the galactic value. Although considering the XRB from first galaxies may further increase J(LW)(crit), we conclude that our investigation supports a scenario in which DCBHs may be more abundant than predicted by models only including galaxies as external radiation sources.