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  6. The formation of a DCBH requires a strong external radiation field to suppress H2 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 NH , we derive the critical field intensity, JLW crit, to suppress H2 formation and cooling. For the SED model with = ´N 1.3 10H 25 cm−2 , 8.0×1024 cm−2 , and 5.0×1024 cm−2 , we obtain »J 22LW crit , 35, and 54, all much smaller than the critical field intensity for normal star- forming galaxies J 1000LW crit X-ray photons from previously formed DCBHs build up a high-z X-ray background (XRB) that may boost the JLW crit. However, we find that in the three SED models, JLW crit only increases to ≈80, 170, and 390, even when r˙• reaches the maximum value allowed by the present-day XRB level (0.22, 0.034, 0.006 Me yr−1 Mpc−3), which is still much smaller than the galactic value. Although considering the XRB from first galaxies may further increase JLW 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.