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Triggering the formation of direct collapse black holes by their congeners

Abstract

Direct collapse black holes (DCBHs) are excellent candidates as seeds of supermassive black holes (SMBHs) observed at z \gsim 6. The formation of a DCBH requires a strong external radiation field to suppress H2\rm H_2 formation and cooling in a collapsing gas cloud. Such strong field is not easily achieved by first stars or normal star-forming galaxies. Here we investigate a scenario in which the previously-formed DCBH can provide the necessary radiation field for the formation of additional ones. Using one-zone model and the simulated DCBH Spectral Energy Distributions (SEDs) filtered through absorbing gas initially having column density NHN_{\rm H}, we derive the critical field intensity, JLWcritJ_{\rm LW}^{\rm crit}, to suppress H2\rm H_2 formation and cooling. For the SED model with NH=1.3Γ—1025N_{\rm H}=1.3\times10^{25} cmβˆ’2^{-2}, 8.0Γ—10248.0\times10^{24} cmβˆ’2^{-2} and 5.0Γ—10245.0\times10^{24} cmβˆ’2^{-2}, we obtain JLWcritβ‰ˆ22J_{\rm LW}^{\rm crit}\approx22, 35 and 54, all much smaller than the critical field intensity for normal star-forming galaxies (J_{\rm LW}^{\rm crit}\simgt 1000). X-ray photons from previously-formed DCBHs build up a high-zz X-ray background (XRB) that may boost the JLWcritJ_{\rm LW}^{\rm crit}. However, we find that in the three SED models JLWcritJ_{\rm LW}^{\rm crit} only increases to β‰ˆ80\approx80, 170 and 390 respectively even when \dt{\rho}_\bullet reaches the maximum value allowed by the present-day XRB level (0.22,0.034,0.006Β MβŠ™0.22, 0.034, 0.006~M_\odotyrβˆ’1^{-1}Mpcβˆ’3^{-3}), still much smaller than the galactic value. Although considering the XRB from first galaxies may further increase JLWcritJ_{\rm LW}^{\rm crit}, we conclude that our investigation supports a scenario in which DCBH may be more abundant than predicted by models only including galaxies as external radiation sources.Comment: 18 pages, 14 figures, 5 tables, ApJ in pres

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