The Formation of Direct Collapse Black Holes at Cosmic Dawn and 21 cm Global Spectrum

Abstract

JWST reveals numerous high-z galaxies and Supermassive Black Holes (SMBHs), suggesting that stars and SMBH seeds formation at $z\gtrsim 10$ may be more efficient than previously derived. One popular SMBH seed scenario is the Direct Collapse Black Holes (DCBHs) formed in pristine atomic-cooling halos irradiated by nearby galaxies. Therefore, the efficient star formation likely facilitates the formation of DCBH. We calculate the first critical $k_{{\mathrm{H}}_2}-k_{{\mathrm{H}}^{-}}$ curves for DCBH formation under the influence of X-ray radiation using the one-zone model. We then build the UV luminosity function consistent with JWST observations and incorporate it into the model that calculates the DCBH-triggering probability. We confirm that enhanced star formation promotes the DCBH formation. However, the DCBH abundance $n_{\mathrm{DCBH}}$ is significantly influenced by the X-ray radiation that is also related to star formation. Since the 21 cm global spectrum is also X-ray dependent, the 21 cm absorption depth $\delta T_b^{\mathrm{trough}}$ at Cosmic Dawn encodes the DCBH abundance information. We provide a tentative trend in the $n_{\mathrm{DCBH}}$ - $\delta T_{\mathrm{b}}^{\mathrm{trough}}$ relation, which could be a useful guide. In our fiducial model, if $\delta T_b^{\mathrm{trough}}\gtrsim -100$ mK, then the DCBH is rather rare; if $-150\mathrm{mK}\lesssim \delta T_b^{\mathrm{trough}}\lesssim -100$ mK, $n_{\mathrm{DCBH}}\sim \mathcal{O}(10^{-2}-10^{-3})$ cMpc${}^{-3}$ (comoving $\mathrm{Mp}{\mathrm{c}}^{-3}$), consistent with the HST/JWST observed SMBHs abundance at $z\gtrsim 6$; if $\delta T_b^{\mathrm{trough}}\lesssim -150$ mK, $n_{\mathrm{DCBH}}$ can largely exceed $\mathcal{O}(10^{-2})$ cMpc${}^{-3}$. The 21 cm global spectrum observations will help to constrain the DCBH abundance.

20 pages, 6 figures, 2 tables. Accepted for publication in ApJ