Speaker
Description
The existence of supermassive black holes (SMBHs) at $z>6$ poses a profound challenge to our understanding of their formation and growth. This long-standing puzzle is intensified by recent JWST discoveries of accreting SMBHs at $z \sim 7-10$ with inferred masses up to $\sim 10^{7-8} \rm M_{\odot}$, which require the formation of massive seeds followed by rapid, sustained accretion. A leading explanation is the direct collapse black hole (DCBH) channel, in which $\sim 10^4-10^{6}\,\rm M_{\odot}$ seeds form from the monolithic collapse of pristine, metal-free gas in atomic-cooling halos ($T_{\rm vir}\geq 10^{4}\,\rm K$). This process requires intense UV radiation from nearby primordial galaxies to suppress H$_2$ formation, rendering DCBHs intrinsically rare. However, the overabundance of high-z galaxies revealed by JWST could facilitate DCBH formation—potentially explaining the unexpectedly abundant faint AGNs (the so-called "Little Red Dots") at $z \sim 4-5$. On the other hand, X-ray radiation from these same galaxies can catalyze H$_2$ formation, potentially suppressing DCBH formation.
In this talk, I will present our work that quantifies the radiative impact of the first galaxies on the abundance of DCBH. Using UV luminosity function models calibrated to the latest JWST observations, we investigate the complex interplay between UV and X-ray radiation that governs DCBH formation in a realistic cosmic environment. Our results show that while more efficient star formation promotes DCBH creation, the accompanying X-rays act as a strong regulator of their final abundance. The most exciting insight from our study is a novel link between the abundance of DCBHs and the 21 cm global signal from Cosmic Dawn. The depth of the 21 cm absorption feature is highly sensitive to the X-ray background, which is dictated by the same intrinsic galactic X-ray properties that modulate DCBH formation. I will present this fundamental connection and highlight the potential of the 21 cm signal to serve as a novel probe of DCBH formation rate, opening a fresh window into the birth of the first giants in the early Universe.
| Session Selection | Astronomy and Astrophysics |
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