# Quasars and the Intergalactic Medium at Cosmic Dawn

**Authors:** Xiaohui Fan, Eduardo Banados, Robert A. Simcoe  
**arXiv:** [2212.06907](https://arxiv.org/abs/2212.06907)  
**Date:** 2022-12-13  
**Summary page:** https://www.summarizepaper.com/en/paper/2212.06907v1/

## Key points

- Quasars provide insights into the formation and growth of supermassive black holes, their connection to galaxy and structure formation, and the evolution of the intergalactic medium during the epoch of reionization.
- Recent observations have revealed hundreds of quasars in the first billion years of cosmic history, with luminous quasars declining exponentially at z>5.
- Billion-solar-mass black holes already exist at z>7.5, requiring them to form and grow in less than 700 million years through a combination of massive early black hole seeds with highly efficient and sustained accretion.
- The rapid growth of quasars is accompanied by strong star formation and feedback activity in their host galaxies, which exhibit diverse morphological and kinetic properties.
- HI absorption in quasar spectra probes the tail end of cosmic reionization at z~5.3-6, indicating an EoR midpoint at 6.9 < z < 7.6 with large spatial fluctuations in IGM ionization.
- Observations suggest that heavy element absorption lines indicate an evolution in ionization structure and metal enrichment during EoR within circumgalactic media.
- These findings provide important insights into early universe physics, including SMBH formation mechanisms, galaxy evolution processes, and IGM reionization history.

## Summary

Quasars are powerful probes of the early universe, providing insights into the formation and growth of supermassive black holes (SMBHs), their connection to galaxy and structure formation, and the evolution of the intergalactic medium (IGM) during the epoch of reionization (EoR). Recent observations have extended the quasar redshift frontier to z~7.6, revealing hundreds of quasars in the first billion years of cosmic history. Observations show that luminous quasars decline exponentially at z>5, suggesting that the earliest quasars emerge at z~10. The lack of strong evolution in their average spectral energy distribution indicates a rapid buildup of the active galactic nucleus (AGN) environment. Billion-solar-mass BHs already exist at z>7.5, requiring them to form and grow in less than 700 million years through a combination of massive early BH seeds with highly efficient and sustained accretion. The rapid growth of quasars is accompanied by strong star formation and feedback activity in their host galaxies, which exhibit diverse morphological and kinetic properties with typical dynamical mass lower than that implied by local BH/galaxy scaling relations. HI absorption in quasar spectra probes the tail end of cosmic reionization at z~5.3-6, indicating an EoR midpoint at 6.9 < z < 7.6 with large spatial fluctuations in IGM ionization. Observations also suggest that heavy element absorption lines indicate an evolution in ionization structure and metal enrichment during EoR within circumgalactic media. These findings provide important insights into early universe physics, including SMBH formation mechanisms, galaxy evolution processes, and IGM reionization history.