The concerted impact of galaxies and QSOs on the ionization and thermal state of the intergalactic medium

Koki Kakiichi, Luca Graziani, Benedetta Ciardi, Avery Meiksin, Michele Compostella, Marius B. Eide, Saleem Zaroubi

Research output: Contribution to journalArticlepeer-review


We present a detailed analysis of the ionization and thermal structure of the intergalactic medium (IGM) around a high-redshift (z = 10) QSO, using a large suite of cosmological, multifrequency radiative transfer simulations, exploring the contribution from galaxies as well as the QSO, and the effect of X-rays and secondary ionization. We show that in high-z QSO environments both the central QSO and the surrounding galaxies concertedly control the reionization morphology of hydrogen and helium and have a non-linear impact on the thermal structure of the IGM. A QSO imprints a distinctive morphology on HII regions if its total ionizing photon budget exceeds that of the surrounding galaxies since the onset of hydrogen reionization; otherwise, the morphology shows little difference from that of HII regions produced only by galaxies. In addition, the spectral shape of the collective radiation field from galaxies and QSOs controls the thickness of the I-fronts. While a UV-obscured QSO can broaden the I-front, the contribution from other UV sources, either galaxies or unobscured QSOs, is sufficient to maintain a sharp I-front. X-ray photons from the QSO are responsible for a prominent extended tail of partial ionization ahead of the I-front. QSOs leave a unique imprint on the morphology of He II/He III regions. We suggest that, while the physical state of the IGM is modified by QSOs, the most direct test to understand the role of galaxies and QSOs during reionization is to perform galaxy surveys in a region of sky imaged by 21 cm tomography.

Original languageEnglish
Pages (from-to)3718-3736
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Jul 2017
Externally publishedYes

Bibliographical note

Funding Information:
We thank James Bolton for kindly providing us the hydrodynamical simulation that is used in this paper and an anonymous referee for constructive comments. KK thanks Michele Sasdelli, Garrelt Mellema and Ilian Iliev for useful conversations, and Andrew Chung and Phillip Busch for helping to improve the manuscript. LG acknowledges the support of the DFG Priority Program 1573 and from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 306476.

Publisher Copyright:
© 2018 The Author(s).


  • Dark ages
  • First stars
  • Intergalactic medium
  • Methods: numerical
  • Radiative transfer
  • Reionization


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