The soft, fluctuating UVB at z ~ 6 as traced by C IV, Si IV, and C II

Kristian Finlator, B. D. Oppenheimer, Romeel S Dave, E. Zackrisson, Robert Thompson, Shuiyao Huang

Research output: Contribution to journalArticle

25 Scopus citations

Abstract

The sources that drove cosmological reionization left clues regarding their identity in the slope and inhomogeneity of the ultraviolet ionizing background (UVB): bright quasars (QSOs) generate a hard UVB with predominantly large-scale fluctuations while Population II stars generate a softer one with smaller scale fluctuations. Metal absorbers probe the UVB's slope because different ions are sensitive to different energies. Likewise, they probe spatial fluctuations because they originate in regions where a galaxy-driven UVB is harder and more intense. We take a first step towards studying the reionization-epoch UVB's slope and inhomogeneity by comparing observations of 12 metal absorbers at z ~ 6 versus predictions from a cosmological hydrodynamic simulation using three different UVBs: a soft, spatially inhomogeneous 'galaxies+QSOs' UVB; a homogeneous 'galaxies+QSOs' UVB, and a 'QSOs-only' model. All UVBs reproduce the observed column density distributions of C II, Si IV, and C IV reasonably well although high-column, high-ionization absorbers are underproduced, reflecting numerical limitations. With upper limits treated as detections, only a soft, fluctuating UVB reproduces both the observed Si IV/C IV and C II/C IV distributions. The QSOs-only UVB overpredicts both C IV/C II and C IV/Si IV, indicating that it is too hard. The Haardt & Madau (2012) UVB underpredicts C IV/Si IV, suggesting that it lacks amplifications near galaxies. Hence current observations prefer a soft, fluctuating UVB as expected from a predominantly Population II background although they cannot rule out a harder one. Future observations probing a factor of 2 deeper in metal column density will distinguish between the soft, fluctuating and QSOs-only UVBs.

Original languageEnglish (US)
Pages (from-to)2299-2310
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Volume459
Issue number3
DOIs
StatePublished - Jul 1 2016
Externally publishedYes

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Keywords

  • Cosmology: theory
  • Galaxies: evolution
  • Galaxies: formation
  • Galaxies: high-redshift
  • Intergalactic medium
  • Quasars: absorption lines

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

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