The Baryon Census in a Multiphase Intergalactic Medium: 30% of the Baryons May Still Be Missing

2011/12/31 by J. Michael Shull, Britton D. Smith, Britton Smith +2 · 3 citations
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Star Formation Studies #Galaxies: Formation, Evolution, Phenomena #astro-ph.CO

paper · pdf · doi:10.1088/0004-637x/759/1/23

14 pages, 9 figures, revised submission to ApJ (new Fig 2, Appendices A,B)

arxiv created 2012/05/14 · openalex publication_date 2012/10/12 · arxiv updated 2015/06/03 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31

Abstract

For low-redshift cosmology and galaxy formation rates, it is important to account for all the baryons synthesized in the Big Bang. Although galaxies and clusters contain 10% of the baryons, many more reside in the photoionized Lyman-alpha forest and shocked-heated warm-hot intergalactic medium (WHIM) at T = 10⁵ to 10⁷ K. Current tracers of WHIM at 10⁵ to 10⁶ K include the O VI 1032, 1038 absorption lines, together with broad Lyman-alpha absorbers (BLAs) and EUV/X-ray absorption lines from Ne VIII, O VII, and O VIII. We improve the O VI baryon surveys with corrections for oxygen metallicity (Z/Zsun) and O VI ionization fraction (f_OVI) using cosmological simulations of heating, cooling, and metal transport in a density-temperature structured medium. Statistically, their product correlates with column density, (Z/Zsun)(f_OVI) = (0.015)(N_OVI/10¹⁴ cm^-2)⁰.70. The N_OVI-weighted mean is 0.01, which doubles previous estimates of WHIM baryon content. We also reanalyze H I data from the Hubble Space Telescope, applying redshift corrections for absorber density, photoionizing background, and proper length, dl/dz. We find substantial baryon fractions in the photoionized Lya forest (28 +/- 11%), O VI/BLA-traced WHIM (25 +/- 8%), and collapsed phase (18 +/- 4%) in galaxies, groups, clusters, and circumgalactic gas. The baryon shortfall is 29 +/- 13%, which may be detected in X-ray absorbers from hotter WHIM or in weaker Lya and O VI absorbers. Further progress will require higher-precision baryon surveys of weak absorbers at column densities N_HI > 10^12.0 cm^-2, N_OVI > 10^12.5 cm^-2, and N_OVII > 10^14.5 cm^-2, with moderate-resolution UV and X-ray spectrographs.

Citations

• Confirmation of X-Ray Absorption by WHIM in the Sculptor Wall
• ChandraDetection of the First X‐Ray Forest along the Line of Sight to Markarian 421
• X‐Ray Absorption Line Spectroscopy of the Galactic Hot Interstellar Medium
• The Local Lyα Forest. IV. Space Telescope Imaging Spectrograph G140M Spectra and Results on the Distribution and Baryon Content of H i Absorbers
• On the Putative Detection ofz> 0 X‐Ray Absorption Features in the Spectrum of Mrk 421
• The chemical composition of the Sun

Cited by

• Cosmological-model independent limits on photon mass from FRB and SNe data
• Gas accretion from minor mergers in local spiral galaxies
• Baryon [wikipedia]