Observations of the missing baryons in the warm–hot intergalactic medium

F. Nicastro (), J. Kaastra, Y. Krongold, S. Borgani, E. Branchini, R. Cen, M. Dadina, C. W. Danforth, M. Elvis, F. Fiore, A. Gupta, S. Mathur, D. Mayya, F. Paerels, L. Piro, D. Rosa-Gonzalez, J. Schaye, J. M. Shull, J. Torres-Zafra, N. Wijers and L. Zappacosta
Additional contact information
F. Nicastro: Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Roma
J. Kaastra: SRON Netherlands Institute for Space Research
Y. Krongold: Instituto de Astronomia Universidad Nacional Autonoma de Mexico
S. Borgani: University of Trieste
E. Branchini: University of Roma Tre
R. Cen: Princeton University
M. Dadina: INAF—Osservatorio di Astrofisica e Scienza dello Spazio di Bologna
C. W. Danforth: University of Colorado
M. Elvis: Harvard–Smithsonian Center for Astrophysics
F. Fiore: Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Roma
A. Gupta: Columbus State Community College
S. Mathur: Ohio State University
D. Mayya: Instituto Nacional de Astrofísica, Óptica y Electrónica
F. Paerels: Columbia University
L. Piro: INAF - Istituto di Astrofisica e Planetologia Spaziali
D. Rosa-Gonzalez: Instituto Nacional de Astrofísica, Óptica y Electrónica
J. Schaye: Leiden Observatory
J. M. Shull: University of Colorado
J. Torres-Zafra: Instituto de Astrofísica de La Plata (IALP-UNLP)
N. Wijers: Leiden Observatory
L. Zappacosta: Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Roma

Nature, 2018, vol. 558, issue 7710, 406-409

Abstract: Abstract It has been known for decades that the observed number of baryons in the local Universe falls about 30–40 per cent short1,2 of the total number of baryons predicted 3 by Big Bang nucleosynthesis, as inferred4,5 from density fluctuations of the cosmic microwave background and seen during the first 2–3 billion years of the Universe in the so-called ‘Lyman α forest’6,7 (a dense series of intervening H i Lyman α absorption lines in the optical spectra of background quasars). A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm–hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas—hydrogen—is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios8,9. Indeed, despite large observational efforts, only a few marginal claims of detection have been made so far2,10. Here we report observations of two absorbers of highly ionized oxygen (O vii) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar’s intrinsic outflow or the host galaxy’s interstellar medium implausible. The O vii systems lie in regions characterized by large (four times larger than average 11 ) galaxy overdensities and their number (down to the sensitivity threshold of our data) agrees well with numerical simulation predictions for the long-sought warm–hot intergalactic medium. We conclude that the missing baryons have been found.

Date: 2018
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DOI: 10.1038/s41586-018-0204-1

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