Measurement of the opacity of ionized helium in the intergalactic medium

摘要: DETERMINATION of the density and ionization state diffuse primordial gas in intergalactic medium (IGM) is an important step towards understanding nature dark matter formation structure Universe. Absorption by neutral hydrogen atoms IGM should produce a large flux deficit spectra high-redshift quasars at wavelengths below that H I Lyman-α line (the Gunn–Peterson effect1). But observations low spectral resolution have revealed only relatively small deficits, higher these appear as numerous discrete absorption features forest). This implies both highly ionized non-uniformly distributed2. Similar expected from helium3, although its detection more challenging evidence for existence was obtained recently4,5. Here we report observation singly helium (He ii) spectrum quasar HS1700 + 64. We measure mean over redshift range 2.2 < z 2.6 yields effective optical depth 1.00 (±0.07). Comparing this result with quasar, argue most measured He II occurs lowest-density regions IGM.