The 304 A He + absorption line in the spectrum of quasar HE 2347-4342

This diagram shows a small part of the far-ultraviolet spectrum of the newly discovered, unusually bright quasar HE 2347-4342 (redshift z = 2.885). It was obtained in June 1996 with the GHRS instrument at the Hubble Space Telescope during a total exposure time of about 6 hours. The abscissa unit is Angstrom (1 A = 10 -10 m); the ordinate is relative intensity (brightness). The spectral resolution is about 0.7 A.

The recorded spectrum in this region is indicated with a fully drawn line. As can be seen, the intensity is virtually zero in several areas, for instance in the 1162 - 1172 A and 1176 - 1182 A intervals (`troughs'). This is due to He + -ions in the foreground of the quasar, strongly absorbing the light from the quasar in the redshifted 304 A helium line. Since a particular wavelength corresponds to a particular redshift and therefore to a certain distance along the line of sight, the amount of absorption at a given wavelength is a direct measure of the number of ions at a given distance from the quasar.

The He + -spectrum has been overlaid (dashed line) by the spectrum of hydrogen (the redshifted 1215 A Lyman-alpha line, as observed at 4700 A with the CASPEC instrument at the ESO 3.6-m telescope), shifted by the appropriate factor (304/1215) in order to allow a direct comparison of the space distributions of hydrogen and helium.

The hydrogen spectrum also displays many absorption minima, but all of these are comparatively narrow. This implies the existence of small and relatively dense clouds with hydrogen, situated along the line of sight. In between, there is little absorption (the relative intensity is near 1) - this corresponds to `voids' in which little hydrogen is present, cf. the areas near 1160 A and 1174-5 A. Here, the absorption by He + -ions is also comparitively small, thus these ions are also less numerous in these voids.

As described in ESO Press Release eso9720 (1 August 1997), a detailed analysis of these data has shown that the observed troughs correspond to large space regions of not yet fully ionized helium.

This is the first time that is has been possible to look back to the re-ionization epoch in the early Universe, a few billion years after the Big Bang .