CHARACTERISTICS of He

The proximity profile in the spectra of z ≈3 quasars, where fluxes extend blueward of the He II Lyα wavelength 304 (1+z) Ä, is one of the most important spectral features in the study of the intergalactic medium (IGM). Based on the Hubble Space Telescope spectra of 24 He II quasars, we find that the majority of them display a proximity profile, corresponding to an ionization radius as large as 20 Mpc in the sources rest frame. In comparison with those in the H I spectra of the quasars at z ≈6, the He II proximity effect is more prominent and is observed over a considerably longer period of reionization. The He II proximity zone sizes decrease at higher redshifts, particularly at z > 3.3. This trend is similar to that for H I, signaling an onset of He II reionization at z ≳ 4. For quasar SDSS1253+6817 (z = 3.48), the He II absorption trough displays a gradual decline and serves as a good case for modeling the He II reionization. To model such a broad profile requires a quasar radiation field whose energy distribution between 4 and 1 Rydberg is considerably harder than normally assumed. The UV continuum of this quasar is indeed exceptionally steep, and the He II ionization level in the quasar vicinity is higher than the average level in the IGM. These results are evidence that a very hard EUV continuum from this quasar produces a large ionized zone around it. Distinct exceptions are the two brightest He II quasars at z ≈2.8, for which no significant proximity profile is present, probably implying that they are very young.