Characteristics of He II Proximity Profiles

TL;DR

This study analyzes He II proximity profiles in quasar spectra at z~3, revealing their size evolution, modeling the reionization process, and highlighting the importance of the quasar's EUV continuum hardness.

Contribution

It provides the first detailed analysis of He II proximity zones at z~3, modeling the reionization process and emphasizing the role of the quasar's EUV spectrum.

Findings

He II proximity zones can extend up to 20 Mpc in the source rest frame.

He II reionization likely begins around redshift 4.

Harder EUV spectra from quasars produce larger ionized zones.

Abstract

The proximity profile in the spectra of z~3 quasars, where fluxes extend blueward of the He II Lya wavelength 304 (1+z) A, is one of the most important spectral features in the study of the intergalactic medium. Based on the HST 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 source's 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 a good case for modeling the He II reionization. To model such a broad profile requires a quasar radiation field whose 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 intergalactic medium. 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, possibly implying that they are young.