Q0302-003 (z=3.2860 +/- 0.0005) was the first quasar discovered that showed a He II Gunn-Peterson trough, a sign of incomplete helium reionization at z > 2.9. We present its HST/Cosmic Origins Spectrograph far-UV medium-resolution spectrum, which resolves many spectral features for the first time, allowing study of the quasar itself, the intergalactic medium, and quasar proximity effects. Q0302-003 has a harder intrinsic extreme-UV spectral index than previously claimed, as determined from both a direct fit to the spectrum (yielding alpha_nu = -0.8) and the helium-to-hydrogen ion ratio in the quasar’s line-of-sight proximity zone. Intergalactic absorption along this sightline shows that the helium Gunn-Peterson trough is largely black in the range 2.87 < z < 3.20, apart from ionization due to local sources, indicating that helium reionization has not completed at these redshifts. However, we tentatively report a detection of nonzero flux in the high-redshift trough when looking at low-density regions, but zero flux in higher-density regions. This constrains the He II fraction to be a few percent, suggesting helium reionization has progressed substantially by z ~ 3.1. The Gunn-Peterson trough recovers to a He II Ly-alpha forest at z < 2.87. We confirm a transmission feature due to the ionization zone around a z = 3.05 quasar just off the sightline, and resolve the feature for the first time. We discover a similar such feature for a luminous z = 3.23 quasar further from the sightline, which suggests that this quasar has been luminous for >34 Myr.
Date added: Tue, 8 Oct 13