Closing in on the Hydrogen Reionization Edge at
z < 7.2 with Deep HST/STIS CCD Parallels

     These are the (preliminary) web-pages of an innovative and large program (PI: Windhorst) with HST/STIS to take CCD spectra of nearly random areas in the sky in parallel to (and offset from) observations with other HST instruments. The main purpose is to contrain the onset of (the second and final) reionization epoch of Hydrogen in the intergalactic medium (IGM), likely due to the rapid formation and prolific star formation and associated UV output of dwarf galaxies.

This epoch would have left a faint (≤ 2×10^-21 erg/cm²/s/Hz/sr at z_ion ≥ 5) but relatively sharp spectral feature on top of the global background that marks the transition from a (partly) neutral to a fully ionized IGM at a predicted redshift z_ion ~ 5-10 (Baltz et al. 1998). This "edge" is due to a rapid change in recombination from the Hydrogen Lyman series.

z_ion is one of the most important remaining unknown quantities in cosmology, and this project is complementary to measurements of the Gunn-Peterson trough toward large numbers of QSOs (i.e., discrete sight-lines).

Our team obtained 28 STIS parallel G750L spectra (5240--10,270Å, where the upper limit corresponds to z ~ 7.2) in Cycle 8 (PID #8260) as a pilot project, and 425 STIS parallel spectra with the same instrumental set-up in an extended Cycle 10 program (PID #9066).