Closing in on the Hydrogen reionization edge signal at z<7.2 with deep STIS/CCD Parallels

We present the first results of a project with the Hubble Space Telescope to take STIS parallel spectra to constrain the onset of reionization. This epoch would have left a faint but sharp spectral feature on top of the global background that marks the transition from a neutral to a fully ionized IGM at a predicted redshift z_ion=5--10. This ``Hydrogen edge'' is due to a rapid change in recombination from the Hydrogen Lyman series at z~= z_ion (Gnedin & Ostriker 1997), and leaves a sharp signal in the recombination spectrum >=3 dex below the Zodiacal background (Baltz et al. 1998). HST is unique in that it can constrain this signal with the STIS CCD spectrograph plus its long-slit and G750L grating, covering the range λ ~=5240--10,270Å or z~=3.3--7.5. The parallel data were scheduled in a fully automated way, and the first data were obtained this summer. The parallels were designed to use only orbital dark-time by padding the start and end of each orbit with direct finder exposures and STIS calibrations. The contemporaneous STIS calibrations are essential to reduce systematics. We found a faint but significant spectral emission band at 8457Å on top of the Zodiacal foreground, which we identify with a weak atmospheric O-I line that entered some of the parallels (despite the use of mostly orbital dark time), rather than coming from the reionization feature. Knowing the wavelength and significance of detection of this atmospheric O-I line, we set tentative limits the real Hydrogen edge signal of <=2x 10^-21 erg/cm²/s/Hz/sr at z_ion>= 5. The result from this first pass in the parallel data reduction is set by the limited S/N obtained so far, and we estimate that it will be ultimately limited by CTE effects plus on-orbit radiation damage in the STIS CCDs, by faint on-orbit O-I lines, and by our knowledge of the Zodiacal foreground spectrum. We outline how these constraints to the Hydrogen reionization edge can be further improved. We acknowledge NASA grants GO-8260.*-98A.