5814( 0) - 07/18/95 18:04 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5814 Version: ********** Check-in Date: ********** 1.Proposal Title: The Optical Depth of the Diffuse Intergalactic Medium ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Quasars AR 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: David Turnshek Dept. of Physics & Astronomy, \ University of Pittsburgh CoI: Valery Khersonsky Dept. of Physics & Astronomy,, University N of Pittsburgh ------------------------------------------------------------------------------------ 6. Abstract -0.1 truein The diffuse intergalactic medium may give rise to weak absorption troughs in the spectra of background quasars, i.e., the so-called Gunn-Peterson effect. The goal of this program is to assemble suitable UV archival HST-FOS spectra of quasars for study of the Gunn-Peterson effect in the HILambda1216 and HeILambda584 LyAlpha lines. By fitting quasar spectral energy distributions and extrapolating to regions of interest, we will measure or place upper limits on diffuse HILambda1216 absorption at redshifts z < 0.9 and diffuse HeILambda584 at redshifts 1.1 < z < 2.7. Archival measurements can be made for as many as 46 ideal objects in the case of HILambda1216 and 14 objects in the case of HeILambda584. We expect our measurements to establish upper limits or evidence for weak absorption. The measurements will be: (1) the best limits that can be derived, (2) significant for models, and (3) complementary to ongoing HST searches for diffuse HeIILambda304 absorption at z > 2.8. Constraints on the Gunn-Peterson effect in the absorption spectra of quasars provide information on the diffuse UV ionizing radiation, which is thought to be mostly generated by quasars and young star-burst galaxies, and on the processes of galaxy and structure formation in the Universe. Some models for the strength of the Gunn-Peterson absorption troughs predict optical depths as high as Tau ~ 0.1. However, the theoretical predictions are very sensitive to several model parameters which are largely unknown. Direct measurements of the Gunn- Peterson effect at these redshifts are therefore very worthwhile. ------------------------------------------------------------------------------------ 5814( 0) - 07/18/95 18:04 - [ 2] Observations Description ------------------------ Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5814( 0) - 07/18/95 18:04 - [ 3] Summary Form for Proposal 5814 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category AR ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Quasars ------------------------------------------------------------------------------------------------------------------------------------