The Optical Depth of the Diffuse Intergalactic Medium

Details The Optical Depth of the Diffuse Intergalactic Medium The Optical Depth of the Diffuse Intergalactic Medium

Jul 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995hst..prop.5814t&link_type=abstract

HST Proposal ID #5814

Computer Science

Hst Proposal Id #5814 Quasars

Scientific paper

-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.

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