IUE Observations of PG 1115+080: The He i Gunn-Peterson Test and a Search for the Lensing Galaxy

Abstract

We have obtained new IUE observations of PG 1115+080 (z_em_ = 1.722), the "triple quasar," in order to carry out the He I Gunn-Peterson test with greater sensitivity and to search for a Lyman-limit discontinuity which could reveal the redshift of the gravitational lens. We find that n_HE I_(z = 1.722) ~ 7x10^11^ h_100_ cm^-3^ at the 3 σ level. We have used this density to set a curve of constraint in the log T_IGM_-log n_HE_ plane according to the shock-heated and photoionized IGM models. The predominantly shock-heated IGM model requires that T_IGM_~> 10^5.3^ K. If the IGM is both photoionized by the standard QSO diffuse radiation field and shock-heated, then the limits on hydrogen Lyman-alpha provide more stringent limits on the density and temperature of the IGM. We have found no evidence of the gravitational lens, placing an upper limit on N_H_ of 3 x 10^16^ cm^-2^ for z_lens_ >~0.39.