Neutral Hydrogen Gunn‐Peterson Absorption from an Overdensity in the Diffuse Intergalactic Medium at Redshift 0.8?

The results of a search for diffuse neutral hydrogen "Gunn-Peterson" absorption, including possible absorption from any overdensity in the diffuse intergalactic medium, at low to moderate redshifts ( z < 1.2) are discussed. The relatively low incidence of Lyα forest lines at these redshifts makes this work more straightforward than would be the case at higher redshifts. Criteria for the selection of QSO spectra leading to conservative estimates of Gunn-Peterson absorption are discussed, including the requirement that spectra shortward of the O VI λ1032 emission line be available. Thus, extrapolation of the continuum from longward to shortward of the Lyα emission line can be replaced by interpolation of the continuum in the spectral interval between Lyα and O VI emission. Analyses of Hubble Space T elescope Faint Object Spectrograph data on two QSOs with z em < 1 are presented here for illustrative purposes. PKS 1354+195 ( z em ≈ 0.72) shows no evidence for an absorption trough to an optical depth limit of τ GP-H I ( z ≈ 0.5) < 0.02, which is the normal case. However, PKS 2145+067 ( z em ≈ 0.99) shows evidence for a rare 12%-14% depression. If this is indeed Gunn-Peterson absorption, there must be an overdensity in the intergalactic medium near redshift z ≈ 0.8 toward PKS 2145+067 which is ≈ 3-7 times the density required for a typical nondetection. The region extends more than 500 h −1 100 Mpc along the sight line and, for J 912 ≈ 10 -22 ergs s -1 Hz -1 cm -2 sr -1 , has typical gas density n g ≈ 10 -6 cm -3 and temperature T ≈ (1-2) × 10 5 K. If the proposed overdense region is spherical and not filamentary in shape, this same effect may be visible in objects near the sight line toward PKS 2145+067. Alternative possibilities are discussed.