High Signal-to-Noise GHRS Observations of H 1821+643: O VI Associated with a Group of Galaxies at z = 0.226 and Complex Lyman α Absorption Profiles at Low Redshift

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As part of a program to study the relationship between QSO Lyman alpha clouds and galaxies at low redshifts, we have obtained a high signal-to-noise far UV spectrum of H 1821+643 [z(em) = 0.297] with the G140L grating of the GHRS. The spectrum, which has a resolution of FWHM = 150 km/s and S/N 100:1, is adequate for detection of weak Ly alpha clouds with equivalent width as low as 50 mA (4sigma) with complete spectral coverage from z(abs) = 0.03 to z(abs) = 0.26. In this paper we present some preliminary results of this study including the following. (1) The absorption profiles of three out of the four strongest extragalactic Ly alpha lines show complex component structure with a main strong component and several weaker outlying components spanning a full velocity range of 1000-1500 km/s. The two-point correlation function does not show any evidence of Ly alpha cloud clustering but suffers from small number statistics. (2) Extragalactic O VI is clearly detected in the intervening absorption system at z(abs) = 0.225. Two galaxies at z = 0.2256 and z = 0.2263 are close to the sight line (the closer galaxy is at a projected distance of 90 kpc), so this O VI absorption could be due to the intracluster medium of a group of galaxies.