The ultraviolet absorption spectrum of the Quasar H1821 + 643 (z = 0.297)

We report measurements of the absorption lines in the ultraviolet spectrum of the nearby luminous quasar H1821 + 643 (z = 0.297). The results were obtained using high-resolution observations with the faint object spectrograph of the Hubble Space Telescope. A total of 51 absorption lines and a number of emission features were detected between 1190 and 3260 Å. A complete sample of 38 absorption lines was constructed with the detection limit defined in terms of a limiting equivalent width as a function of wavelength. The identifications with interstellar and with extra-galactic lines were made with the aid of software that utilizes the limiting detection sensitivity as a function of wavelength and the known atomic properties of cosmologically abundant ions. The majority of the absorption features are Galactic interstellar lines. The interstellar absorption lines and the quasar emission lines present in the spectrum of H1821 + 643 are similar to the lines observed in other small-redshift quasars. There are five strong extragalactic Lyα absorption lines in the spectrum of H1821 + 643, all with observed equivalent widths greater than 0.45 Å. Combining the HST observations of 3C 273 and H1821 + 643, we estimate the local number density of Lyα systems with rest equivalent widths larger than 0.32 Å to be (dN/dz)₀ ≈ 13 ± 5 Lyα lines per unit redshift. Some of the Lyα systems with redshifts significantly different from the quasar appear to be associated with galaxies or with clusters of galaxies. Two of the Lyα lines have the same redshift within cΔz = 400 km s^-1 as that of an emission-line galaxy located at a projected separation from the quasar of approximately 90 kpc (H₀ = 100 km s^-1 Mpc^-1). This system at z = 0.226 is part of a triplet of Lyα lines that have a total redshift separation of cz = ± 450 km s^-1. There is also statistical evidence that some of the Lyα systems are clumped more than would be expected if they were distributed randomly along the line of sight to H1821 + 643. Many of the Lyα systems observed at small redshifts may be associated with large hydrogen halos of galaxies or groups and clusters of galaxies. One of the Lyα systems in H1821 + 643 occurs at an absorption redshift approximately equal to the emission-line redshift of the quasar and is accompanied by absorption from the C IV and O VI doublets; this is an example of what has been called associated absorption. z_abs ≈ z_em, for large-redshift quasars. No lines from low-ionization ions are detected in this system. The observations of H1821 + 643 fail to confirm previous suggestions that associated absorption is correlated with strong radio emission (H1821 + 643 is radio-quiet) or with low optical luminosity (H1821 + 643 is comparable in luminosity to 3C 273). The observations are consistent with two previously proposed conjectures, that associated absorption is produced in clusters of galaxies containing the quasars or that the absorption is produced in the gas associated with the quasars.