Lyalpha absorption in the nearby Universe: the sightline to Q1821+643

Abstract

We present the first results of a survey designed to understand the origin of Lyalpha-forest absorption systems at low redshift. Using the WYFFOS and HYDRA multifibre spectrographs on the William Herschel and Wisconsin Indiana Yale NOAO (WIYN) telescopes, we have identified 51 galaxies brighter than b_j:=:18.5 within 30 arcmin of the sightline of the QSO 1821+643. We find three galaxies within 500 h^-1 kpc of the QSO sightline; the nearest galaxy is 104 h^-1 kpc away from the line of sight, and is at the same redshift as a strong (W_r:=:0.63Angstroms) Lyalpha absorption line. The remaining two galaxies have no corresponding absorption to extremely low equivalent width limits (<0.05 Angstroms). Beyond 500 h^-1 kpc, Lyalpha absorption lines are found at redshifts similar to those of several galaxies, but we show that these coincidences are likely to be accidental. Half of the Lyalpha systems for which we could have found at least an L^* galaxy have no galaxies at the redshifts of the absorbers. For the majority of the remainder, we show that any apparent association with galaxies is probably coincidental. These Lyalpha systems are characterized by their weak equivalent widths (W_r:<:0.2Angstroms), and we conclude that this population of absorbers is uncorrelated, or at best weakly correlated, with galaxies.