Ly α absorption in the nearby universe : the sightline to Q 1821 + 643

We present the first results of a survey designed to understand the origin of the Lyα-forest absorption system at low redshift. Using the WYFFOS and HYDRA multifibre spectrographs on the WHT and WIYN telescopes, we have identified 51 galaxies brighter than bj = 18.5 within 30 arcmins of the sightline of the QSO 1821+643. This line of sight is of particular interest due to the detection of a population of low redshift, low equivalent width Lyα systems in several high quality, high resolution HST spectra. We identify three galaxies within 500 h kpc of the QSO sightline; the nearest galaxy is 104 h kpc away from the line of sight, and is at the same redshift as a strong (Wr = 0.63 Å) Lyα absorption line. The remaining two galaxies have no corresponding absorption to extremely low equivalent width limits (< 0.05 Å). Beyond 500 h kpc Lyα absorption lines are found at similar redshifts as several galaxies, but we show that these coincidences are likely to be accidental. Half of the Lyα systems for which we could have found at least an L∗ galaxy have no galaxies at the absorbers’ redshifts. For the majority of the remainder, we show that any apparent association with galaxies is probably coincidental. These Lyα systems are characterised by their weak equivalent widths (Wr < 0.2 Å), and we conclude that this population of absorbers is uncorrelated, or at best, weakly correlated, with galaxies. However, since we detect few Lyα lines with equivalent widths Wr > 0.2 Å in the present survey, the nature of these stronger lines is less clear. The detection of strong absorption both from the single galaxy in our survey at an impact parameter of 104 h kpc, and those already shown by Lanzetta et al. (1995) to be within 200 h kpc of bright galaxies, supports the ‘two-population model’ proposed by other researchers, whereby strong lines are associated with galaxies while weak ones are intergalactic in origin. However, the absence of bright galaxies responsible for strong lines beyond these distances (as found in other published surveys) means that low luminosity galaxies must also be associated with strong lines if this model is to be valid.