Lyman Alpha Absorption in the Nearby Universe: the Sightline to Q1821+643

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  • 28 July 1997
Abstract
We present the first results of a survey designed to understand the origin of the Lya-forest absorption system at low redshift. Using the WYFFOS and HYDRA multi-fibre spectrographs on the WHT and WIYN telescopes, we have identified 51 galaxies brighter than b_j = 18.5 within 30 arcmins of the sightline of the QSO 1821+643. We identify three galaxies within 500 h-1 kpc of the QSO sightline; the nearest galaxy is 104 h-1 kpc away, and is at the same redshift as a strong Lya absorption line. The remaining two galaxies have no corresponding absorption to extremely low equivalent width limits. Beyond 500 h-1 kpc Lya absorption lines are found at similar redshifts as several galaxies, but we show that these coincidences are likely to be accidental. Half of the Lya 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 Lya systems are characterised by their weak equivalent widths (W_r < 0.2A), and we conclude that this population of absorbers is uncorrelated, or at best, weakly correlated, with galaxies. The detection of strong absorption both from a single galaxy at an impact parameter of 104 h-1 kpc, and those already shown by Lanzetta et al. (1995) to be within 200 h-1 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.

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