Reobservation of Close QSO Groups: The Size Evolution and Shape of Lyman Alpha Forest Absorbers

Abstract

Probing the size and shape of the absorbers that lie in front of the QSOs, in particular the Ly alpha forest, we analyze 785 absorption lines in spectra of five QSOs in close groupings: a pair and a triplet. Both QSO groups have been observed before, but these data are a drastic improvement over earlier data, and provide a qualitatively different view of the nature of the absorbers. The pair samples a scale critical in determining the size upper bound of Ly alpha absorbers, with significant leverage in redshift compared to previous studies. The triplet data represents the spatially densest sample of Ly alpha forest ever studied, and an almost ideally-suited probe of the shape of absorbers. We observe a significant number of Ly alpha lines in common between the triplet sightlines, for lines stronger than rest equivalent width W > 0.4A (and no detected metal lines) and velocity differences up to 200 km/s, corresponding to a two-point correlation function xi = 1.88 +0.78/-0.50 on scales 0.5 to 0.8 Mpc/h with < z > = 2.14, and inconsistent at the 99.999% level with the absence of any clustering. These data also show that a significant fraction of the W > 0.4A Ly alpha forest absorbers span all three sightlines to the triplet, indicating that the strong-lined absorbers are consistent with nearly round shapes, chosen from a range of possible cylinders of different elongations. This may be inconsistent with results from hydrodynamic/gravitational simulations of H I in the early Universe indicating that the theoretical counterparts of Ly alpha forest clouds are long and filamentary. We conclude, tentatively, that the W > 0.4A Ly alpha forest objects are sheetlike. This is supported by the uniformity of linestrengths between the three sightlines, for W > 0.4A. (Abstract continues.)

95 pages LaTeX file; 21 figures contained in 1 postscript file. This is the final version as will appear in The Astrophysical Journal. This version is significantly revised from original and expanded with new results. The latest change is the addition of another QSO pair in absorber size estimates. A previous version corrected a small calculational error

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• Sizes, Shapes, and Correlations of Lyman Alpha Clouds and Their Evolution in the CDM$+\Lambda$ Universe
• The Weakness of C IV Absorber Clustering in Keck HIRES Spectra of Adjacent QSO Sightlines

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