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We present the first large sample of metal absorption systems in pairs of QSOs with sightlines separated by about 1 Mpc at z= 2. We found 690 absorption systems in the spectra of 310 QSOs in 170 pairings. Most systems show C IV or Mg II absorption.

When we see absorption in one QSO, the probability that we see absorption in the paired QSO, within about 500 km s−1, is at least ∼50 per cent at <100 >kpc, declining rapidly to ∼8 per cent at 200–400 kpc, ∼0.8 per cent by 1–2 Mpc proper distance. Although we may occasionally see an individual absorbing halo in two sightlines, the absorber–absorber correlation is primarily a probe of the distribution of metals around galaxies and Mpc scale galaxy clustering. QSO absorption lines give redshifts errors of ∼23 km s−1, almost 10 times smaller than the error for galaxy spectra at these redshifts, hence we can measure clustering on small scales, around 0.5 Mpc proper, with a small sample. The distribution of 23 absorber–absorber coincidences separated by

We see an excess of C IV absorbers, with an a posteriori probability of 0.0003, when a line of sight passes a foreground QSO. We see 16 absorbers where we expect 5.8 at 0–600 km s−1, on the front side of the partner QSO. At these velocities, we see an excess absorber in ∼6 per cent of sightlines that pass within 0.1–2.5 Mpc of a QSO, but inus, the 3D distribution of 59 absorbers around 313 QSOs is approximately isotropic, except for the 1.5–2σ tendency for the excess C IV absorbers to be on the front side of the QSOs. Our QSO redshifts may be too large by ∼300 km s−1, or there might be a real asymmetry coming from a hypothetical anisotropy in the QSO UV emission, or from isotropic UV emission that lasted less than ∼1 Myr, possibilities suggested by the excess H I behind these QSOs. The velocity dispersion of the excess absorbers near the QSOs is small, ∼250 km s−1, suggesting that both these absorbers and the QSOs are in the blue sequence of galaxies. The probability of seeing absorption when a sightline passes a QSO rises only slowly as the impact parameter drops from 2.5 to 0.1 Mpc, perhaps because the UV radiation from QSOs destroys many nearby absorbers.


© 2009, The Royal Astronomical Society. Original published version available at

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Monthly Notices of the Royal Astronomical Society





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