HST Observations of the QSO pair Q1026-0045A,B

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The spatial distribution of the Lyα forest is studied using new HST data for the quasar pair Q 1026--0045 A and B at zem = 1.438 and 1.520 respectively. The angular separation is 36 arcsec and corresponds to transverse linear separations between lines of sight of 300h⁻¹₅₀ kpc (qₒ = 0.5) over the redshift range 0.833 < z < 1.438. From the observed numbers of coincident and anti-coincident Lyα absorption lines, we conclude that, at this redshift, the Lyα structures have typical dimensions of 500h⁻¹₅₀ kpc, larger than the mean separation of the two lines of sight. The velocity difference, ΔV, between coincident lines is surprisingly small (4 and 8 pairs with ΔV < 50 and 200 km/s respectively). Metal line systems are present at zabs = 1.2651 and 1.2969 in A, zabs = 0.6320, 0.7090, 1.2651 and 1.4844 in B. In addition we tentatively identify a weak Mg II system at zabs = 0.11 in B. It is remarkable that the zabs = 1.2651 system is common to both lines of sight. The system at zabs = 1.4844 %is probably associated and has strong O VI absorption. There is a metal-poor associated system at zabs = 1.4420 along the line of sight to A with complex velocity profile. We detect a strong Lyα absorption along the line of sight to B redshifted by only 300 km/s relatively to the associated system. It is tempting to interpret this as the presence of a disk of radius larger than 300h⁻¹₅₀ kpc surrounding quasar A.