A SPECTROSCOPIC STUDY OF UM-673-A AND UM-673-B - ON THE SIZE OF LYMAN-ALPHA CLOUDS

We present a study of the Ly-alpha forest spectra (2 angstrom resolution) obtained for the A and B images of the gravitationally lensed high-redshift quasar UM 673. We also present higher resolution data of the brightest (A) image. In the 2 angstrom resolution spectra, all the absorption lines detected at 5-sigma in the spectrum of the fainter B image are present in the A image; however, we find two anticoincidences, i.e., two lines in A which do not have a counterpart in B at more than a 3-sigma confidence level. Given the fact that corresponding Ly-alpha lines in the spectra of A and B have their equivalent widths well correlated, this proves that both light beams actually cross the same clouds. Most of the velocity differences between corresponding lines are compatible with 0 km s-1 within the error bars, with a standard deviation of 17 km s-1. As the comoving linear separation increases from virtually Oh50(-1) kpc (H-0 = 50h50 km s-1 Mpc-1, q0 = 0) at the redshift of the QSO to 2h50(-1) kpc, we derive a best value of 12h50(-1) kpc for the 2-sigma lower limit and of 160h50(-1) kpc for the 2-sigma upper limit of the diameter of spherical Ly-alpha clouds in the redshift range 2.1-2.7, by means of Monte Carlo simulations. However, if we interpret the two anticoincidences as due to a Mg II doublet at z = 0.4261, we find in this case a best value of 23h50(-1) kpc to the 2-sigma lower limit of the Ly-alpha cloud diameter, but we cannot derive any upper limit. For the two major heavy-element systems detected in the spectrum of UM 673, we do not find any significant difference between the corresponding lines in the two spectra, indicating that these systems do not show dramatic variations over scales of 0.8h50(-1) and 2.1h(50)-1 kpc, respectively.