THE SIZE AND GEOMETRY OF THE LY-ALPHA CLOUDS

Spectra of the QSO pair Q0307-195A,B have been obtained in the Ly alph a forest (3660-3930 Angstrom) and C Iv (4720-4850 Angstrom) regions wi th a FWHM resolution between 0.7 and 0.5 Angstrom. 46 lines have been detected in the spectrum of object A while 36 in the spectrum of objec t B, of them 29 and 20 were identified as Ly alpha absorptions respect ively. The present observations have been supplemented with data of co mparable quality on other 7 QSO pairs available in the literature to g ive an enlarged sample of 217 Ly alpha lines with rest equivalent widt h W-o greater than or equal to 0.3 Angstrom. The analysis of the hits (i.e. when an absorption line appears in both QSO spectra) and misses (i.e. when a line is seen in any of the QSO spectra, but no line is se en in the other), carried out with an improved statistical approach, i ndicates that the absorbers have typically a large size(1): R = 362 h( -1) kpc, with 95% confidence limits 298 < R < 426 h(-1) kpc and R = 41 2 h(-1) kpc, with 95% confidence limits 333 < R < 514 h(-1) kpc for th e radius of idealized spherical and disc geometries, respectively. The present data do not allow to establish any correlation of the typical inferred size with the proper separation or with the redshift of the pairs. The correlation between the observed equivalent widths of the a bsorbers in the adjacent lines of sight becomes poorer and poorer with increasing proper separation. A disc geometry with a column density p rofile N(r) proportional to (r/R-0)(-gamma), gamma = 4, is found to re asonably reproduce the data with R-0 similar or equal to 100 - 200 h(- 1) kpc, but also spherical clouds with the same column density profile and a power-law distribution of radii may give a satisfactory represe ntation of the observations.