Structure of the MgII and damped Lyman-alpha systems along the line of sight to APM 08279+5255

A study of the absorption systems toward the gravitationally lensed quasar APM 08279+5255 is presented. Most of the Mg II systems in the redshift rang e z similar to 1.2-2.07, although saturated, show large residuals at the bo ttom of the lines. The most likely interpretation is that individual clouds within Mg II halos do cover only one of the two brightest QSO images. The separation between the two lines of sight decreases from 1.7 to 0.7 h(75)(- 1) kpc (q(o) = 0.5 z(lens) = 1)between z = 1.22 and 2.07, This reveals that Mg II halos are made of a collection of clouds of radius smaller than abou t 1 h(75)(-1) kpc Two strong Mg II absorbers at z(abs) = 1.062 and 1.181 are studied in detai l. This is the first time that the Na I lambda 3303 doubler is detected in such high redshift systems. Together with the detection of the Mg I lambda 2852 transition, this strongly constrains the physical characteristics of t he gas. The N(Na I)IN(Mg I) ratio is found to be larger than unity, implyin g that the gas is cool and neutral. The Doppler parameters measured in indi vidual and well detached components are probably as small as 1 km s(-1). Th e column densities of Na I, Ca II, Mg I, Ti II, Mn II and Fe II observed at z(abs) = 1.1801 are very close to that observed along the line of sight to wards 23 Ori in our Galaxy. The shape of the QSO continuum is consistent wi th attenuation by dust at z similar to 1 (A(V) similar to 0.5 mag). Altoget her it is found that the H I column density at z = 1 is of the order of 1 t o 5 10(21) cm(-2), the corresponding metallicity is in the range 1-0.3 Z., the overall dust-to-metal ratio is about half that in our Galaxy and the re lative depletion of iron, titanium, manganese and calcium is similar to wha t is observed in cool gas in the disk of our Galaxy. The objects associated with these two systems could both contribute to the lens together with ano ther possible strong system z(abs) = 1.1727 and the strong Lyman-alpha syst em at z(abs) = 2.974. The probable damped Lyman-alpha system at z(abs) = 2.974 has 19.8 < log N(H I) < 20.3. The transverse dimension of the absorber is larger than 200 h(7 5)(-1) pc. Column densities of Al Ir, Fe II, Si II, C II and O I indicate a bundances relative to solar of -2.31, -2.26, -2.10, -2.35 and -2.37 for, re spectively, Fe, Al, Si, C and O (for log N(H I) = 20.3). These surprizingly similar values indicate that the amount of dust in the cloud is very small as are any deviations from relative solar abundances. It seems likely that the upper limits found for the zinc metallicity of several damped Lyman-al pha systems z > 3 in previous surveys is indicative of a true cosmological evolution of the metallicity in individual systems.