The Lyman-alpha forest and heavy-element systems of GB1759+7539

We present observations of the high-redshift QSO GB1759+7539 (z(em) = 3.05) obtained with HIRES on the 10-m Keck telescope. The spectrum has a resolut ion of FWHM = 7 km s(-1), and a typical signal-to-noise ratio per 2 km s(-1 ) pixel of similar to 25 in the Ly alpha forest region, and similar to 60 l ongward of the Ly alpha emission. The observed Ly alpha forest systems have a mean redshift of < z > = 2.7. T he H I column density distribution is well described by a power-law distrib ution with index beta = 1.68 +/- 0.15 in the range 13.5 < log N < 14.5. The Doppler width distribution is consistent with a Gaussian distribution of m ean b = 26 km s(-1), and standard deviation sigma = 12 km s(-1), with a cut -off at b = 20 km s(-1). There is marginal evidence of clustering along the line of sight over the velocity range 100 < Delta v < 250 km s(-1). The on e-point and two-point joint probability distributions of the transmitted fl ux for the Ly alpha forest are calculated, and shown to be very insensitive to the heavy-element contamination. We could find no evidence of Voigt pro file departures caused by infalling gas, as observed in the simulated fores t spectra. 12 heavy-element absorption systems were identified, including damped Lyman -alpha (DLA) systems at z(abs) = 2.62 and 2.91. The C, N, O, Al, Si, P, S, Mg, Fe and Ni absorption features of these systems were studied, and the el emental abundances calculated for the weak unsaturated lines. The systems h ave metallicities of Z(2.62) similar or equal to 1/20 Z. and Z(2.91) simila r or equal to 1/45 Z.. Both systems appear to have a low dust content. They show an overabundance of alpha-elements relative to Fe-peak elements, and an underabundance of odd atomic number elements relative to even. Nitrogen was observed in both systems, and found to be underabundant relative to oxy gen, in line with the time-delay model of primary nitrogen production. C ii * was also seen, allowing us to determine an upper limit to the cosmic micr owave background temperature at z = 2.62 of T-CMB < 12.9 K.