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

We present observations of the high-redshift QSO GB1759+7539 (zem=3.05) obtained with HIRES on the 10-m Keck telescope. The spectrum has a resolution of FWHM=7 km s-1, and a typical signal-to-noise ratio per 2 km s−1 pixel of ∼25 in the Lyα forest region, and ∼60 longward of the Lyα emission. The observed Lyα forest systems have a mean redshift of 〈z〉=2.7. The H i column density distribution is well described by a power-law distribution with index β=1.68±0.15 in the range 13.5<log N<14.5. The Doppler width distribution is consistent with a Gaussian distribution of mean b=26 km s-1, and standard deviation σ=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<Δv<250 km s-1. The one-point and two-point joint probability distributions of the transmitted flux for the Lyα forest are calculated, and shown to be very insensitive to the heavy-element contamination. We could find no evidence of Voigt profile departures caused by infalling gas, as observed in the simulated forest spectra. 12 heavy-element absorption systems were identified, including damped Lyman-alpha (DLA) systems at zabs=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 elemental abundances calculated for the weak unsaturated lines. The systems have metallicities of Z2.62≃1/20 Z⊙ and Z2.91≃1/45 Z⊙. Both systems appear to have a low dust content. They show an overabundance of α-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 oxygen, 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 microwave background temperature at z=2.62 of TCMB<12.9 K.