A high-resolution study of the absorption spectrum of PKS 2126-158.

Observations of the QSO PKS 2126--158 (Z/sub em/=3.280) at 0.8 A resolution from 4l53 A to 6807 A reveal 113 absorption lines. There are two certain absorption-line systems at z/sub abs/=2.6381 and 2.7685, and a possible third system at z/sub abs/=2.3938. The ions H I, C II, C IV, O I, Al II, Si II, Si III, Si IV, and Fe II are observed in these systems; no excited fine-structure lines Si II* are seen, but C II* is almost certainly present in the z/sub abs/=2.7685 system. These three systems lead to the identification of all 12 lines longward of the L..cap alpha.. emission line. However, only 22 out of 101 absorption lines shortward of L..cap alpha.. are thereby identified.Among the otherwise unidentiied lines shortward of L..cap alpha.. emission we find at most three clear L..cap alpha../..beta.. pairs. A cross-correlation between the regions of spectrum below L..cap alpha.. and L..beta.. emission, respectively, shows a strong peak at the L..cap alpha../..beta.. ratio. The peak has the strength expected on the hypothesis that > or approx. =90% of the lines shortward of L..cap alpha.. emission are L..cap alpha.. absorptions with the corresponding L..beta.. absorption buried in the noise.The analysis of the spectrummore » of PKS 2126--158 is used to develop criteria for defining particular types of redshift systems which can then form a basis for statistical analyses of the distribution of absorption systems with redshift in QSOs generally.« less