Is the Probability of Occurrence of Absorption Lines in QSOs a Function of Red-shift?

Abstract

IT has been noted^1–3 that the occurrence of absorption lines in quasi-stellar objects appears to increase with increasing red-shift. This has suggested that QSOs with high red-shift have intrinsically different properties than those with low red-shift. McCrea², for example, found that 53 per cent of the QSOs with emission red-shift z_em > 1.8 have absorption lines; 17 per cent with 1.0 < z_em < 1.8 have absorption lines, and only 8 per cent with z_em < 1.0. This correlation, however, has been found, using all the observed absorption lines, and it is possible that the correlation is merely the result of stronger lines being brought into the observable spectrum as the red-shift increases. A proper test for such a correlation requires the use of a single line, or else the difference in strengths of lines in different parts of the spectrum should be allowed for.