A new measure of the clustering of QSO heavy-element absorption-line systems

We examine the line-of-sight clustering of QSO heavy-element absorption-line systems, using a new measure of clustering, called the reduced second moment measure, K(r), that directly measures the mean overdensity of absorbers on scales less than or similar to r. This measure-while closely related to other second-order measures such as the correlation function or the power spectrum-has a number of distinct statistical properties that make possible a continuous exploration of clustering as a function of scale. From a sample of 352 C Iv absorbers with median redshift [z] = 2.2, drawn from the spectra of 274 QSOs, we find that the absorbers are strongly clustered on scales from 1-20 h(-1) Mpc. Furthermore, there appears to be a sharp break at 20 h(-1) Mpc, with significant clustering on scales up to 100 h(-1) Mpc in excess of that which would be expected from a smooth transition to homogeneity. There is no evidence of clustering on scales greater than 100 h(-1) Mpc. These results suggest that strong C Iv absorbers along a line of sight are indicators of clusters and possibly superclusters, a relationship that is supported by recent observations of "Lyman break" galaxies.