Pixel correlation searches for OVI in the Lyman α forest and the volume filling factor of metals in the intergalactic medium at z∼2-3.5

Artificial absorption spectra are used to test a variety of instrumental and physical effects on the pixel correlation technique for the detection of weak O VI absorption. At H I optical depths less than or similar to0.3-1, the apparent O VI detections are spurious coincidences due to H I absorption at other redshifts. In this range, the apparent O VI optical depth is independent of H I optical depth. At larger H I optical depths, the apparent O VI optical depth and H I optical depth are correlated. Detailed modelling is required in order to interpret the significance of this relation. High-resolution spectra of four quasi-stellar objects together with a large suite of synthetic spectra are used to show that the detection of O VI in individual spectra is only statistically significant for overdensities greater than or similar to5. These overdensities are larger than would be naively inferred from the onset of the correlation and a tight optical depth-density relation. The lower limit for the volume filling factor of regions which are enriched by O VI is 4 per cent at 95 per cent confidence. This is no larger than the observed volume filling factor of the winds from Lyman break galaxies. Previous claims that the observed O VI absorption extends to underdense regions and requires a universal metal enrichment with large volume filling factor, as may be expected from Population III star formation at very high redshift, appear not to be warranted.