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

Abstract

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 ≲0.3–1, 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 ≳5. 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.