Agnostic stacking of intergalactic doublet absorption: measuring the Ne VIII population

We present a blind search for doublet intergalactic metal absorption with a method dubbed 'agnostic stacking'. Using a forward-modelling framework, we combine this with direct detections in the literature to measure the overall metal population. We apply this novel approach to the search for Ne VIII absorption in a set of 26 high-quality COS spectra. We probe to an unprecedented low limit of log N> 12.3 at 0.47 <= z <= 1.34 over a path-length Delta z = 7.36. This method selects apparent absorption without requiring knowledge of its source. Stacking this mixed population dilutes doublet features in composite spectra in a deterministic manner, allowing us to measure the proportion corresponding to Ne VIII absorption. We stack potential Ne VIII absorption in two regimes: absorption too weak to be significant in direct line studies (12.3 < log N < 13.7), and strong absorbers (log N > 13.7). We do not detect Ne VIII absorption in either regime. Combining our measurements with direct detections, we find that the Ne VIII population is reproduced with a power-law column density distribution function with slope beta = -1.86(0.26)(+0.18) and normalization log f(13.7) = -13.99(-0.23)(+0.20), leading to an incidence rate of strong Ne VIII absorbers dn/dz = 1.38(-0.82)(+0.97). We infer a cosmic mass density for Ne VIII gas with 12.3 < log N < 15.0 of Omega(Ne VIII) = 2.2(-1.2)(+1.6) x 10(-8), a value significantly lower that than predicted by recent simulations. We translate this density into an estimate of the baryon density Omega(b) approximate to 1.8 x 10(-3), constituting 4 per cent of the total baryonic mass.