Agnostic Stacking of Intergalactic Doublet Absorption: Measuring the NeVIII 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 NeVIII absorption in a set of 26 high-quality COS spectra. We probe to an unprecedented low limit of log N$>$12.3 at 0.47$\le z\le$1.34 over a pathlength $\mathrm{\Delta }$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 NeVIII absorption. We stack potential NeVIII 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 NeVIII absorption in either regime. Combining our measurements with direct detections, we find that the NeVIII population is reproduced with a power law column density distribution function with slope $\beta =-1.86\begin{array}{c}+0.18\\ -0.26\end{array}$ and normalisation log $f_{13.7} = -13.99 \substack{+0.20 \ -0.23}$, leading to an incidence rate of strong NeVIII absorbers $dn\mathrm{/}dz=1.38\begin{array}{c}+0.97\\ -0.82\end{array}$. We infer a cosmic mass density for NeVIII gas with 12.3 $<$ log N $<$ 15.0 of $\Omega _{NeVIII} = 2.2 \substack{+1.6 \ _-1.2} \times 10^{-8}$, a value significantly lower that than predicted by recent simulations. We translate this density into an estimate of the baryon density ${\mathrm{\Omega }}_b\approx 1.8\times {10}^{-3}$, constituting 4% of the total baryonic mass.