Agnostic Stacking of Intergalactic Doublet Absorption: Measuring the NeVIII Population

TL;DR

This paper introduces an agnostic stacking method to detect intergalactic NeVIII absorption, finds no significant detections, and estimates the NeVIII cosmic mass density, which is lower than recent simulations predict.

Contribution

The paper presents a novel agnostic stacking approach for detecting intergalactic metal absorption without source knowledge, applied to NeVIII, providing new constraints on its population.

Findings

No NeVIII absorption detected in stacked spectra.

NeVIII population follows a power-law column density distribution.

Estimated NeVIII cosmic mass density is lower than simulation predictions.

Abstract

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$>$12.3 at 0.47$\leq z \leq$1.34 over a pathlength $\Delta$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 $\beta = -1.86 \substack{+0.18 \\ -0.26}$ and normalisation log $f_{13.7} = -13.99 \substack{+0.20 \\ -0.23}$, leading to an incidence rate of strong NeVIII absorbers $dn/dz =1.38 \substack{+0.97 \\ -0.82}$. 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 $\Omega _{b} \approx 1.8 \times 10^{-3}$, constituting 4\% of the total baryonic mass.