http://arxiv.org/abs/1710.05023

We present a blind search for doublet intergalactic metal absorption with a method we dub `agnostic stacking’. Using forward-modelling we combine it with direct detections in the literature to measure the overall metal population. Here we apply this novel approach to the search for NeVIII in 26 high-quality COS spectra of QSOs at z>0.7. We probe an unprecedented low limit of log N>12.3 at 0.47<z<1.34 with a total pathlength ${\Delta}$z = 7.36. The method selects absorption without requiring knowledge of its source, be it observing noise, artifacts, or any line transition. Stacking this mixed population with NeVIII absorption dilutes doublet features in composite spectra in a deterministic manner. We stack potential NeVIII absorption in two regimes: absorption too weak to be statistically significant in direct line studies (12.3 < log N< 13.7), and strong absorbers (log N> 13.7). We do not detect NeVIII in either regime, and place upper limits on the population using agnostic stacking alone. Combining our measurements with direct line detections, the NeVIII population is reproduced with a single power law column density distribution of slope \b{eta} = -1.86 and normalisation log f_{13.7} = -13.99, leading to an incidence rate of strong NeVIII absorbers of dn/dz =1.38. Comparing our results with a group of 3 systems in PG1148+549, these have a 0.024% probability of arising by chance. We infer a cosmic mass density for NeVIII in the column density range 12.3 < log N < 15.0 of ${\Omega}$(NeVIII) = 2.2×10^{-8}. We translate this inferred density into an estimate of the baryon density of the NeVIII-bearing gas, and arrive at ${\Omega}b~1.8×10^{-3}$, which constitutes only 4% of the total baryonic mass. The measured NeVIII column density distribution function and cosmic density here are inconsistent with predictions of the EAGLES simulations at ${\sigma}>2.0$ significance. (abridged)

S. Frank, M. Pieri, S. Mathur, et. al.

Mon, 16 Oct 17

2/59

Comments: 17 pages, 12 figures