Coincidences between OVI and OVII Lines: Insights from High Resolution Simulations of the Warm-Hot Intergalactic Medium

TL;DR

This study uses high-resolution simulations to analyze the properties and coincidences of OVI and OVII absorption lines in the warm-hot intergalactic medium, providing insights into their origins and observational signatures.

Contribution

The paper presents new high-resolution simulations that accurately reproduce observed properties of OVI absorbers and predicts the presence of broad, shallow lines missed by current observations.

Findings

Simulations agree with observed OVI properties like line incidence and Doppler widths.

Velocity structures significantly influence Doppler widths of OVI and OVII absorbers.

A notable fraction of OVI and OVII clouds originate from gas below 10^5K.

Abstract

With high resolution (0.46kpc/h), adaptive mesh-refinement Eulerian cosmological hydrodynamic simulations we compute properties of O VI and O VII absorbers from the warm-hot intergalactic medium (WHIM). Our new simulations are in broad agreement with previous simulations, with ~40% of the intergalactic medium being in the WHIM at z=0. It is found (1) The amount of gas in the WHIM at temperature below and above 10^6K is about equal within uncertainties. (1) Our simulations are in excellent agreement with observed properties of O VI absorbers, with respect to the line incidence rate and Doppler width-column density relation. (2) Velocity structures within absorbing regions are a significant, and for large Doppler width clouds, a dominant contributor to the Doppler widths of both O VI and O VII absorbers. A non-negligible fraction (in number and mass) of O VI and O VII clouds can arise from gas of temperature lower than 10^5, until the Doppler width is well in excess of 100km/s. (3) Strong O VI absorbers are predominantly collisionally ionized. About (61%, 57%, 39%) of O VI absorbers in the column density ranges of log N(OVI) cm^2=(12.5-13,13-14,>14) have temperature lower than 10^5K. (4) Quantitative prediction is made for the presence of broad and shallow O VI lines, which current observations may have largely missed. Upcoming observations by COS may be able to provide a test. (5) The reported 3 sigma upper limit on the mean column density of coincidental O VII lines at the location of detected O VI lines by Yao et al is above the predicted value by a factor of 2.5-4. (6) The claimed observational detection of O VII lines by Nicastro et al, if true, is 2 sigma above what our simulations predict.