Origin of the Underpopulated Si IV and O VI Absorption

Determine the physical origin of the weak Si IV and O VI absorption in synthetic spectra generated from the CGOLS SFR simulations, distinguishing unresolved collisionally ionized gas from deficiencies in the assumed photoionizing background and other missing physics.

Background

The synthetic Si IV and O VI equivalent widths are substantially lower than observationally reported values. The paper examines insufficient resolution, missing intermediate-temperature gas, and the use of the Haardt–Madau 2012 metagalactic background instead of a spatially varying radiation field from young stellar clusters. The authors argue that the ionizing background is probably responsible for much of the discrepancy, but the exact contribution of each mechanism remains unresolved and requires a more physically complete treatment.

References

Although the lines are clearly underpopulated, the source of the discrepancy is not immediately obvious.

— Studying Outflows with Synthetic Absorption Line Spectra from High Resolution Simulations  (2609.10868 - Magee et al., 9 Sep 2026) in Section 6.2, Level Population and Ionizing Background

While the discrepancies in the EWs can be addressed by tuning the various parameters we have investigated in this work, it is not clear that we can adequately address the tension by tuning these parameters alone, and there is likely some missing physics in our analysis pipeline (and in the simulations) that must be accounted for in order for the mock observables to be in agreement with empirical predictions.

— Studying Outflows with Synthetic Absorption Line Spectra from High Resolution Simulations  (2609.10868 - Magee et al., 9 Sep 2026) in Section 6.1, Comparison to Observations