The Observed O VI is Just the Tip of the Iceberg: Estimating the Hidden Material in Clouds

How much material is really in a cloud? The answer is important for multiple astronomical topics yet has been hard to estimate accurately. Previously, the ionized material in a cloud was calculated from observations of O VI and the estimated O VI/oxygen ratio. This ratio was typically chosen to be the upper limit determined from static collisional ionization equilibrium (CIE) and non-equilibrium ionization (NEI) models. However, most ionized clouds are dynamic and out of CIE. We introduce an improved method for estimating the amount of material in ionized clouds. The improvement comes from finding a more realistic O VI/oxygen ratio for dynamic clouds. To do so, we simulate a variety of clouds with the FLASH hydrodynamic code. These simulations reveal that dynamic NEI gas has a substantially different O VI/oxygen ratio compared to static gas. We also find that OVI exists across a broad temperature range in the clouds.

We then apply our methodology to two example high velocity clouds (HVC)s: the Magellanic Stream (MS) and Complex C. We estimate the total hydrogen column density along several observed sight lines and find that they are significantly greater than previously thought. This suggests that Complex C and the MS are more massive by factors of 3 and 7. We also analyze intergalactic clouds, finding that they have several times more baryonic material than previously thought. Finally, we provide a mechanism that can be used to find an appropriate O VI/oxygen ratio, given observations of H I and O VI column densities, for a wide range of clouds.