The Observed O vi Is Just the Tip of the Iceberg: Estimating the Hidden Material in Circumgalactic and Intergalactic Clouds

Abstract

This paper proposes a new method for estimating the total quantity of material in moving circumgalactic and intergalactic clouds from O vi measurements. We simulate high-velocity clouds (HVCs) with the FLASH hydrodynamic code and track the ionization and recombination of all ionization levels of oxygen as a function of time. We calculate the O vi /oxygen ratio ( f _O _VI ) in our dynamic nonequilibrium ionization clouds, finding that it differs significantly from that in static gas. We find that O vi exists in cool, medium, and hot gas in the clouds. As such, it traces all of the hydrogen rather than merely the ionized hydrogen. The total quantity of hydrogen along a typical observed line of sight through a cloud can be estimated from the observed O vi column density, metallicity, and our f _O _VI . We provide the simulations’ f _O _VI values, a prescription for finding f _O _VI for observed dynamic clouds, and a methodology for calculating the total hydrogen column density from f _O _VI and an observed O vi column density. As examples, we use our f _O _VI values to estimate the total hydrogen column densities along various observed sight lines through two HVCs, Complex C and the Magellanic Stream, finding that these clouds contain more material than previous lower limits. We also extend this analysis to low-redshift intergalactic O vi clouds, finding that they contain several times more baryonic material than previously thought and therefore may account for a significant fraction of the Universe’s baryons.

Keywords

• High-velocity clouds
• Magellanic Stream
• Interstellar clouds
• Intergalactic clouds
• Hydrodynamical simulations
• Hot ionized medium