The Cosmic Baryon and Metal Cycles

TL;DR

This paper reviews the cosmic baryon and metal cycles, highlighting the evolution of gas and metals in galaxies over cosmic time, and finds little evidence for missing metals in current censuses.

Contribution

It compiles and synthesizes contemporary data on baryons, metals, and dust in galaxies, revealing new insights into their evolution and distribution.

Findings

H I mass density is well determined up to z ~ 5 with minor evolution.

Molecular hydrogen evolution mirrors star formation rate density.

Most metals at z > 2.5 are in cold gas, while at lower redshifts they are mostly in stars.

Abstract

Characterizing the relationship between stars, gas, and metals in galaxies is a critical component of understanding the cosmic baryon cycle. We compile contemporary censuses of the baryons in collapsed structures, their chemical make-up and dust content. We show that: The H I mass density of the Universe is well determined to redshifts z ~ 5 and shows minor evolution with time. New observations of molecular hydrogen reveal its evolution mirrors that of the global star formation rate density. The constant cosmic molecular gas depletion timescale points to a universal relationship between gas reservoirs and star formation. The metal mass density in cold gas ($T < 10^4$ K) contains virtually all the metals produced by stars for z > 2.5. At lower redshifts, the contributors to the total amount of metals are more diverse; at z < 1, most of the observed metals are bound in stars. Overall there is little evidence for a "missing metals problem" in modern censuses. We characterize the dust content of neutral gas over cosmic time, finding the dust-to-gas and dust-to-metals ratios fall with decreasing metallicity. We calculate the cosmological dust mass density in the neutral gas up to z ~ 5. There is good agreement between multiple tracers of the dust content of the Universe.