The Cosmic Baryon and Metal Cycles

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 and their chemical makeup and dust content. We show the following: ▪ The [Formula: see text] mass density of the Universe is well determined to redshifts [Formula: see text] and shows minor evolution with time. New observations of molecular hydrogen reveal its evolution mirrors that of the global star-formation rate density, implying a universal cosmic molecular gas depletion timescale. The low-redshift decline of the star-formation history is thus driven by the lack of molecular gas supply due to a drop in net accretion rate related to the decreased growth of dark matter halos. ▪ The metal mass density in cold gas ([Formula: see text] K) contains virtually all the metals produced by stars for [Formula: see text]. At lower redshifts, the contributors to the total amount of metals are more diverse; at [Formula: see text], 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 [Formula: see text]. There is good agreement between multiple tracers of the dust content of the Universe.