Evolution of giant molecular clouds across cosmic time
  Giant molecular clouds (GMCs) are well-studied in the local Universe,
however, exactly how their properties vary during galaxy evolution is poorly
understood due to challenging resolution requirements, both observational and
computational. We present the first time-dependent analysis of giant molecular
clouds in a Milky Way-like galaxy and an LMC-like dwarf galaxy of the FIRE-2
(Feedback In Realistic Environments) simulation suite, which have sufficient
resolution to predict the bulk properties of GMCs in cosmological galaxy
formation self-consistently. We show explicitly that the majority of star
formation outside the galactic center occurs within self-gravitating gas
structures that have properties consistent with observed bound GMCs. We find
that the typical cloud bulk properties such as mass and surface density do not
vary more than a factor of 2 in any systematic way after the first Gyr of
cosmic evolution within a given galaxy from its progenitor. While the median
properties are constant, the tails of the distributions can briefly undergo
drastic changes, which can produce very massive and dense self-gravitating gas
clouds. Once the galaxy forms, we identify only two systematic trends in bulk
properties over cosmic time: a steady increase in metallicity produced by
previous stellar populations and a weak decrease in bulk cloud temperatures.
With the exception of metallicity we find no significant differences in cloud
properties between the Milky Way-like and dwarf galaxies. These results have
important implications for cosmological star and star cluster formation and put
especially strong constraints on theories relating the stellar initial mass
function to cloud properties.
