Thermal Regulation and the Star-Forming Main Sequence
  We argue that the interplay between cosmic rays, the initial mass function,
and star formation plays a crucial role in regulating the star-forming "main
sequence". To explore these phenomena we develop a toy model for galaxy
evolution in which star formation is regulated by a combination of a
temperature-dependent initial mass function and heating due to starlight,
cosmic rays and, at very high redshift, the cosmic microwave background. This
produces an attractor, near-equilibrium solution which is consistent with
observations of the star-forming main sequence over a broad redshift range.
Additional solutions to the same equations may correspond to other observed
phases of galaxy evolution including quiescent galaxies. This model makes
several falsifiable predictions, including higher metallicities and dust masses
than anticipated at high redshift and isotopic abundances in the Milky Way. It
also predicts that stellar mass-to-light ratios are lower than produced using a
Milky Way-derived IMF, so that inferences of stellar masses and star formation
rates for high redshift galaxies are overestimated. In some cases, this may
also transform inferred dark matter profiles from core-like to cusp-like.
