  Star formation in galaxies is regulated by the heating and cooling in the
interstellar medium. In particular, the processing of molecular gas into stars
will depend strongly on the ratio of gas heating to gas cooling in the neutral
gas around sites of recent star-formation. In this work, we combine
mid-infrared (mid-IR) observations of Polycyclic Aromatic Hydrocarbons (PAHs),
the dominant heating mechanism of gas in the interstellar medium (ISM), with [C
II], [O I], and [Si II] fine-structure emission, the strongest cooling channels
in dense, neutral gas. The ratio of IR cooling line emission to PAH emission
measures the photoelectric efficiency, a property of the ISM which dictates how
much energy carried by ultraviolet photons gets transferred into the gas. We
find that star-forming, IR luminous galaxies in the Great Observatories All-Sky
LIRG Survey (GOALS) with high IR surface densities have low photoelectric
efficiencies. These systems also have, on average, higher ratios of radiation
field strength to gas densities, and larger average dust grain size
distributions. The data support a scenario in which the most compact galaxies
have more young star-forming regions per unit area, which exhibit less
efficient gas heating. These conditions may be more common at high-z, and may
help explain the higher star-formation rates at cosmic noon. We make
predictions on how this can be investigated with JWST.

Astrophysics of Galaxies