The Interstellar Medium of Dusty, Star-Forming Galaxies: Low Heating Efficiencies in Compact Systems

Star formation in galaxies is regulated by the heating and cooling of gas in the interstellar medium. In particular, the balance of energy gains and losses within photodissociation regions, the boundary layer between hot, ionized gas and cold molecular clouds, is key for future star-formation in a galaxy. Through combined observations of strong cooling line emission in the far-infrared with dust signatures of photoelectric heating, we measure the cooling-to-heating ratio in luminous, infrared star-forming galaxies selected from the GOALS survey. The most compact systems exhibit low heating efficiencies when the galaxy-integrated radiation field is strong, and also have larger dust grain sizes on average. The data supports a scenario in which the most compact galaxies have more young star-forming regions per unit area, which exhibit less efficient heating because of changes to the properties of small dust grains. Low heating efficiencies could raise the star-formation efficiency by lowering the energy transfer from stellar photons into the gas. The star-forming conditions within local, compact, dusty galaxies may be more common at high-redshift, and may help explain the higher star-formation rates at cosmic noon.