Regulating Star Formation in Nearby Dusty Galaxies: Low Photoelectric Efficiencies in the Most Compact Systems

TL;DR

This study investigates how star formation is regulated in dusty, compact galaxies by examining the efficiency of gas heating via photoelectric effects, revealing lower efficiencies in the most compact systems and implications for high-redshift galaxies.

Contribution

It combines mid-IR PAH observations with fine-structure emission lines to measure photoelectric efficiency in galaxies, highlighting its dependence on galaxy compactness and star-forming activity.

Findings

High IR surface density galaxies have low photoelectric efficiencies.

Compact galaxies exhibit larger radiation field to density ratios and bigger dust grains.

Results suggest less efficient gas heating in dense, young star-forming regions.

Abstract

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.