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

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.

Contribution of polycyclic aromatic hydrocarbon ionization to neutral gas heating in galaxies: model versus observations

“…R e for ULIRGs is also in good agreement with the model. ULIRGs have a lower heating efficiency as compared to starforming galaxies of the local universe, which is well explained in the model by the higher value of the ionization parameter γ in these galaxies due to intense massive star formation, increasing the average integrated intensity of the radiation field G 0 (Díaz-Santos et al 2017;McKinney et al 2021). The R e value of the high redshift galaxy GS IRS20 is also in good agreement with the model.…”

“…For star-forming galaxies in the local Universe, we use the ratio of the [CII] emission to the integrated flux in the 5 to 10 µm range provided in Helou et al (2001). Following McKinney et al (2021), we apply a factor 0.4 to this ratio to obtain R e , to account for the fact that PAH emission is responsible for only about 40% of the mid-IR emission of galaxies (Helou et al 2001). The range of values of γ for the galaxies are taken from McKinney et al ( 2021) using the same approach as for ULIRGs, which is based on the relationship of Díaz-Santos et al ( 2017) and presented in the following section.…”

“…Appendix B.5: Ultra luminous infrared galaxies McKinney et al (2021) present measurements of the ratio between the PAH to [CII] emissions, which we adopt to derive R e . These authors also provide the values of G 0 /n H in a large sample of ultra luminous infrared galaxies (ULIRGs).…”