The Spatially Resolved [CII] Cooling Line Deficit in Galaxies

TL;DR

This study analyzes [CII] 158um emission across over 15,000 regions in 54 nearby galaxies, revealing how the [CII] cooling line deficit correlates with star formation rate density, metallicity, and local physical conditions, providing insights into interstellar gas processes.

Contribution

It offers the first large-scale resolved analysis of the [CII] line deficit, establishing its dependence on star formation rate density and metallicity, and providing a method to estimate SFRD from [CII]/TIR ratios.

Findings

[CII]/TIR ratio varies from >1% to <0.1% across regions.

Star formation rate density correlates with the depth of the [CII] deficit.

Metallicity influences the deficit, with higher metallicity showing deeper deficits.

Abstract

We present [CII] 158um measurements from over 15,000 resolved regions within 54 nearby galaxies of the KINGFISH program to investigate the so-called [CII] "line cooling deficit" long known to occur in galaxies with different luminosities. The [CII]/TIR ratio ranges from above 1% to below 0.1% in the sample, with a mean value of 0.48+-0.21%. We find that the surface density of 24um emission dominates this trend, with [CII]/TIR dropping as nuInu{24um} increases. Deviations from this overall decline are correlated with changes in the gas phase metal abundance, with higher metallicity associated with deeper deficits at a fixed surface brightness. We supplement the local sample with resolved [CII] measurements from nearby luminous infrared galaxies and high redshift sources from z=1.8-6.4, and find that star formation rate density drives a continuous trend of deepening [CII] deficit across six orders of magnitude in SFRD. The tightness of this correlation suggests that an approximate star formation rate density can be estimated directly from global measurements of [CII]/TIR, and a relation is provided to do so. Several low-luminosity AGN hosts in the sample show additional and significant central suppression of [CII]/TIR, but these deficit enhancements occur not in those AGN with the highest X-ray luminosities, but instead those with the highest central starlight intensities. Taken together, these results demonstrate that the [CII] cooling line deficit in galaxies likely arises from local physical phenomena in interstellar gas.