The Spatially Resolved [CII] Cooling Line Deficit in Galaxies

Abstract

We present [C II] 158 $\mu$m measurements from over 15,000 resolved regions within 54 nearby galaxies of the KINGFISH program to investigate the so-called [C II] “line-cooling deficit” long known to occur in galaxies with different luminosities. The [C II]/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 24 $\mu$m emission dominates this trend, with [C II]/TIR dropping as $\nu$$I_\nu$ (24 $\mu$m) 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 [C II] 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 [C II] deficit across six orders of magnitude in $\sum_{SFR}$. The tightness of this correlation suggests that an approximate $\sum_{SFR}$ can be estimated directly from global measurements of [C II]/TIR, and a relation is provided to do so. Several low-luminosity active galactic nucleus (AGN) hosts in the sample show additional and significant central suppression of [C II]/TIR, but these deficit enhancements occur not in those AGNs with the highest X-ray luminosities, but instead those with the highest central starlight intensities. Taken together, these results demonstrate that the [C II] line-cooling line deficit in galaxies likely arises from local physical phenomena in interstellar gas.