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.This work is based in part on observations made with Herschel, a European Space Agency Cornerstone Mission with significant participation by NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. We thank Steve Hailey-Dunsheath, T. Rawle, and Tanio Diaz-Santos for advanced access to their compiled [C II] data sets. J.D.S. gratefully acknowledges visiting support from the Alexander von Humboldt Foundation and the Max Planck Institute für Astronomie as well as support from the Research Corporation for Science Advancement through its Cottrell Scholars program