The Spatially Resolved Cooling Line Deficit in Galaxies

Abstract

We present [C II] 158 μm 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 μm emission dominates this trend, with [C II]/TIR dropping as n m n I 24 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 highredshift 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 S SFR . The tightness of this correlation suggests that an approximate S 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.

The Dust and [C ii] Morphologies of Redshift ∼4.5 Sub-millimeter Galaxies at ∼200 pc Resolution: The Absence of Large Clumps in the Interstellar Medium at High-redshift

“…core regions compared with the outer annuli is expected, but the same expectation is not true for the [ ] L C II /L FIR ratio. The fact that our SMGs follow the same trend seen by, for example, Smith et al (2017), implies that the [C II] deficit in SMGs is also due to a local process, where the core regions have a higher [C II] deficit than the regions further out from the core. Smith et al (2017) suggest that the [C II] deficit is related to the metallicity of the gas, where a low metallicity results in a high [ ] L C II /L FIR ratio.…”

“…The fact that our SMGs follow the same trend seen by, for example, Smith et al (2017), implies that the [C II] deficit in SMGs is also due to a local process, where the core regions have a higher [C II] deficit than the regions further out from the core. Smith et al (2017) suggest that the [C II] deficit is related to the metallicity of the gas, where a low metallicity results in a high [ ] L C II /L FIR ratio. Alternatively, a recent study by Muñoz & Oh (2016) explored the possibility of [C II] saturation.…”

“…All four of the galaxies show the same behavior: the core has lower [ ] L C II /L FIR and higher star formation rate surface density than the surrounding annulus. This follows the relation seen by Smith et al (2017) for local star-forming galaxies. A higher star formation rate surface density in the L C II to L FIR ratio as a function of the star formation rate density for local galaxies in the KINGFISH and GOALS (Armus et al 2009;Lutz et al 2016;Díaz-Santos et al 2017), a sample of high-redshift galaxies (Wang et al 2013;Capak et al 2015;Gullberg et al 2015;Kimball et al 2015;Lutz et al 2016;Nesvadba et al 2016;Díaz-Santos et al 2017;Jones et al 2017;Umehata et al 2017;Venemans et al 2017), and our sample of four z∼4.5 SMGs.…”

“…L C II /L FIR ratio as a function of the star formation rate surface density (S SFR ) in spatially resolved local galaxies in the KINGFISH sample, Smith et al (2017) identify a declining relation of the [ ] L C II /L FIR ratio as a function of S SFR . The authors suggest that the [C II] deficit is driven by local physical processes of interstellar gas (e.g., [C II] self-absorption, dust extinction, and dust grain charge), not related to the global properties of the galaxies.…”

“…Among the various explanations proposed for this behavior are: [C II] self-absorption, strong continuum extinction at 158 μm; collisional quenching of [C II] emission; high ionization parameters; and metallicity dependence (see Smith et al 2017 for an extensive discussion).…”