Using [CII] 158 mu m Emission from Isolated ISM Phases as a Star Formation Rate Indicator

Abstract

The brightest observed emission line in many star-forming galaxies is the $[{\rm{C}}\,{\rm{II}}]$ 158 μm line, making it detectable up to z ∼ 7. In order to better understand and quantify the $[{\rm{C}}\,{\rm{II}}]$ emission as a tracer of star formation, the theoretical ratio between the $[{\rm{N}}\,{\rm{II}}]$ 205 μm emission and the $[{\rm{C}}\,{\rm{II}}]$ 158 μm emission has been employed to empirically determine the fraction of $[{\rm{C}}\,{\rm{II}}]$ emission that originates from the ionized and neutral phases of the interstellar medium (ISM). Sub-kiloparsec measurements of the $[{\rm{C}}\,{\rm{II}}]$ 158 μm and $[{\rm{N}}\,{\rm{II}}]$ 205 μm lines in nearby galaxies have recently become available as part of the Key Insights in Nearby Galaxies: a Far Infrared Survey with Herschel (KINGFISH) and Beyond the Peak programs. With the information from these two far-infrared lines along with the multi-wavelength suite of KINGFISH data, a calibration of the $[{\rm{C}}\,{\rm{II}}]$ emission line as a star formation rate (SFR) indicator and a better understanding of the $[{\rm{C}}\,{\rm{II}}]$ deficit are pursued. $[{\rm{C}}\,{\rm{II}}]$ emission is also compared to polycyclic aromatic hydrocarbon (PAH) emission in these regions to compare photoelectric heating from PAH molecules to cooling by $[{\rm{C}}\,{\rm{II}}]$ in the neutral and ionized phases of the ISM. We find that the $[{\rm{C}}\,{\rm{II}}]$ emission originating in the neutral phase of the ISM does not exhibit a deficit with respect to the infrared luminosity and is therefore preferred over the $[{\rm{C}}\,{\rm{II}}]$ emission originating in the ionized phase of the ISM as an SFR indicator for the normal star-forming galaxies included in this sample.