[CII] 158 $\mu$m Emission as a Star Formation Tracer

TL;DR

This study demonstrates that [CII] 158 μm emission is a reliable tracer of star formation rates in normal galaxies, with calibrated relations that account for IR color, covering a wide range of galaxy types and SFR intensities.

Contribution

The paper provides a calibrated [CII]-based SFR relation using Herschel data, including IR color adjustments, applicable across diverse galaxy types and SFR levels.

Findings

[CII] surface brightness correlates with SFR with ±0.21 dex scatter.

IR color adjustment reduces scatter in the [CII]-SFR relation.

Surface brightness relations outperform luminosity-based ones for SFR estimation.

Abstract

The [CII] 157.74 $\mu$m transition is the dominant coolant of the neutral interstellar gas, and has great potential as a star formation rate (SFR) tracer. Using the Herschel KINGFISH sample of 46 nearby galaxies, we investigate the relation of [CII] surface brightness and luminosity with SFR. We conclude that [CII] can be used for measurements of SFR on both global and kiloparsec scales in normal star-forming galaxies in the absence of strong active galactic nuclei (AGN). The uncertainty of the $\Sigma_{\rm [CII]}-\Sigma_{\rm SFR}$ calibration is $\pm$0.21 dex. The main source of scatter in the correlation is associated with regions that exhibit warm IR colors, and we provide an adjustment based on IR color that reduces the scatter. We show that the color-adjusted $\Sigma_{\rm[CII]}-\Sigma_{\rm SFR}$ correlation is valid over almost 5 orders of magnitude in $\Sigma_{\rm SFR}$, holding for both normal star-forming galaxies and non-AGN luminous infrared galaxies. Using [CII] luminosity instead of surface brightness to estimate SFR suffers from worse systematics, frequently underpredicting SFR in luminous infrared galaxies even after IR color adjustment (although this depends on the SFR measure employed). We suspect that surface brightness relations are better behaved than the luminosity relations because the former are more closely related to the local far-UV field strength, most likely the main parameter controlling the efficiency of the conversion of far-UV radiation into gas heating. A simple model based on Starburst99 population-synthesis code to connect SFR to [CII] finds that heating efficiencies are $1\%-3\%$ in normal galaxies.