2013MNRAS.434.2051R

We present observations of the nearby spiral galaxy IC 342 with the Herschel Spectral and Photometric Imaging Receiver (SPIRE) Fourier transform spectrometer. The spectral range afforded by SPIRE, 196-671µm, allows us to access a number of 12CO lines from J = 4-3 to J = 13-12 with the highest J transitions observed for the first time. In addition we present measurements of 13CO, [Ci] and [Nii]. We use a radiative transfer code coupled with Bayesian likelihood analysis to model and constrain the temperature, density and column density of the gas. We find two 12CO components, one at 35K and one at 400K with CO column densities of 6.3 x 10¹⁷ and 0.4x10¹⁷/cm2 and CO gas masses of 1.26x10⁷ and 0.15x10⁷M_☉ for the cold and warm components, respectively. The inclusion of the high-J 12CO line observations indicate the existence of a much warmer gas component ( ∼ 400K) confirming earlier findings from H2 rotational line analysis from Infrared Space Observatory and Spitzer. The mass of the warm gas is 10percent of the cold gas, but it likely dominates the CO luminosity. In addition, we detect strong emission from [Nii] 205µm and the 3P1 ⟶ 3P0 and 3P2 ⟶ 3P1 [Ci] lines at 370 and 608µm, respectively. The measured 12CO line ratios can be explained by photon-dominated region (PDR) models although additional heating by e.g. cosmic rays cannot be excluded. The measured [Ci] line ratio together with the derived [C] column density of 2.1x10¹⁷cm-2 and the fact that [Ci] is weaker than CO emission in IC 342 suggests that [Ci] likely arises in a thin layer on the outside of the CO emitting molecular clouds consistent with PDRs playing an important role.