Herschel-SPIRE Fourier transform spectroscopy of the nearby spiral galaxy IC 342

Abstract

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, 196-671 μm, 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, [C I] and [N II]. 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 35 K and one at 400 K with CO column densities of 6.3 × 10¹⁷ and 0.4 × 10¹⁷ cm^-2 and CO gas masses of 1.26 × 10⁷ and 0.15 × 10⁷ M_⊙ 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 (∼400 K) confirming earlier findings from H₂ rotational line analysis from Infrared Space Observatory and Spitzer. The mass of the warm gas is 10 per cent of the cold gas, but it likely dominates the CO luminosity. In addition, we detect strong emission from [N II] 205 μm and the ³P₁ → ³P₀ and ³P₂ → ³P₁ [C I] lines at 370 and 608 μm, 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 [C I] line ratio together with the derived [C] column density of 2.1 × 10¹⁷ cm^-2 and the fact that [C I] is weaker than CO emission in IC 342 suggests that [C I] likely arises in a thin layer on the outside of the CO emitting molecular clouds consistent with PDRs playing an important role.