Herschel-SPIRE Fourier transform spectroscopy of the nearby spiral galaxy IC 342
2013
We present observations of the nearby spiral galaxy IC342 with the Herschel Spectral and Photometric Imaging Receiver (SPIRE) Fourier Transform Spectrometer. The spectral range afforded by SPIRE, 196-671 m, allows us to access a number of 12 CO 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 13 CO, [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 12 CO components, one at 35 K and one at 400 K with CO column densities of 6.3 10 17 cm 2 and 0.4 10 17 cm 2 and CO gas masses of 1.26 10 7 M and 0.15 10 7 M , for the cold and warm components, respectively. The inclusion of the high-J 12 CO line observations, indicate the existence of a much warmer gas component ( 400 K) confirming earlier findings from H2 rotational line analysis from ISO and Spitzer. The mass of the warm gas is 10% of the cold gas, but it likely dominates the CO luminosity. In addition, we detect strong emission from [NII] 205 m and the 3 P1! 3 P0 and 3 P2! 3 P1 [CI] lines at 370 and 608 m, respectively. The measured 12 CO 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 2.1 10 17 cm 2 and the fact that [CI] is weaker than CO emission in IC342 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.
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