We present [ C i ] ^ { 3 } P _ { 1 } – ^ { 3 } P _ { 0 } spectra at four spiral arm positions and the nuclei of the nearby galaxies M83 and M51 obtained at the JCMT .
The spiral arm positions lie at galacto-centric distances of between 2 kpc and 6 kpc .
This data is complemented with maps of CO 1–0 , 2–1 , and 3–2 , and ISO/LWS far-infrared data of [ C ii ] ( 158 \mu m ) , [ O i ] ( 63 \mu m ) , and [ N ii ] ( 122 \mu m ) allowing for the investigation of a complete set of all major gas cooling lines .
From the intensity of the [ N ii ] line , we estimate that between 15 % and 30 % of the observed [ C ii ] emission originate from the dense ionized phase of the ISM .
The analysis indicates that emission from the diffuse ionized medium is negligible .
In combination with the FIR dust continuum , we find gas heating efficiencies below \sim 0.21 \% in the nuclei , and between 0.25 and 0.36 % at the outer positions .
Comparison with models of photon-dominated regions ( PDRs ) of Kaufman et al .
( 1999 ) with the standard ratios [ O i ] ( 63 ) / [ C ii ] _ { PDR } and ( [ O i ] ( 63 ) + [ C ii ] _ { PDR } ) vs. TIR , the total infrared intensity , yields two solutions .
The physically most plausible solution exhibits slightly lower densities and higher FUV fields than found when using a full set of line ratios , [ C ii ] _ { PDR } / [ C i ] ( 1–0 ) , [ C i ] ( 1–0 ) /CO ( 3–2 ) , CO ( 3–2 ) /CO ( 1–0 ) , [ C ii ] /CO ( 3–2 ) , and , [ O i ] ( 63 ) / [ C ii ] _ { PDR } .
The best fits to the latter ratios yield densities of 10 ^ { 4 } cm ^ { -3 } and FUV fields of \sim G _ { 0 } = 20–30 times the average interstellar field without much variation .
At the outer positions , the observed total infrared intensities are in perfect agreement with the derived best fitting FUV intensities .
The ratio of the two intensities lies at 4–5 at the nuclei , indicating the presence of other mechanisms heating the dust .
[ C i ] area filling factors lie below 2 % at all positions , consistent with low volume filling factors of the emitting gas .
The fit of the model to the line ratios improves significantly if we assume that [ C i ] stems from a larger region than CO 2–1 .
Improved modelling would need to address the filling factors of the various submm and FIR tracers , taking into consideration the presence of density gradients of the emitting gas by including cloud mass and size distributions within the beam .