Mapping of Nearby Galaxies in [CI] 370 μm and CO (7→6) 371 μm

The fine structure line of [CI] at 370 μm, and the CO (7→6) rotational line at 371 μm are important coolants which can be used to probe the temperature and density of the warm gas component in regions of enhanced star formation. These lines can be used to provide key constrains on the physical properties of the warm molecular gas component and to deduce the excitation mechanism that heats the molecular gas. We have used the Cornell submillimeter spectrometer, SPIFI, on the JCMT to map the [CI] and CO (7→6) line emission from the nuclear regions of the nearby galaxies NGC 253 and M82, and from the overlap region of NGC4038/9. In both NGC 253 and NGC 4038/9 the [CI] emission appears fairly constant, while the CO (7→6) emission peaks at the nucleus and the most active star-forming region, respectively. The observations of NGC 253 suggest that the majority of the nuclear molecular gas is most likely heated by cosmic rays. The spatial distribution of the line emission in the overlap region of NGC 4038/9 is suggestive of a PDR, with a decrease in density from the most active region to the outside. In contrast, in M82 both the CO (7→6) and [CI] line intensities vary over the mapped region, with the line ratios remaining fairly constant. This suggests a change in the beam filling factor of PDRs with a higher PDR number density in the most active regions.