CI and CO in nearby galaxy centers. The star-burst galaxies NGC 278, NGC
  660, NGC 3628, NGC 4631, and NGC 4666
  Maps and measurements of the J=1-0, J=2-1, J=3-2, J=4-3 12CO, the J=1-0,
J=2-1 and J=3-2 13CO lines in the central arcminute squared of NGC 278, NGC
660, NGC 3628, NGC 4631, and NGC 4666, as well as 492 GHz [CI] maps in three of
these are used to model the molecular gas. All five objects exhibit bright CO
emission in the inner regions, with strong central concentrations in NGC 660,
NGC 3628, and NGC 4666, but not in the weakest CO emitters NGC 278 and NGC
4631. In all cases, the observed lines could be modeled only with at least two
distinct gas components. The physical condition of the molecular gas is found
to differ from galaxy to galaxy. Relatively tenuous (density 100-1000 cm-3) and
high kinetic temperature (100-150 K) gas occurs in all galaxies, except perhaps
NGC 3628, and is mixed with cooler (10-30 K) and denser (3000-10000 cm-3) gas.
In all galaxy centers, the CO-to-H2 conversion factor X is typically an order
of magnitude smaller than the `standard' value for the Solar Neighborhood. The
molecular gas is constrained within radii between 0.6 and 1.5 kpc from the
nuclei. Within these radii, H2 masses are typically 0.6-1.5 x 10**8 M(O), which
corresponds to no more than a few per cent of the dynamical mass in the same
region.
