CI and CO in nearby galaxy centers The star-burst galaxies NGC 278, NGC 660, NGC 3628, NGC 4631, and NGC 4666

Aims. We study the physical properties and mass of molecular gas in the central regions of galaxies with active nuclei. Methods. Maps and measurements of the J = 1-0, J = 2-1, J = 3-2, J = 4-3 (CO)-C-12, the J = 1-0, J = 2-1 and J = 3-2 (CO)-C-13 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. Results. All five objects contain bright CO emission in the central regions. Clear central concentrations were found 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 occur in all galaxies, except perhaps NGC 3628, mixed with cooler (10-30 K) and denser (0.3-1.0 x 10(4) cm(-3)) gas. The CO-to-H-2 conversion factor X is typically an order of magnitude less than the standard value in the Solar Neighborhood in all galaxy centers. The molecular gas is constrained within radii between 0.6 and 1.5 kpc from the nuclei. Within these radii, H-2 masses are typically 0.6-1.5 x 10(8) M-circle dot, which corresponds to no more than a few per cent of the dynamical mass in the same region.