| Galaxies are defined by a cumulative history of converting of gas into stars. Molecular gas is the fuel for this star formation. Because H2 in molecular clouds cannot be observed directly we observe the "tracer", CO, and derive the H2 mass indirectly via a "conversion" factor.; To move beyond global correlations and the limitations of undersampled, limited coverage maps, this study has used the NRAO 12 meter telescope and its "on the fly" observing mode, to efficiently and reliably map cold, CO emission over very large areas in 5 spiral galaxies. The galaxies were selected based on previous CO detections, large angular size, low inclination, and morphology, to form an illustrative sample, ranging from big to small, starburst to nonstarburst, actively interacting to passively quiescent. CO was mapped in roughly 15' x 15' regions covering the optical disks of the selected galaxies.; This experiment has the following major results: (1) We now have large scale, fully sampled maps of CO in IC 342, M83, NGC 1097, NGC 4736 and NGC 6946. The first large scale, total neutral gas (H2 + HI) surface density maps, Sigmagas, of these galaxies are now available. (2) The CO maps confirm the correlation with the optical disks in overall extent and gross morphology, although significant morphological differences exist. Interarm molecular gas is common in the sample. (3) Widespread, high r12 (CO(2-1)/CO(1-0)) was found in two of the galaxies, indicating large scale optically thin CO may be present. This is significantly different from the large scale properties of Milky Way molecular gas. (4) In comparisons between CO, neutral gas, and star formation tracers, the best correlation in overall morphology and the uniformity of the ratio from galaxy to galaxy (at least for the later galaxy types), is between Sigma gas and non-thermal continuum. (5) Toomre's Q parameter was evaluated for the disks of these galaxies in order to see if gas instability theory could predict molecular cloud formation. No correlation was found; the gas disks are found to be sub-critical (stable). Alternatively, there is a correlation between the total mid-plane gas partial-pressure, Pism, and the fraction of neutral gas disk that is molecular, as predicted by theory. But the scatter in this incomplete evaluation of Pism is large, and the quantities required to improve the evaluation are not currently available. |
