The Large‐Scale Distribution and Properties of Carbon Monoxide in a Sample of Nearby Spiral Galaxies

Abstract

Galaxies are defined by a cumulative history of converting of gas into stars, and molecular gas (H2) is the fuel for this star formation. Because H2 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 m telescope and its “on-the-fly” observing mode to efficiently and reliably map cold CO emission over very large areas in five 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, and actively interacting to passively quiescent. CO was mapped in roughly regions covering the optical disks of the selected ′ ′ 15 # 15 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 (Fig. 1). The first largescale, total neutral gas (H2 H i) surface density maps Sgas 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. In the case of NGC 1097, interarm regions in the grand-design pattern,