The Detection of Massive Molecular Complexes in the Ring Galaxy System Arp 143

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Abstract
We have imaged the kiloparsec-scale distribution of 12CO(J=1-0) emission in the ring galaxy system Arp 143 (NGC 2444/2445) using the OVRO millimeter array. We find two giant molecular complexes in the ring component (NGC 2445) and a bright central source. The ring complexes represent 20%-60% of the detected MH2, depending on the relative ICO-NH2 for the ring and nucleus. Their individual H2 masses and surface densities (ΣH2) exceed typical spiral arm giant molecular clouds and associations regardless of the conversion factor. Both are associated with a 6 kpc ridge of peak ΣH I and massive star formation activity. Hα imaging shows a patchy ring of H II regions situated along the outer edge of the H I ring. The kinematics of the H I ring show clear signs of expansion. A simple rotating-expanding ring model (Vexp=118±30 km s−1) fits the data reasonably well, which implies a ring age of 60±15 Myr. NGC 2445's ring is able to form very large molecular complexes promptly in a metal-poor ISM and trigger massive star formation. Nearly 80% of the detected 12CO(1-0) flux originates in a resolved central source that is slightly offset from NGC 2445's starburst nucleus. We find an ordered velocity field in this component. Assuming an inclined disk, we argue that it is dynamically stable. The central ΣH2 (9 × 10 M☉ pc-2) significantly exceeds ΣH2 values commonly found in normal spirals but is much smaller than values derived in similar sized regions of IR-luminous galaxies. The nuclear H2 may be the result of a previous encounter with NGC 2444.12CO(1-0) emission in ring galaxies may be dominated by the nucleus, which could bias the interpretation of single-dish measurements.