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J. L. Higdon (Kapteyn Institute), R. J. Rand, M. Nord (University of New Mexico)
Highly evolved ring galaxies provide unique environments to study starburst triggering and regulation, yet little is known of the molecular ISM in these objects. We have mapped the large southern ring galaxy AM0644-741 in the 12CO(2-1) line with the SEST sub-mm telescope. The 35 kpc diameter starburst outer ring is rich in molecular hydrogen (MH_2=1.5x1010 Msolar), with only upper-limits in the nucleus and enclosed disk. These data are combined with optical H-alpha images and interferometric HI & 20cm maps to provide a more complete picture of the ISM in a ring galaxy. The collision with the companion has concentrated essentially all of the ISM into the expanding ring, and very high atomic and molecular gas surface densities are derived (peak SHI+H2 = 150 Msolar pc-2, 3 kpc scales). The outer ring is thus far above the threshold for robust star formation as defined by Kennicutt (1989), while star formation is totally quenced in the sub-critical disk. Using L(H-alpha)/MH_2 to measure star formation efficiency, we find values more than an order of magnitude larger than are commonly found in non-interacting spirals. We also address the question of a universal Schmidt Law in the outer ring of this galaxy. Finally, both HI & CO line profiles appear noticeably non-gaussian in parts of the ring. We tentatively attribute this to more complicated kinematics in a non-planar ``ribbon''-shaped ring (Mihos & Hernquist 1994).
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