CO in Toomre Sequence

Molecular Gas in Optically Selected Mergers

M. S. Yun & J. E. Hibbard
To be published in The Astrophysical Journal
accepted 10/20/00.

We have imaged the 2.6 mm CO(1-0) emission in three optically selected ``Toomre Sequence" mergers (NGC 520, NGC 3921, NGC 4676). The molecular gas distributions are well resolved by the observations, and they are as compact as in IR selected mergers (R<2 kpc). For NGC 520 and NGC 4676A, the nuclear gas concentrations form a disk- or a ring-like structure, and the gas kinematics are regular and are consistent with simple rotation. Discrete molecular gas complexes are found along the stellar bar in NGC4676B, and the gas kinematics is consistent with the disk rotation traced in Halpha. The molecular gas distribution in NGC3921 is asymmetric about the stellar remnant, and both the distribution and kinematics suggest that the molecular gas has not settled into the center of the remnant. Molecular gas clouds are detected outside the central regions of NGC3921 and NGC4676, and they may be associated with the tidal tails and bridges mapped in HI. Departures from the canonical scenario for a merger involving two large spiral galaxies are found in all three Toomre Sequence mergers studied. Our data suggest that one of the progentor disks in NGC520 and NGC3921 were relatively gas-poor. A detailed comparison of these optically selected mergers and more luminous IR selected mergers is deferred to a companion paper (Paper II).

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