Molecular Gas in Optically Selected Mergers: Do All Mergers Undergo an Ultraluminous Phase?

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  • 30 March 1999
Abstract
We have mapped the 2.6 mm CO(1-0) emission in three optically selected "Toomre Sequence" mergers (NGC520, NGC3921, NGC4676). We examine these data in order to address whether there are any fundamental differences between these optically selected mergers and more luminous IR selected mergers. The molecular gas distributions are well resolved by the observations, and they are as compact as in IR selected mergers. For NGC520 and NGC4676 the gas kinematics are regular and are consistent with simple rotation, and the nuclear gas concentrations form a disk- or a ring-like structure. 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. These optically selected mergers fall on the same relationship between the mean gas density and FIR luminosity defined by the IR luminous mergers, extending it to lower luminosity and lower mean gas density. This is similar to the "global Schmidt law" of Kennicutt, and all merger systems appear to follow the same star formation "law" as the field galaxies. We discuss the relevance of these data to the presence of buried AGN, the transfer of angular momentum from the molecular gas disks, and the evolution of merger remnants. [ABRIDGED]

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