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We have performed 2$^{\prime\prime}$-resolution aperture synthesis observations of a sample of luminous infrared galaxies in the CO(1--0) and HCN(1--0) transitions with the Owens Valley Millimeter Array. Five of the six galaxies are classified by their optical morphology as mergers, while the other one is classified as a member of an interacting pair; all have infrared luminosities equal to or greater than $10^{11.5}$~$L_{\sun}$. Our observations show that the mergers have very large concentrations of molecular gas in their nuclear regions, while the weaker interaction shows a much more extensive gas distribution. As part of our analysis, we have re-evaluated the behavior of the molecular gas mass to CO luminosity ratio for a variety of different geometries in order to better understand the uncertainties involved in the use of the standard mass conversion factor.
The objects classified as mergers all have over $10^{10}$~$M_{\sun}$ of gas contained within a radius of 1.6~kpc, and one, Mrk~231, has this gas within a radius of 0.5 kpc. The inferred molecular hydrogen column densities of these objects range from $2 \times 10^{23}$ to $2 \times 10^{24}$~cm$^{-2}$. Since the nuclei can be clearly seen at 2~$\mu$m, the gas must either be very clumpy or in a flattened, disk-like distribution. Further evidence for a disk-like distribution for the molecular gas comes from Mrk~231, whose derived gas mass is over six times the dynamical mass estimated from its CO line width. We argue that it is unlikely that the standard conversion factor is overestimating the gas mass by such a large factor and thus that the gas is Mrk~231 resides in a roughly face-on rotationally-supported disk.
In contrast to the merger objects, the galaxy NGC~7674, a member of an interacting pair, shows a comparable amount of gas, but in a 11-kpc-long bar-like feature. It is unclear whether this object represents a merger event early in its evolution or a less violent interaction. In either case, our observations of luminous infrared galaxies are consistent with models of interacting galaxies, such as those of Barnes \& Hernquist, which show a rapid accumulation of gas in the central regions during a merger event.
