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

We have mapped the 2.6 mm CO J = 1 → 0 emission in three optically selected “Toomre Sequence” mergers (NGC 520, NGC 3921, NGC 4676). We examine these data in order to address if 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 (R ≤ 2 kpc). For NGC 520 and NGC 4676 the gas kinematics are regular and are consistent with simple rotation, and the nuclear gas concentrations form a diskor a ring-like structure. The molecular gas distribution in NGC 3921 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 (ΣFIR ∝ Σ 1.55±0.12 H2 ), and all merger systems appear to follow the same star formation “law” as the field galaxies. If AGN fueling is largely decoupled from the gas distribution on kpc scales, then deviations from the tight LFIR −ΣH2 relationship may constrain the contribution of AGN to the FIR luminosity. The response of the gas during the merger process is quantified by comparing the specific angular momentum, J/M , of the merger systems to normal spiral disks. We find that the molecular gas disks of the merger systems have ∼5 times less J/M than field spirals with the same gas mass. There is little difference between the optically selected mergers and IR selected mergers suggesting that angular momentum shedding by molecular gas is a universal process among all major merger systems. Lastly, an examination of the radial gas density profiles suggests that some of the less luminous mergers such as NGC 7252 may have had a dense central molecular core in the past, which has since been eroded by starburst activity. Subject headings: galaxies: individual (NGC 520, NGC 3921, NGC 4676) — galaxies: interactions — galaxies: evolution — ISM: molecules — infrared: galaxies