New Observations of Extra-Disk Molecular Gas in Interacting Galaxy Systems, Including a Two-Component System in Stephan's Quintet

Abstract

We present new CO (1 - 0) observations of eleven extragalactic tails and bridges in nine interacting galaxy systems, almost doubling the number of such features with sensitive CO measurements. Eight of these eleven features were undetected in CO to very low CO/HI limits, with the most extreme case being the NGC 7714/5 bridge. This bridge contains luminous H II regions and has a very high HI column density (1.6 X 10^21 cm^-2 in the 55" CO beam), yet was undetected in CO to rms T(R)* = 2.4 mK. The HI column density is higher than standard H2 and CO self-shielding limits for solar-metallicity gas, suggesting that the gas in this bridge is metal-poor and has an enhanced N(H2)/I(CO) ratio compared to the Galactic value. Only one of the eleven features in our sample was unambiguously detected in CO, a luminous HI-rich star formation region near an optical tail in the compact group Stephan's Quintet. We detect CO at two widely separated velocities in this feature, at ~6000 km/s and ~6700 km/s. Both of these components have HI and H-alpha counterparts. These velocities correspond to those of galaxies in the group, suggesting that this gas is material that has been removed from two galaxies in the group. The CO/HI/H-alpha ratios for both components are similar to global values for spiral galaxies.