2009ApJ...706..941Z

We present a new observational study of the ¹²CO(1-0) line emission as an H₂ gas mass tracer under extreme conditions in extragalactic environments. Our approach is to study the full neutral interstellar medium (H₂, H I, and dust) of two galaxies whose bulk interstellar medium (ISM) resides in environments that mark (and bracket) the excitation extremes of the ISM conditions found in infrared luminous galaxies, the starburst NGC 3310, and the quiescent spiral NGC 157. Our study maintains a robust statistical notion of the so-called X = N(H₂)/I_CO factor (i.e., a large ensemble of clouds is involved) while exploring its dependence on the very different average ISM conditions prevailing within these two systems. These are constrained by fully sampled ¹²CO(3-2) and ¹²CO(1-0) observations, at a matched beam resolution of half-power beam width ∼15'', obtained with the James Clerk Maxwell Telescope (JCMT) on Mauna Kea (Hawaii) and the 45 m telescope of the Nobeyama Radio Observatory in Japan, combined with sensitive 850 µm and 450 µm dust emission and H I interferometric images which allow a complete view of all the neutral ISM components. Complementary ¹²CO(2-1) observations were obtained with the JCMT toward the center of the two galaxies. We found an X factor varying by a factor of 5 within the spiral galaxy NGC 157 and about two times lower than the Galactic value in NGC 3310. In addition, the dust emission spectrum in NGC 3310 shows a pronounced submillimeter "excess." We tried to fit this excess by a cold dust component but very low temperatures were required (T_C∼ 5-11 K) with a correspondingly low gas-to-dust mass ratio of ∼5-43. We furthermore show that it is not possible to maintain the large quantities of dust required at these low temperatures in this starburst galaxy. Instead, we conclude that the dust properties need to be different from Galactic dust in order to fit the submillimeter "excess." We show that the dust spectral energy distribution can be fitted by an enhanced abundance of very small grains and discuss different alternatives.