Tracing molecular gas mass in extreme extragalactic environments: an observational study Zhu, Ming Papadopoulos, Padeli P. Xilouris, Emmanuel M. Kuno, N. Lisenfeld , Ute Dust Extinction Galaxies NGC 157 NGC 3310 ISM Spiral Staburst Radio lines We present a new observational study of the CO(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, HI 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 NGC3310 and the quiescent spiral NGC157. Our study maintains a robust statistical notion of the so-called X factor (i.e. a large ensemble of clouds is involved) while exploring its dependency on the very different average ISM conditions prevailing within these two systems. These are constrained by fully-sampled CO(3-2) and CO(1-0) observations, at a matched beam resolution of Half Power Beam Width 15", obtained with the JCMT the Nobeyama 45-m telescope, combined with sensitive 850 and 450 micron dust emission and HI interferometric images which allow a complete view of all the neutral ISM components. We found an X factor varying by a factor of 5 within the spiral galaxy NGC157 and about 2 times lower than the Galactic value in NGC3310. In addition, the dust emission spectrum in NGC3310 shows a pronounced submm "excess". We explain and fit this by an enhanced abundance of Very Small Grains (VSGs). The alternative approach to interpret this excess as very cold dust would yield a cold dust temperature of 5-11 K with a correspondingly very low gas-to-dust mass ratio of 5-43, very unlikely for this starburst galaxy with low metallicity. 2013-10-09T11:29:56Z 2013-10-09T11:29:56Z 2009 info:eu-repo/semantics/preprint Zhu, M.; et al. Tracing molecular gas mass in extreme extragalactic environments: an observational study. Astrophysical Journal, 706(2): 941-959 (2009). [http://hdl.handle.net/10481/28391] 0004-637X 1538-4357 arXiv:0908.1600v2 http://hdl.handle.net/10481/28391 10.1088/0004-637X/706/2/941 eng http://creativecommons.org/licenses/by-nc-nd/3.0/ info:eu-repo/semantics/openAccess Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License American Astronomical Society; Institute of Physics (IOP)