1202.1803 (Padelis P. Papadopoulos et al.)

The molecular gas in Luminous Infrared Galaxies II: extreme physical conditions, and their effects on the X_{co} factor    [PDF]

Padelis P. Papadopoulos, Paul van der Werf, E Xilouris, Kate G. Isaak, Yu Gao

In this work we conclude the analysis of our CO line survey of Luminous Infrared Galaxies (LIRGs: L_{IR}>=10^{11}L_{sol}) in the local Universe (Paper\,I), by focussing on the influence of their average ISM properties on the total molecular gas mass estimates via the so-called X_{co}=M(H_2)/L_{co,1-0} factor. One-phase radiative transfer models of the global CO Spectral Line Energy Distributions (SLEDs) yield an X_{co} distribution with: \sim(0.6+/-0.2) M_{sol}(K km s^{-1} pc^2)^{-1} over a significant range of average gas densities, temperatures and dynamical states. The latter emerges as the most important parameter in determining X_{co}, with unbound states yielding low values and self-gravitating states the highest ones. Nevertheless in many (U)LIRGs where available higher-J CO lines (J=3--2, 4--3, and/or J=6--5) or HCN line data from the literature allow a separate assesment of the gas mass at high densities (>=10^{4}cm^{-3}) rather than a simple one-phase analysis a near-Galactic, X_{co}\sim(3-6) M_{sol} (K\,km s^{-1} pc^2)^{-1} become possible..... ...and may have thus resulted to systematic underestimates of molecular gas mass in ULIRGs.

View original: http://arxiv.org/abs/1202.1803