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

TL;DR

This study investigates the extreme physical conditions of molecular gas in Luminous Infrared Galaxies and how these conditions influence the X_{co} factor used to estimate molecular gas mass, revealing that turbulence and density significantly affect mass estimates.

Contribution

It provides a detailed analysis of the X_{co} factor in LIRGs using radiative transfer models and high-J CO lines, highlighting the impact of physical conditions on molecular mass estimates.

Findings

X_{co} varies with gas density, temperature, and dynamical state.

Unbound gas states yield lower X_{co} values, while self-gravitating states yield higher.

High-density components in ULIRGs can dominate X_{co}, leading to potential underestimates of molecular gas mass.

Abstract

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 focusing 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: <X_{co}>\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 assessment of the gas mass at high densities (>=10^{4} cm^{-3}) rather than a simple one-phase analysis we find that {\it near-Galactic X_{co} (3-6)\, M_sol\,(K\,km^{-1}\,pc^2)^{-1} values become possible.} We further show that in the highly turbulent molecular gas in ULIRGs a high-density component will be common and can be massive enough for its high X_{co} to dominate the average value for the entire galaxy. ......... ...this may have thus resulted to systematic underestimates of molecular gas mass in ULIRGs.