||In the past, considerable efforts have been made to observe the infrared, submillimeter, and millimeter line emission of the neutral gas in the central regions of nearby starburst galaxies. Several physical models (large-velocity-gradient, PDR, LTE radiative transfer) have been used to interpret the far-infrared/submm/mm line emission. All of these models have successfully demonstrated spatially (1) the origin of the FIR/submm/mm line emission; (2) the relations between the degree of molecular excitation measured by different line ratios and the concentration, distribution of different gas components, as well as the efficiency of star-forming activity; (3) a statistical, time-independent value of the CO-to-H2 conversion factor X that is 4-10 times lower than the conventional X derived from the gas in our Galaxy; and (4) the physical states of the interstellar medium (ISM), such as gas density, far-ultraviolet flux, and gas kinetic temperature, are enhanced in starburst regions. However, none of these models was able to (1) temporally relate the observed line emission properties of molecular gas in a starburst galaxy to its age and star formation history; (2) provide a better understanding of the physical conditions in the progenitor giant molecular clouds (GMCs), and how they relate to massive star formation, as well as the chemical evolution of molecular species in starburst environments; and (3) calculate a time-dependent X-factor as a function of starburst age. In this study, we introduce a new set of physical models, called Evolving Starburst Models, that will allow us to achieve these goals.
Our new starburst models consist of a standard dynamical model of the bubble/shell structure around a young star cluster, a time-dependent stellar population-synthesis model, a fully time-dependent chemistry model for the photo-dissociation regions, and a one-dimensional non-LTE line radiative-transfer model. Few previous models, if any, have all these physical elements included at the same time. This work pioneers a basic understanding of a few challenging and important issues and hopes to provide some potential answers to these questions. They are (1) Can we follow the evolution of an ensemble of GMCs and the swept-up shells, and model the FIR/submm/mm line emission in the ISM of a starburst galaxy? (2) Can we use these line emissions from giant molecular cloud fossils (the shells) to date the starburst? and (3) Are the ages predicted from our studies consistent with those measured from the use of red supergiant spectral features, or the absorption/emission spectral properties of dust in H II regions?
Finally, the predictions made by our theoretical modeling are useful for the interpretation of future ALMA high resolution maps of molecular gas on small and large scales in starburst galaxies, in order to provide a deeper understanding of the structure, dynamics, and evolution of the neutral ISM and its relationship with active star formation.