The Star-Forming Molecular Gas in High Redshift Submillimeter Galaxies

TL;DR

This paper models CO molecular line emission in high-redshift Submillimeter Galaxies, explaining their observed properties as transient merger phases and highlighting the complexities in deriving physical conditions from CO observations.

Contribution

It introduces a combined simulation and radiative transfer model to interpret CO emissions in SMGs, revealing potential biases in common observational techniques.

Findings

CO line widths may overestimate dynamical mass by 1.5-2 times.

Assuming unity line ratios can underestimate gas mass.

SMGs are analogous to local ULIRGs as scaled-up merger systems.

Abstract

We present a model for the CO molecular line emission from high redshift Submillimeter Galaxies (SMGs). By combining hydrodynamic simulations of gas rich galaxy mergers with the polychromatic radiative transfer code, Sunrise, and the 3D non-LTE molecular line radiative transfer code, Turtlebeach, we show that if SMGs are typically a transient phase of major mergers, their observed compact CO spatial extents, broad line widths, and high excitation conditions (CO SED) are naturally explained. In this sense, SMGs can be understood as scaled-up analogs to local ULIRGs. We utilize these models to investigate the usage of CO as an indicator of physical conditions. We find that care must be taken when applying standard techniques. The usage of CO line widths as a dynamical mass estimator from SMGs can possibly overestimate the true enclosed mass by a factor ~1.5-2. At the same time, assumptions of line ratios of unity from CO J=3-2 (and higher lying lines) to CO (J=1-0) will oftentimes lead to underestimates of the inferred gas mass. We provide tests for these models by outlining predictions for experiments which are imminently feasible with the current generation of bolometer arrays and radio-wave spectrometers.