2021ApJ...908...95H

Dusty star-forming galaxies at high redshift (1 < z < 3) represent the most intense star-forming regions in the universe. Key aspects to these processes are the gas heating and cooling mechanisms, and although it is well known that these galaxies are gas-rich, little is known about the gas excitation conditions. Only a few detailed radiative transfer studies have been carried out owing to a lack of multiple line detections per galaxy. Here we examine these processes in a sample of 24 strongly lensed star-forming galaxies identified by the Planck satellite (LPs) at z ∼ 1.1-3.5. We analyze 162 CO rotational transitions (ranging from J_up = 1 to 12) and 37 atomic carbon fine-structure lines ([C I]) in order to characterize the physical conditions of the gas in the sample of LPs. We simultaneously fit the CO and [C I] lines and the dust continuum emission, using two different non-LTE, radiative transfer models. The first model represents a two-component gas density, while the second assumes a turbulence-driven lognormal gas density distribution. These LPs are among the most gas-rich, IR-luminous galaxies ever observed (µ_LL_IR(8-1000µm)_∼10^13–14.6 L_☉; <µ_LM_ISM>= (2.7 ± 1.2) x 10¹² M_☉, with µ_L ∼ 10-30 the average lens magnification factor). Our results suggest that the turbulent interstellar medium present in the LPs can be well characterized by a high turbulent velocity dispersion (<ΔV_turb>∼ 100 km s^–1) and ratios of gas kinetic temperature to dust temperature <T_kin/T_d>∼ 2.5, sustained on scales larger than a few kiloparsecs. We speculate that the average surface density of the molecular gas mass and IR luminosity, Σ_MISM_ ∼ 10^3–4 M_☉ pc^–2 and Σ_LIR_ ∼ 10^11–12 L_☉ kpc^–2, arise from both stellar mechanical feedback and a steady momentum injection from the accretion of intergalactic gas.