2020ApJ...905..152D

We present new observations with the Atacama Large Millimeter/submillimeter Array of the 122 and 205 µm fine-structure line emission of singly ionized nitrogen in a strongly lensed starburst galaxy at z = 2.6. The 122/205 µm [N II] line ratio is sensitive to electron density, n_e, in the ionized interstellar medium, and we use this to measure n_e ≃ 300 cm^–3, averaged across the galaxy. This is over an order of magnitude higher than the Milky Way average, comparable to localized Galactic star-forming regions. Combined with observations of the atomic carbon (C I) and carbon monoxide (CO J = 4-3) in the same system, we reveal the conditions in this intensely star-forming system. The majority of the molecular interstellar medium has been driven to high density, and the resultant conflagration of star formation produces a correspondingly dense ionized phase, presumably colocated with myriad H II regions that litter the gas-rich disk.