[N ii] Fine-structure Emission at 122 and 205 μm in a Galaxy at z = 2.6: A Globally Dense Star-forming Interstellar Medium

Abstract

We present new observations with the Atacama Large Millimeter/submillimeter Array of the 122 and 205 μ m fine-structure line emission of singly ionized nitrogen in a strongly lensed starburst galaxy at z  = 2.6. The 122/205 μ m [N ii ] line ratio is sensitive to electron density, , 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.