[N Ii] Fine-Structure Emission At 122 And 205 Mu M In A Galaxy At Z=2.6: A Globally Dense Star-Forming Interstellar Medium

We present new observations with the Atacama Large Millimeter/submillimeter Array of the 122 and 205 mu m finestructure line emission of singly ionized nitrogen in a strongly lensed starburst galaxy at z.=.2.6. The 122/205 mu m [N II] line ratio is sensitive to electron density, ne, in the ionized interstellar medium, and we use this to measure n(e) approximate to 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 starforming 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.