[NII] fine-structure emission at 122 and 205um in a galaxy at z=2.6: a globally dense star-forming interstellar medium

TL;DR

This study uses ALMA observations of [NII] lines in a galaxy at z=2.6 to measure high electron densities in its star-forming interstellar medium, revealing conditions similar to dense Galactic star-forming regions.

Contribution

First measurement of [NII] 122/205um line ratio in a high-redshift galaxy, indicating a dense ionised interstellar medium comparable to local star-forming regions.

Findings

Electron density ~300 cm^-3 in the galaxy's ionised medium.

The molecular gas is driven to high densities.

Star formation occurs in dense, co-located HII regions.

Abstract

We present new observations with the Atacama Large Millimeter/sub-millimeter Array of the 122um and 205um fine-structure line emission of singly-ionised nitrogen in a strongly lensed starburst galaxy at z=2.6. The 122/205um [NII] line ratio is sensitive to electron density, n_e, in the ionised interstellar medium, and we use this to measure n_e~300cm^-3 averaged across the galaxy. This is over an order of magnitude higher than the Milky Way average, but comparable to localised Galactic star-forming regions. Combined with observations of the atomic carbon (CI(1-0)) and carbon monoxide (CO(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 ionised phase, presumably co-located with myriad HII regions that litter the gas-rich disk.