We report interferometric measurements of [ N ii ] 205 { \mu m } fine-structure line emission from a representative sample of three galaxies at z = 5 – 6 using the Atacama Large ( sub ) Millimeter Array ( ALMA ) .
These galaxies were previously detected in [ C ii ] and far-infrared continuum emission and span almost two orders of magnitude in star formation rate ( SFR ) .
Our results show at least two different regimes of ionized inter-stellar medium properties for galaxies in the first billion years of cosmic time , separated by their L _ { [ CII ] } / L _ { [ NII ] } ratio .
We find extremely low [ N ii ] emission compared to [ C ii ] ( L _ { [ CII ] } / L _ { [ NII ] } = 68 ^ { +200 } _ { -28 } ) from a “ typical ” \sim L ^ { * } _ { UV } star-forming galaxy , likely directly or indirectly ( by its effect on the radiation field ) related to low dust abundance and low metallicity .
The infrared-luminous modestly star-forming Lyman Break Galaxy ( LBG ) in our sample is characterized by an ionized-gas fraction ( L _ { [ CII ] } / L _ { [ NII ] } \lesssim 20 ) typical of local star-forming galaxies and shows evidence for spatial variations in its ionized-gas fraction across an extended gas reservoir .
The extreme SFR , warm and compact dusty starburst AzTEC-3 shows an ionized fraction higher than expected given its star-formation rate surface density ( L _ { [ CII ] } / L _ { [ NII ] } = 22 \pm 8 ) suggesting that [ N ii ] dominantly traces a diffuse ionized medium rather than star-forming H ii regions in this type of galaxy .
This highest redshift sample of [ N ii ] detections provides some of the first constraints on ionized and neutral gas modeling attempts and on the structure of the inter-stellar medium at z = 5 –6 in “ normal ” galaxies and starbursts .