ALMA Reveals Weak [NII] Emission in "Typical" Galaxies and Intense Starbursts at z=5-6

TL;DR

This study uses ALMA to measure [NII] emission in galaxies at z=5-6, revealing diverse ionized gas properties and low [NII] emission in typical galaxies, providing insights into early galaxy interstellar medium conditions.

Contribution

First ALMA measurements of [NII] 205 um in z=5-6 galaxies, revealing different ionized gas regimes and their relation to galaxy properties in the early universe.

Findings

Low [NII] emission in typical galaxies suggests low metallicity and dust.

Starburst galaxy shows higher ionized gas fraction, indicating diffuse ionized medium.

Diverse ionized gas properties observed across galaxy types at high redshift.

Abstract

We report interferometric measurements of [NII] 205 um 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 [CII] 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 [NII] emission compared to [CII] (L_ [CII]/L_[NII]=68 [+200/-28]) from a "typical" 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]<=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+/-8) suggesting that [NII] dominantly traces a diffuse ionized medium rather than star-forming HII regions in this type of galaxy. This highest redshift sample of [NII] 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.