# CNO emission of an unlensed submillimeter galaxy at z=4.3

## TL;DR

This study uses ALMA to analyze ionized and molecular gas in a high-redshift galaxy, revealing detailed interstellar medium conditions, ionization sources, and chemical maturity at z=4.3.

## Contribution

First detailed spatially-resolved multi-line analysis of an unlensed galaxy at z=4.3, providing insights into its ISM conditions and chemical evolution.

## Key findings

- CII emission is more extended than dust and ionized gas.
- Ionized gas shares kinematics with molecular gas, indicating a common gravitational potential.
- The galaxy has near-solar metallicity, indicating advanced chemical evolution at high redshift.

## Abstract

We present the results from ALMA observations of [NII]205 micron, [CII]158 micron, and [OII]88 micron lines in an unlensed submillimeter galaxy at z=4.3, COSMOS-AzTEC-1, hosting a compact starburst core with an effective radius of 1 kpc. The [CII] and [NII] emission are spatially-resolved in 0.3 arcsec-resolution (1 kpc in radius). The kinematic properties of the [NII] emission are consistent with those of the CO(4-3) and [CII] emission, suggesting that the ionized gas feels the same gravitational potential as the associated molecular gas and photodissociation regions (PDRs). On the other hand, the spatial extent is different among the lines and dust continuum: the [CII] emitting gas is the most extended and the dust is the most compact, leading to a difference of the physical conditions in the interstellar medium. We derive the incident far-ultraviolet flux and the hydrogen gas density through PDR modeling by properly subtracting the contribution of ionized gas to the total [CII] emission. The observed [CII] emission is likely produced by dense PDRs with nH(PDR)=10^(5.5-5.75) cm^-3 and G0=10^(3.5-3.75) in the central 1 kpc region and nH(PDR)=10^(5.0-5.25) cm^-3 and G0=10^(3.25-3.5) in the central 3 kpc region. We have also successfully measured the line ratio of [OIII/[NII] in the central 3 kpc region of COSMOS-AzTEC-1 at z=4.3, which is the highest redshift where both nitrogen and oxygen lines are detected. Under the most likely physical conditions, the measured luminosity ratio of L([OIII])/L([NII])=6.4+-2.2 indicates a near solar metallicity with Zgas=0.7-1.0 Zsol, suggesting a chemically evolved system at z=4.3.