# Extended ionised and clumpy gas in a normal galaxy at z=7.1 revealed by ALMA

## TL;DR

This study uses ALMA to reveal complex, clumpy, and offset ionized and neutral gas structures in a typical galaxy at redshift 7.1, highlighting diverse star formation and assembly processes in early galaxies.

## Contribution

First high-resolution ALMA observations of multiple far-IR lines in a z=7.1 galaxy, showing complex gas morphology and star formation activity.

## Key findings

- [CII] emission is clumpy and diffuse on >1kpc scales.
- [OIII] emission is offset and indicates in-situ star formation.
- Detected two star-forming companion systems within 20 kpc.

## Abstract

We present new ALMA observations of the [OIII]88$\mu$m line and high angular resolution observations of the [CII]158$\mu$m line in a normal star forming galaxy at z$=$7.1. Previous [CII] observations of this galaxy had detected [CII] emission consistent with the Ly$\alpha$ redshift but spatially slightly offset relative to the optical (UV-rest frame) emission. The new [CII] observations reveal that the [CII] emission is partly clumpy and partly diffuse on scales larger than about 1kpc. [OIII] emission is also detected at high significance, offset relative to the optical counterpart in the same direction as the [CII] clumps, but mostly not overlapping with the bulk of the [CII] emission. The offset between different emission components (optical/UV and different far-IR tracers) is similar to what observed in much more powerful starbursts at high redshift. We show that the [OIII] emitting clump cannot be explained in terms of diffuse gas excited by the UV radiation emitted by the optical galaxy, but it requires excitation by in-situ (slightly dust obscured) star formation, at a rate of about 7 M$_{\odot}$/yr. Within 20 kpc from the optical galaxy the ALMA data reveal two additional [OIII] emitting systems, which must be star forming companions. We discuss that the complex properties revealed by ALMA in the z$\sim$7.1 galaxy are consistent with expectations by recent models and cosmological simulations, in which differential dust extinction, differential excitation and different metal enrichment levels, associated with different subsystems assembling a galaxy, are responsible for the different appearance of the system when observed with different tracers.