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