  We present spatially resolved ALMA [CII] observations of the bright (flux
density S=400 mJy at 350 microns), gravitationally lensed, starburst galaxy
PLCK G045.1+61.1 at z=3.427, the "Garnet". This source is part of our set of
"Planck's Dusty GEMS", discovered with the Planck's all-sky survey. Two
emission-line clouds with a relative velocity offset of ~600 km/s extend
towards north-east and south-west, respectively, of a small, intensely
star-forming clump with a star-formation intensity of 220 Msun/yr/kpc^2, akin
to maximal starbursts. [CII] is also seen in absorption, with a redshift of
+350 km/s relative to the brightest CO component. [CII] absorption has
previously only been found in the Milky Way along sightlines toward bright
high-mass star-forming regions, and this is the first detection in another
galaxy. Similar to Galactic environments, the [CII] absorption feature is
associated with [CI] emission, implying that this is diffuse gas shielded from
the UV radiation of the clump, and likely at large distances from the clump.
Since absorption can only be seen in front of a continuum source, the gas in
this structure can definitely be attributed to gas flowing towards the clump.
The absorber could be part of a cosmic filament or merger debris being accreted
onto the galaxy. We discuss our results also in light of the on-going debate of
the origin of the [CII] deficit in dusty star-forming galaxies.
