The assembly of "normal" galaxies at z=7 probed by ALMA
  We report new deep ALMA observations aimed at investigating the [CII]158um
line and continuum emission in three spectroscopically confirmed Lyman Break
Galaxies at 6.8<z<7.1, i.e. well within the re-ionization epoch. With Star
Formation Rates of SFR ~ 5-15 Msun/yr these systems are much more
representative of the high-z galaxy population than other systems targeted in
the past by millimeter observations. For the galaxy with the deepest
observation we detect [CII] emission at redshift z=7.107, fully consistent with
the Lyalpha redshift, but spatially offset by 0.7" (4 kpc) from the optical
emission. At the location of the optical emission, tracing both the Lyalpha
line and the far-UV continuum, no [CII] emission is detected in any of the
three galaxies, with 3sigma upper limits significantly lower than the [CII]
emission observed in lower reshift galaxies. These results suggest that
molecular clouds in the central parts of primordial galaxies are rapidly
disrupted by stellar feedback. As a result, [CII] emission mostly arises from
more external accreting/satellite clumps of neutral gas. These findings are in
agreement with recent models of galaxy formation. Thermal far-infrared
continuum is not detected in any of the three galaxies. However, the upper
limits on the infrared-to-UV emission ratio do not exceed those derived in
metal- and dust-poor galaxies.
