Obscured star-formation in bright z ~ 7 Lyman-break galaxies
  We present Atacama Large Millimeter/Submillimeter Array observations of the
rest-frame far-infrared (FIR) dust continuum emission of six bright Lyman-break
galaxies (LBGs) at $z \simeq 7$. One LBG is detected ($5.2\sigma$ at peak
emission), while the others remain individually undetected at the $3\sigma$
level. The average FIR luminosity of the sample is found to be $L_{\rm FIR}
\simeq 2 \times 10^{11}\,{\rm L}_{\odot}$, corresponding to an obscured
star-formation rate (SFR) that is comparable to that inferred from the
unobscured UV emission. In comparison to the infrared excess (IRX$\,=L_{\rm
FIR}/L_{\rm UV}$)-$\beta$ relation, our results are consistent with a
Calzetti-like attenuation law (assuming a dust temperature of T = 40-50 K). We
find a physical offset of 3 kpc between the dust continuum emission and the
rest-frame UV light probed by Hubble Space Telescope imaging for galaxy ID65666
at $z = 7.17^{+0.09}_{-0.06}$. The offset is suggestive of an inhomogeneous
dust distribution, where 75% of the total star formation activity (SFR$
\,\simeq 70\,{\rm M}_{\odot}/{\rm yr}$) of the galaxy is completely obscured.
Our results provide direct evidence that dust obscuration plays a key role in
shaping the bright-end of the observed rest-frame UV luminosity function at $z
\simeq 7$, in agreement with cosmological galaxy formation simulations. The
existence of a heavily-obscured component of galaxy ID65666 indicates that
dusty star-forming regions, or even entire galaxies, that are "UV-dark" are
significant even in the $z \simeq 7$ galaxy population.
