ALMA Lensing Cluster Survey: a strongly lensed multiply imaged dusty
  system at $z\geq$6
  We report the discovery of an intrinsically faint, quintuply-imaged, dusty
galaxy MACS0600-z6 at a redshift $z=$6.07 viewed through the cluster
MACSJ0600.1-2008 ($z$=0.46). A $\simeq4\sigma$ dust detection is seen at 1.2mm
as part of the ALMA Lensing Cluster Survey (ALCS), an on-going ALMA Large
program, and the redshift is secured via [C II] 158 $\mu$m emission described
in a companion paper. In addition, spectroscopic follow-up with
GMOS/Gemini-North shows a break in the galaxy's spectrum, consistent with the
Lyman break at that redshift. We use a detailed mass model of the cluster and
infer a magnification $\mu\gtrsim$30 for the most magnified image of this
galaxy, which provides an unprecedented opportunity to probe the physical
properties of a sub-luminous galaxy at the end of cosmic reionisation. Based on
the spectral energy distribution, we infer lensing-corrected stellar and dust
masses of $\rm{2.9^{+11.5}_{-2.3}\times10^9}$ and
$\rm{4.8^{+4.5}_{-3.4}\times10^6}$ $\rm{M_{\odot}}$ respectively, a star
formation rate of $\rm{9.7^{+22.0}_{-6.6} M_{\odot} yr^{-1}}$, an intrinsic
size of $\rm{0.54^{+0.26}_{-0.14}}$ kpc, and a luminosity-weighted age of
200$\pm$100 Myr. Strikingly, the dust production rate in this relatively young
galaxy appears to be larger than that observed for equivalent, lower redshift
sources. We discuss if this implies that early supernovae are more efficient
dust producers and the consequences for using dust mass as a probe of earlier
star formation.
