Disentangling a group of lensed submm galaxies at z~2.9
  MS$\,$0451.6$-$0305 is a rich galaxy cluster whose strong lensing is
particularly prominent at submm wavelengths. We combine new SCUBA-2 data with
imaging from Herschel SPIRE and PACS and HST in order to try to understand the
nature of the sources being lensed. In the region of the "giant submm arc," we
uncover seven multiply imaged galaxies (up from the previously known three), of
which six are found to be at a redshift of $z\sim2.9$, and possibly constitute
an interacting system. Using a novel forward-modelling approach, we are able to
simultaneously deblend and fit SEDs to the individual galaxies that contribute
to the giant submm arc, constraining their dust temperatures, far infrared
luminosities and star formation rates. The submm arc first identified by SCUBA
can now be seen to be composed of at least five distinct sources, four of these
within the galaxy group at $z\sim2.9$. The total unlensed luminosity for this
galaxy group is $(3.1\pm0.3) \times 10^{12}\,\mathrm{L}_\odot$, which gives an
unlensed star formation rate of $(450\pm50)$ M$_\odot$ yr$^{-1}$. From the
properties of this system, we see no evidence of evolution towards lower
temperatures in the dust temperature versus far-infrared luminosity relation
for high redshift galaxies.
