  We directly detect dust emission in an optically-detected, multiply-imaged
galaxy lensed by the Frontier Fields cluster MACSJ0717.5+3745. We detect two
images of the same galaxy at 1.1mm with the AzTEC camera on the Large
Millimeter Telescope leaving no ambiguity in the counterpart identification.
This galaxy, MACS071_Az9, is at z>4 and the strong lensing model (mu=7.5)
allows us to calculate an intrinsic IR luminosity of 9.7e10 Lsun and an
obscured star formation rate of 14.6 +/- 4.5 Msun/yr. The unobscured star
formation rate from the UV is only 4.1 +/- 0.3 Msun/yr which means the total
star formation rate (18.7 +/- 4.5 Msun/yr) is dominated (75-80%) by the
obscured component. With an intrinsic stellar mass of only 6.9e9Msun,
MACS0717_Az9 is one of only a handful of z>4 galaxies at these lower masses
that is detected in dust emission. This galaxy lies close to the estimated star
formation sequence at this epoch. However, it does not lie on the dust
obscuration relation (IRX-beta) for local starburst galaxies and is instead
consistent with the Small Magellanic Cloud (SMC) attenuation law. This
remarkable lower mass galaxy showing signs of both low metallicity and high
dust content may challenge our picture of dust production in the early
Universe.
