An SMG with A_V>~5

One of the brightest sub-mm galaxies (SMGs) in the GOODS-N field is completely undetected in deep optical/NIR imaging.  Guessing that this object might be part of a known protocluster at z=4.05, Daddi et al. have used the IRAM Plateau de Bure Interferometer to search for CO emission in the expected wavelength range.  This figure shows that they found it.

This is quite a remarkable object: the sub-mm emission corresponds to a star formation rate of order 1000 Msun/yr, and the SED suggests stellar mass of roughly 10^11 Msun and A_V in the range 5-7.5.  That's a lot of extinction.