Constraints on the galaxy "main sequence" at z>5: the stellar mass of HDF850.1

We present rest-frame optical and near-infrared detections of one of the highest redshift submm-selected galaxies to date, HDF850.1. We do not detect the previously proposed counterpart HDF850.1K in new deep J and H-band HST WFC3 data, placing a strong limit of H-K>3.8, concluding that the K-band source is spurious. However, we detect 5.8um and 8um emission co-located with the submm in deblended images. After modelling and removing the flux contributions from another foreground galaxy, we constrain the stellar mass of HDF850.1 to be (2.5+/-1)x10^{11} Msun/mu for a lensing magnification mu=1.9+/-0.3, with a specific star formation rate of 8.5 Gyr^{-1}, faster than the 1-4 Gyr^{-1} observed for UV-selected galaxies at this epoch.