We discuss the rest-frame ultraviolet emission from the starbursting galaxy HFLS3 at a redshift of 6.34 .
The galaxy was discovered in Herschel /SPIRE data due to its red color in the sub-mm wavelengths from 250 to 500 \mu m. The apparent instantaneous star-formation rate of HFLS3 inferred from the total far-IR luminosity measured with over 15 photometric data points between 100 and 1000 \mu m is 2900 M _ { \odot } yr ^ { -1 } .
Keck/NIRC2 K _ { s } -band adaptive optics imaging data showed two potential near-IR counterparts near HFLS3 .
Previously , the northern galaxy was taken to be in the foreground at z = 2.1 while the southern galaxy was assumed to HFLS3 ’ s near-IR counterpart .
The recently acquired Hubble /WFC3 and ACS imaging data show conclusively that both optically-bright galaxies are in the foreground at z < 6 .
A new lensing model based on the Hubble imaging data and the mm-wave continuum emission yields a magnification factor of 2.2 \pm 0.3 .
The lack of multiple imaging constrains the lensing magnification to be lower than either 2.7 or 3.5 at the 95 % confidence level for the two scenarios , which attribute one or two components to HFLS3 in the source plane .
Once accounting for the possibility of gravitational lensing , the instantaneous star-formation rate is 1320 M _ { \sun } yr ^ { -1 } with the 95 % confidence lower limit around 830 M _ { \sun } yr ^ { -1 } .
Using models for the rest-frame UV to far-IR spectral energy distribution ( SED ) we determine the average star-formation rate over the last 100 Myr to be around 660 M _ { \odot } yr ^ { -1 } .
The dust and stellar masses of HFLS3 from the same SED models are at the level of 3 \times 10 ^ { 8 } M _ { \odot } and \sim 5 \times 10 ^ { 10 } M _ { \odot } , respectively , with large systematic uncertainties on assumptions related to the SED model .
With Hubble /WFC3 images we also find diffuse near-IR emission about 0.5 arcseconds ( \sim 3 kpc ) to the South-West of HFLS3 that remains undetected in the ACS imaging data .
The emission has a photometric redshift consistent with either z \sim 6 or a dusty galaxy template at z \sim 2 .
If at the same redshift as HFLS3 the detected diffuse emission could be part of the complex merger system that could be triggering the starburst .
Alternatively , it could be part of the foreground structure at z \sim 2.1 that is responsible for lensing of HFLS3 .