2005ApJ...635L...5C

We report on Spitzer IRAC observations of the spectroscopically confirmed z=6.56 lensed Lyα emitting source HCM 6A that was found behind the cluster Abell 370. Detection of the source at 3.6 and 4.5 µm, corresponding to rest-frame optical emission, allows us to study the stellar population of this primeval galaxy. The broadband flux density at 4.5 µm is enhanced compared to the continuum at other wavelengths, likely due to the presence of strong Hα in emission. The derived Hα line flux corresponds to a star formation rate of ≳140 M_☉/yr, more than an order of magnitude larger than estimates from the ultraviolet continuum and Lyα emission line. The dust extinction required to explain the discrepancy is A_V∼1 mag. The inference of dust at such high redshifts is surprising and implies that the first epoch of star formation in this galaxy occurred at z∼20.