Near-infrared observations of a redshift 4.92 galaxy:Evidence for significant dust absorption

Near-infrared imaging and spectroscopy have been obtained of the gravitationally lensed galaxy at z = 4.92 discovered in Hubble Space Telescope (HST) images by Franx et al. Images at 1.2, 1.6, and 2.2 mu m show the same are morphology as the HST images. The spectrum with resolution lambda/Delta lambda similar to 70 shows no emission lines with equivalent width stronger than 100 Angstrom in the rest-frame wavelength,range 0.34-0.40 mu m. In particular, [O II] lambda 3727 and [Ne III] lambda 3869 are not seen. The energy distribution is quite blue, as expected for a young stellar population with the observed Ly alpha flux. The spectral energy distribution can be fitted satisfactorily for such a young stellar population when absorption by dust is included. The models imply a reddening 0.1 mag <E(B - V) <0.4 mag. The stellar mass of the lensed galaxy lies in the range of 2-16 x 10(9) M-circle dot. This is significantly higher than estimates based on the HST data alone. Our data imply that absorption by dust is important to redshifts of similar to 5.