Near-Infrared Observations of a Redshift 4.92 Galaxy: Evidence for Significant Dust Absorption

Abstract

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 μm show the same arc morphology as the HST images. The spectrum with resolution λ/Δλ~70 shows no emission lines with equivalent width stronger than 100 Å in the rest-frame wavelength range 0.34-0.40 μm. In particular, [O II] λ3727 and [Ne III] λ3869 are not seen. The energy distribution is quite blue, as expected for a young stellar population with the observed Lyα 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 2-16×109 M☉. This is significantly higher than estimates based on the HST data alone. Our data imply that absorption by dust is important to redshifts of ~5.