Spectroscopy of the optical Einstein ring 0047-2808

We present optical and near-infrared spectroscopic observations of the optical Einstein ring 0047-2808. We detect both [OIII] lines 4959, 5007 near 2.3 micron, confirming the redshift of the lensed source as z=3.595. The Ly-a line is redshifted relative to the [OIII] line by 140+-20 km/s. Similar velocity shifts have been seen in nearby starburst galaxies. The [OIII] line is very narrow, 130 km/s FWHM. If the ring is the image of the centre of a galaxy the one-dimensional stellar velocity dispersion sigma=55 km/s is considerably smaller than the value predicted by Baugh et al. (1998) for the somewhat brighter Lyman-break galaxies. The Ly-a line is significantly broader than the [OIII] line, probably due to resonant scattering. The stellar central velocity dispersion of the early-type deflector galaxy at z=0.485 is 250+-30 km/s. This value is in good agreement both with the value predicted from the radius of the Einstein ring (and a singular isothermal sphere model for the deflector), and the value estimated from the D_n-sigma relation.