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 λλ4959, 5007 near ∼ 2.3μ, confirming the redshift of the lensed source as z = 3.595. The Lyα 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 σ = 55 km s is considerably smaller than the value predicted by Baugh et al. (1998) for the somewhat brighter Lyman–break galaxies. The Lyα 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 Dn − σ relation.