The Redshift of the Gravitationally Lensed Radio Source PKS 1830–211

We report on the spectroscopic identification and the long-awaited redshift measurement of the heavily obscured, gravitationally lensed radio source PKS 1830-211, which was first observed as a radio Einstein ring. The northeast component of the doubly imaged core is identified, in our infrared spectrum covering the wavelength range 1.5-2.5 μm, as an impressively reddened quasar at z=2.507±0.002. The mass contained within the Einstein ring radius is M(r<2.1 h−1 kpc)=6.3×1010 h−1 M☉ for ΩM=1 or M(r<2.4 h−1 kpc)=7.4×1010 h−1 M☉ for ΩM=0.3. Our redshift measurement, together with the recently measured time delay (Lovell et al.), means that we are a step closer to determining H0 from this lens. Converting the time delay into H0 by using existing models leads to high values of the Hubble parameter, H0=65+15−9 for ΩM=1 and H0=76+18−10 for ΩM=0.3. Since the lensing galaxy lies very close to the center of the lensed ring, improving the error bars on H0 will require not only a more precise time delay measurement but also very precise astrometry of the whole system.