The redshift of the gravitationally lensed radio source PKS1830-211

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 mu m, 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 x 10(10) h(-1) M-. for Omega(M)= 1 or M(r< 2.4 h(-1) kpc) = 7.4 x 10(10) h(-1) M-. for Omega(M) = 0.3. Our redshift measurement, together with the recently measured time delay (Lovell et al.), means that we are a step closer to determining H-0 from this lens. Converting the time delay into H-0 by using existing models leads to high values of the Hubble parameter, H-0 = 65(-19)(+15) for Omega(M)= 1 and H-0= 76(-10)(+18) for Omega(M) = 0.3. Since the lensing galaxy lies very close to the center of the lensed ring, improving the error bars on H-0 will require not only a more precise time delay measurement but also very precise astrometry of the whole system.