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 Ω_M = 1 or M(r < 2.4 h^-1 kpc) = 7.4 × 10¹⁰ h^-1 M⊙ for Ω_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 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 Ω_M = 1 and H₀ = 76¹⁸_-10, for Ω_M = 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.