P35	January 1999

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
² Institut d'Astrophysique et de Géophysique - Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
³ URA 173 CNRS-DAEC, Observatoire de Paris, F-92195 Meudon Principal Cédex, France
⁴ European Southern Observatory,Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany
⁵ Berkeley Astronomy Department, University of California, Berkeley, CA 94720 U.S.A.

⁺ Based on observations collected at the European Southern Observatory, La Silla, Chile (ESO Program 61.B-0413)

Abstract

     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 NE component of the doubly imaged core is identified, in our infrared spectrum covering the wavelength range 1.5-2.5 m, as an impressively reddened quasar at z = 2.507 ± 0.002. The mass contained within the Einstein ring radius is M(r < 2.1h^-1Kpc) = 6.3 × 10¹⁰h^-1M for _M = 1 or M(r < 2.4h^-1Kpc) = 7.4 × 10¹⁰h^-1M for _M = 0.3. Our redshift measurement, together with the recently measured time delay (Lovell et al. 1998), means that we are a step closer to determining H₀ from this lens. Converting the time delay intoH₀ by using existing models leads to high values of the Hubble parameter,   for _M = 1 and    _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.