Resolving Redshifted Molecular Absorption toward the Gravitational Lens PKS 1830−211

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Abstract
Using the high-resolution configuration of the BIMA array, we have spatially resolved molecular absorption at z = 0.89 toward the gravitational lens PKS 1830-211. Two continuum sources are detected at λ3 mm separated by 098 at the known positions of the doubly lensed radio core. Broad molecular absorption of width 40 km s-1 is found toward the southwest component only, where surprisingly it does not reach the base of the continuum, despite the large optical depth inferred from the molecular transitions (Wiklind & Combes). This implies incomplete coverage of the southwest component, ~70%, by the molecular gas, despite the small projected size of the source, less than 8 h-1 pc at the absorption redshift. Similar saturated but unfilled columns of low-density molecular gas are found in absorption through the spiral arms of the Milky Way, with large variations in column depth on parsec scales, indicating that the southwest component of PKS 1830-211 may be occulted by an ordinary spiral arm. At spectral resolution better than 5 km s-1, we might expect the broad molecular absorption of PKS 1830-211 to resolve into multiple narrow lines.