Spatially resolved millimeter spectroscopy of the gravitational lens PKS 1830-211

This paper presents data from the BIMA interferometer showing spatially resolved absorption spectra of the gravitationally lensed quasar PKS 1830-211. High-resolution (1.2 km s^-1) spectra were taken in two spectral windows centered on the redshifted frequencies of the HCO⁺(2 ← 1) and HCN(2 ← 1) molecular transitions. There is no molecular absorption in the northeast image, but the southwest image reveals optically thick absorbing gas at these transition frequencies. Further analyses conclude that the spectra are consistent with completely saturated absorption in the southwest image, and the line profiles suggest that the absorbing medium is complex, perhaps containing multiple components and small-scale structure. The absorption occurs along a pencil beam through the lensing galaxy which is thought to be a late-type spiral oriented almost face-on. However, the spectra show absorption spanning more than 60 km s^-1, which is difficult to explain for this scenario.