Formaldehyde Absorption at [ITAL]z[/ITAL] = 0.685 toward the “Einstein Ring” B0218+357

Abstract

We report VLA observations of absorption in the 211 → 212 transition of formaldehyde (H2CO) at a redshift of 0.68466 toward the Einstein ring gravitational lens system B0218+357. We observe the H2CO line at 8.6 GHz only toward the stronger of the two compact images of this system. The column densities of H2CO and other molecules previously observed at millimeter wavelengths, as well as their relative abundances, are comparable to the values one finds for molecular clouds within the Milky Way. We argue that the material seen in absorption has a molecular hydrogen column density between 5 × 1021 and 5 × 1022 cm-2.