Formaldehyde Anti-Inversion at z=0.68 in the Gravitational Lens B0218+357
Abstract
We report new observations of the 110-111 (6 cm) and 211-212 (2 cm) transitions of ortho-formaldehyde (o-H2CO) in absorption at z=0.68466 toward the gravitational lens system B0218+357. Radiative transfer modeling indicates that both transitions are anti-inverted relative to the 4.6 K cosmic microwave background (CMB), regardless of the source covering factor, with excitation temperatures of ~1 K and 1.5-2 K for the 110-111 and 211-212 lines, respectively. Using these observations and a large velocity gradient radiative transfer model that assumes a gradient of 1 km s-1 pc-1, we obtain a molecular hydrogen number density of 2x103 cm-3 < n(H2) < 1x104 cm-3 and a column density of 2.5x1013 cm-2 < N(o-H2CO) < 8.9x1013 cm-2, where the allowed ranges conservatively include the range of possible source covering factors in both lines. The measurements suggest that H2CO excitation in the absorbing clouds in the B0218+357 lens is analogous to that in Galactic molecular clouds: it would show H2CO absorption against the CMB if it were not illuminated by the background quasar or if it were viewed from another direction.