Formaldehyde Anti-Inversion at z=0.68 in the Gravitational Lens B0218+357

TL;DR

This study observes formaldehyde absorption lines in a gravitational lens system, revealing anti-inversion relative to the CMB and providing insights into molecular cloud conditions at z=0.68.

Contribution

First detailed measurement of formaldehyde excitation and physical conditions in a gravitational lens at z=0.68, demonstrating anti-inversion relative to the CMB.

Findings

Formaldehyde lines are anti-inverted relative to the CMB.

Molecular hydrogen density is estimated between 2x10^3 and 1x10^4 cm^-3.

H2CO excitation resembles that in Galactic molecular clouds.

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 2x10^3 cm^-3 < n(H2) < 1x10^4 cm^-3 and a column density of 2.5x10^13 cm^-2 < N(o-H2CO) < 8.9x10^13 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.