SIMBAD references

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 microwave background, 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/pc, we obtain a molecular hydrogen number density of 2x10³/cm3 < n(H₂) < 1x10⁴/cm3 and a column density of 2.5x10¹³/cm2 < N(o-H₂CO) < 8.9x10¹³/cm2, where the allowed ranges conservatively include the range of possible source covering factors in both lines. The measurements suggest that H₂ CO 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 cosmic microwave background if it were not illuminated by the background quasar or if it were viewed from another direction.