Redshifted molecular absorption systems toward PKS 1830-211 and B0218+357: Submillimeter CO, C I, and H₂O data

We have detected the J = 4 ← 3 rotational transition of ¹²CO in absorption at z = 0.89 toward the quasar PKS 1830-211, but not the ¹²CO (5 ← 4) or the ³/₁ ← ³/P₀ fine-structure line of neutral carbon. The intervening molecular medium thus has a total ¹²CO column density of 10¹⁸ cm^-2 ≤ N(CO) ≤ 5 × 10¹⁸ cm^-2 with a most likely value of N(CO) ≃ 2 × 10^-8 cm^-2, which corresponds to the large column density of molecular hydrogen of N(H₂) = 2.5 × 10²² cm^-2 and a reddening of A_ν. = 25 mag. The ¹²CO excitation temperature is low, below 15 K. Comparison with existing molecular absorption results shows that the absorbing material has molecular abundances similar to Galactic dark clouds. We find an upper limit for atomic carbon of N(C I) ≤ 10¹⁸ cm^-2, which again would be the case for most Galactic dark clouds. We also report new observations of the absorbing system toward B0218+357 at z = 0.68. We have tentatively detected the ¹³CO (4 ← 3) line, but for H₂O, although a feature is seen at the correct velocity, because of the inadequate signal-to-noise ratio we report only an upper limit for the fundamental line of ortho-water vapor. The tentative detection of the ¹³CO J = 4 ← 3 line implies that the ¹³CO excitation temperature is lower than 20 K and the column density is fairly large, 4 × 10¹⁶ cm^-2 ≤ N (CO¹³) ≤ 2.2 × 10¹⁷ cm^-2, with a likely value of N(CO¹³) ≃ 10¹⁷ cm^-2, giving rise to saturated absorption in the J = 2 ← 1 transition. The total column density of molecular gas is again large in this source, N(H₂) ≥ 2 × 10²² cm^-2, which corresponds to a reddening larger than 20 mag.