We have detected the J =4←3 rotational transition of 12 CO in absorption at z = 0.89 toward the quasar PKS 1830-211, but not the 12 CO (5←4) or the 3 P 1 ← 3 P 0 fine-structure line of neutral carbon. The intervening molecular medium thus has a total 12 CO column density of 10 18 cm −2 ≤ N (CO)≤5 × 10 18 cm -2 with a most likely value of N (CO)≃2 × 10 18 cm -2 , which corresponds to the large column density of molecular hydrogen of N (H 2 )=2.5 × 10 22 cm -2 and a reddening of A v = 25 mag. The 12 CO 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)≤10 18 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 13 CO (4←3) line, but for H 2 O, 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 13 CO J =4←3 line implies that the 13 CO excitation temperature is lower than 20 K and the column density is fairly large, 4 × 10 16 cm −2 ≤ N (CO 13 )≤2.2 × 10 17 cm -2 , with a likely value of N (CO 13 )≃10 17 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 (H 2 )≥2 × 10 22 cm -2 , which corresponds to a reddening larger than 20 mag.
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A 1997 study studied this question.