Redshifted molecular absorption systems towards PKS1830-211 and B0218+357: submillimeter CO, CI and H2O data

Abstract

We have detected the J=4-3 rotational transition of 12CO in absorption at z=0.89 towards the quasar PKS1830-211, but not the 12CO 5-4 or the 3P1-3P0 fine structure line of neutral carbon. The intervening molecular medium thus has a total 12CO column density of N(CO)~2x10^18 cm^-2, which corresponds to the large column density of molecular hydrogen of N(H2)=2.5x10^22 cm^-2 and a reddening of Av=25 magnitudes. The 12CO excitation temperature is low, below 15 K. Comparison with the molecular absorption results of Wiklind and Combes (1996) shows that the absorbing material has similar molecular abundances to Galactic dark clouds. We find an upper limit for atomic carbon of N(CI)<10^18 cm^-2, which again would be the case for most Galactic dark clouds. We also report new observations of the absorbing system towards 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, due to 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 20K and the column density is fairly large, N(13CO)~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(H2)>2x10^22 cm^-2, corresponding to a reddening >20 magnitudes.