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We present the detection of carbon monoxide molecules (CO) at z=1.6408towards the quasar SDSSJ160457.50+220300.5 using the VLT Ultraviolet and VisualEchelle Spectrograph. CO absorption is detected in at least two components inthe first six A-X bands and one d-X(5-0) inter-band system. This is the seconddetection of this kind along a quasar line of sight. The CO absorption profilesare well modelled assuming a rotational excitation of CO in the range 6<Tex<16K, which is consistent with or higher than the temperature of the CMB radiationat this redshift. We derive a total CO column density of N(CO)=4e14 cm^-2. Themeasured column densities of SI, MgI, ZnII, FeII and SiII indicate a dustdepletion pattern typical of cold gas in the Galactic disc. The backgroundquasar spectrum is significantly reddened (u-K~4.5 mag) and presents apronounced 2175 A dust absorption feature at the redshift of the CO absorber.Using a control sample of ~500 quasars we find the chance probability for thisfeature to be spurious is ~0.3%. We show that the SED of the quasar is wellfitted with a QSO composite spectrum reddened with a LMC supershell extinctionlaw at the redshift of the absorber. It is noticeable that this quasar isabsent from the colour-selected SDSS quasar sample. This demonstrates ourcurrent view of the Universe may be biased against dusty sightlines. Thesedirect observations of carbonaceous molecules and dust open up the possibilityof studying physical conditions and chemistry of diffuse molecular gas in highredshift galaxies.

Diffuse molecular gas at high redshift: Detection of CO molecules and the 2175 {\AA} dust feature at z=1.64