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

TL;DR

This paper reports the first detection of CO molecules and the 2175 Å dust feature at high redshift (z=1.64), revealing the presence of diffuse molecular gas and dust in the early universe, and highlighting potential observational biases.

Contribution

It presents the second detection of CO molecules at high redshift and characterizes the dust extinction, providing insights into the physical conditions of diffuse molecular gas at z=1.64.

Findings

Detection of CO molecules at z=1.64 with multiple absorption components.

Identification of the 2175 Å dust feature in the quasar spectrum.

Evidence that dusty sightlines may be underrepresented in current surveys.

Abstract

We present the detection of carbon monoxide molecules (CO) at z=1.6408 towards the quasar SDSSJ160457.50+220300.5 using the VLT Ultraviolet and Visual Echelle Spectrograph. CO absorption is detected in at least two components in the first six A-X bands and one d-X(5-0) inter-band system. This is the second detection of this kind along a quasar line of sight. The CO absorption profiles are well modelled assuming a rotational excitation of CO in the range 6<Tex<16 K, which is consistent with or higher than the temperature of the CMB radiation at this redshift. We derive a total CO column density of N(CO)=4e14 cm^-2. The measured column densities of SI, MgI, ZnII, FeII and SiII indicate a dust depletion pattern typical of cold gas in the Galactic disc. The background quasar spectrum is significantly reddened (u-K~4.5 mag) and presents a pronounced 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 this feature to be spurious is ~0.3%. We show that the SED of the quasar is well fitted with a QSO composite spectrum reddened with a LMC supershell extinction law at the redshift of the absorber. It is noticeable that this quasar is absent from the colour-selected SDSS quasar sample. This demonstrates our current view of the Universe may be biased against dusty sightlines. These direct observations of carbonaceous molecules and dust open up the possibility of studying physical conditions and chemistry of diffuse molecular gas in high redshift galaxies.