We present the results of a 7 mm spectral survey of molecular absorption lines originating in the disk of a z =0.89 spiral galaxy located in front of the quasar PKS 1830 - 211 .
Our survey was performed with the Australia Telescope Compact Array and covers the frequency interval 30–50 GHz , corresponding to the rest-frame frequency interval 57–94 GHz .
A total of 28 different species , plus 8 isotopic variants , were detected toward the south-west absorption region , located about 2 kpc from the center of the z =0.89 galaxy , which therefore has the largest number of detected molecular species of any extragalactic object so far .
The results of our rotation diagram analysis show that the rotation temperatures are close to the cosmic microwave background temperature of 5.14 K that we expect to measure at z =0.89 , whereas the kinetic temperature is one order of magnitude higher , indicating that the gas is subthermally excited .
The molecular fractional abundances are found to be in-between those in typical Galactic diffuse and translucent clouds , and clearly deviate from those observed in the dark cloud TMC 1 or in the Galactic center giant molecular cloud Sgr B2 .
The isotopic ratios of carbon , nitrogen , oxygen , and silicon deviate significantly from the solar values , which can be linked to the young age of the z =0.89 galaxy and a release of nucleosynthesis products dominated by massive stars .
Toward the north-east absorption region , where the extinction and column density of gas is roughly one order of magnitude lower than toward the SW absorption region , only a handful of molecules are detected .
Their relative abundances are comparable to those in Galactic diffuse clouds .
We also report the discovery of several new absorption components , with velocities spanning between - 300 and +170 km s ^ { -1 } .
Finally , the line centroids of several species ( e.g. , CH _ { 3 } OH , NH _ { 3 } ) are found to be significantly offset from the average velocity .
If caused by a variation in the proton-to-electron mass ratio \mu with redshift , these offsets yield an upper limit | \frac { \Delta \mu } { \mu } | < 4 \times 10 ^ { -6 } , which takes into account the kinematical noise produced by the velocity dispersion measured from a large number of molecular species .