The inhomogeneous ISM toward PKS1830-211 SW - A detailed view on
  molecular gas at a look-back time of 7.5 Gyr
  Based on measurements with the Effelsberg 100-m telescope, a multi-line study
of molecular species is presented toward the south-western source of the
gravitational lens system PKS 1830-211, which is by far the best known target
to study molecular gas in absorption at intermediate redshift. Determining line
parameters and optical depths and performing Large Velocity Gradient radiative
transfer calculations, the aims of this study are (1) to evaluate physical
parameters of the absorbing foreground gas at z~0.89, in particular its
homogeneity, and (2) to monitor the spectroscopic time variability caused by
fluctuations of the z~2.5 background continuum source. We find, that the gas is
quite inhomogeneous with n(H2)~2 x 10^3 cm^-3 for most molecular species but
with higher values for H2CO and lower ones for SO. Measuring the CS J=1-0
transition during a time interval of more than a decade, from 2001 to 2012, the
peak absorption depth of the line remains approximately constant, while the
line shape undergoes notable variations. Covering the time between 1996 and
2013, CS, HCO+, and CH3OH data indicate maximal integrated optical depths in
~2001 and 2011/2012. This is compatible with a ~10 yr periodicity, which,
however, needs confirmation by substantially longer time monitoring. Comparing
molecular abundances with those of different types of Galactic and nearby
extragalactic clouds we find that the observed cloud complex does not
correspond to one particular type but to a variety of cloud types with more
diffuse and denser components as can be expected for an observed region with a
transverse linear scale of several parsec and a likely larger depth along the
line-of-sight. A tentative detection of Galactic absorption in the c-C3H2
1(10)-1(01) line at 18.343 GHz is also reported.
