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TwitterDOI: 10.1051/0004-6361/200811475
The density, the cosmic microwave background, and the proton-to-electron mass ratio in a cloud at redshift 0.9
C. Henkel1, K. M. Menten1, M. T. Murphy2, N. Jethava3, V. V. Flambaum4, J. A. Braatz5, S. Muller6, 7, J. Ott5, 8, and R. Q. Mao91 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: chenkel@mpifr-bonn.mpg.de
2 Centre for Astrophysics and Supercomputing, Swinburne University, PO Box 218, Victoria 3122, Australia
3 National Institute of Standards and Technology, M.S. 817.03, 325 Broadway, 80305 Boulder, USA
4 School of Physics, University of New South Wales, Sydney, N.S.W. 2052, Australia
5 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
6 Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23–141, Taipei, 106, Taiwan
7 Onsala Space Observatory, 439–92 Onsala, Sweden
8 CalTech, 1200 E. California Blvd., Caltech Astronomy, 105–24, Pasadena, CA 91125–2400, USA
9 Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China
Received 4 December 2008 / Accepted 26 March 2009
Abstract
Based on measurements with the Effelsberg 100-m telescope, a multi-line study of molecular species is presented
toward the gravitational lens system PKS 1830–211, which is by far the best known target to study dense cool gas
in absorption at intermediate redshift. Determining average radial velocities and performing Large Velocity Gradient
radiative transfer calculations, the aims of this study are (1) to determine the density of the gas, (2) to constrain
the temperature of the cosmic microwave background (CMB), and (3) to evaluate the proton-to-electron mass ratio at
redshift
. Analyzing data from six rotational HC3N transitions (this includes the
line, which is likely detected for the first time in the interstellar medium) we obtain n(H2) ~ 2600 cm-3
for the gas density of the south-western absorption component, assuming a background source covering factor, which
is independent of frequency. With a possibly more realistic frequency dependence proportional to 
(the maximal
exponent permitted by observational boundary conditions), n(H2) ~ 1700 cm-3. Again toward the south-western
source, excitation temperatures of molecular species with optically thin lines and higher rotational constants are,
on average, consistent with the expected temperature of the cosmic microwave background,
K.
However, individually, there is a surprisingly large scatter which far surpasses expected uncertainties. A comparison
of CS
and
optical depths toward the weaker north-western absorption component
results in
K and a 1-
error of 3 K. For the main component, a comparison of velocities
determined from ten optically thin NH3 inversion lines with those from five optically thin rotational transitions
of HC3N, observed at similar frequencies, constrains potential variations of the proton-to-electron mass ratio 
to
with 3- confidence. Also including optically thin rotational lines
from other molecular species, it is emphasized that systematic errors are
km s-1, corresponding
to
.
Key words: galaxies: abundances -- galaxies: ISM -- galaxies: individual: PKS 1830-211 -- gravitational lensing -- radio lines: galaxies -- elementary particles
© ESO 2009