Molecular Gas in the Interstellar Medium of the Milky Way
Introduction
H2 is the most abundant molecule in the universe.
However, we still know only little on its distribution in our Galaxy.
Unlike the CO molecule, from which emission lines are vastly observed
with earthbound radio telescopes in the submillimeter domain of the
spectrum, H2 emission from the cold cores of interstellar
clouds is not observed:
The H2 molecule consists of just two identical hydrogen
atoms, thus it is perfectly symmetric in contrast to, e.g., the CO
molecule.
Due to this structure H2 has no permanent dipole moment,
which prevents it from emitting energy in the form of dipole radiation.
Quadrupole and higher multipole radiation can, in principle, be
observed, but this requires a significantly higher excitation of the
molecule. This is not observed in the cold interstellar gas.
Luckily, the H2 molecule shows a large number of absorption
lines in the far ultraviolet range of the spectrum, at wavelengths
between 90 and 120 nm (or, in the units typically used in astronomy,
900 to 1200 Å).
This region of the electromagnetic spectrum is completely absorbed by
the earth's atmosphere, thus spectra can only be taken from space-bound
observatories like the ORFEUS and the FUSE satellite or the Hubble Space
Telescope.
Observations of interstellar H2 require a background source
with a large photon flux in the ultraviolet.
To examine gas within the Galaxy, young O or early B type stars are
used, in some cases also white dwarfs or Wolf-Rayet stars.
Gas in the Galactic halo and in other galaxies (like the Magellanic
Clouds) can be analysed by using the spectra of distant quasars,
(rare) supernovae and bright star formation regions.
In contrast to emission line spectroscopy, where large areas in the
sky are simply mapped for emission and so provide information on the
extent of the observed matter, absorption line spectroscopy can only
probe the gas along single lines of sight towards point-shaped
background sources.
Information on the radial distribution of the gas along the line of
sight is deduced from the Doppler shift of the observed absorption
line with respect to the transition's rest wavelength.
Information on the distribution of the gas in the sky has to be
derived from absorption towards many closely located background
sources, which is seldomly found.
First observations of interstellar H2 were made with the
Copernicus satellite in the 1970s, mainly from gas in the disk of the
Milky Way.
With the end of the Copernicus mission no observation of molecular
hydrogen was possible until 1996, when the ORFEUS platform was carried
into space by the Space Shuttle.
In two weeks of observing time valuable data was gathered, leading to
important scientific results, among others the detection of molecular
hydrogen in some of the High Velocity Clouds in the Galactic Halo.
Since 1999, the FUSE satellite is collecting data in the far
ultraviolet.
Its mission is planned to last at least for three years.
Far UV spectra, however, do not only provide information on
interstellar H2.
In this wavelength range we observe the full Lyman absorption line
series of the neutral atomic hydrogen and deuterium and a vast amount
of interstellar absorption lines from, e.g., iron, silicon, carbon,
oxygen and many other.
In addition, absorption from the CO molecule is found here.
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