Molecules may be seen as playing two essential roles in the Universe: as
the cosmic material which forms stars; and as a first step in building
complexity which culminates in life. For the second aspect of
astrobiology, it is obvious that, because of the enormous distances,
galaxies are not the best hunting ground to try to understand the
numerous and complex steps which could lead from the simple interstellar
molecules to the genetic code. On the other hand, the panoramic views
provided by molecular signals from galaxies are one of the best sources
of information to grasp the essential features of star formation
at galactic scales. CO is the best tracer of normal, cold molecular
gas. In various types of local galaxies, its millimetre lines reveal the
standard giant molecular clouds, home of `normal' massive star formation
such as in most of the Milky Way. CO lines also deeply probe the most
extreme, massive, dusty starbursts, especially at high redshift, without
being blocked and much distorted by extinction. CO and better HCN line
intensities yield good estimates of the global star formation rates. The
high resolution velocity profiles of CO lines, routinely offered by
heterodyne techniques, provide molecular and dynamical masses of
starburst galaxies, key parameters for learning about their past and
future star formation history. The great news is that such a
comprehensive information about star formation is available not only for
our local neighbours, but more and more up to the most distant galaxies
that we see in their youth and infancy. This will be soon extended by
ALMA at z
10, practically
up to the frontier of the `dark ages' when the first stars and galaxies
formed and the Universe was only a few percents of its present age. We
are indeed living the dream of the first explorers of stellar
populations in the Milky Way, by directly catching the action of
successive starbursts which formed most of the stars in Milky Way
sisters at various epochs of their past life up to z
2. Even farther
out in space and time, through molecular lines, we watch and get insight
in the most powerful starbursts in the Universe, progenitors of the most
massive (elliptical) galaxies today. Disentangling their history is a
basic clue for understanding the joint evolution of the most fascinating
objects in the Universe: their massive dark matter halos, their stellar
spheroids, their central super-massive black-holes, and the massive
clusters of galaxies surrounding today the most massive ones. However,
the power of molecular lines to trace star formation in galaxies is far
from being limited to such climaxes, but it is almost universal even for
much milder episodes in various kinds of galaxies. Even much less
spectacular, some of these episodes may be of great importance for their
hosts, either dwarf such as the Magellanic Clouds, or giant such as
elliptical galaxies harbouring cooling flows. While CO traces the cold
bulk of molecular gas, observations of rotational lines of H2
itself are spectacularly progressing with the development of space
mid-infrared spectroscopy. They trace patches of warm gas in
particularly active regions of starbursts, mergers, vicinities of AGN,
and various types of shocks even at extra-galactic scales. CO and
H2 lines should thus be major tracers of violent events in
galaxy lifes, such as mergers, accretion shocks, outflows and other
effects of feedback action from supernovae and AGN.
Star formation is not by far the only fascinating question of
contemporary astronomy that the young extragalactic molecular
astrophysics may address. Large aromatic molecules (PAHs) are recognized
as an ubiquitous major component and a reservoir of carbon of the
interstellar medium of most types of galaxies. They are now currently
studied up to redshift at least ~ 2 in the most massive galaxies; but we
have still to learn about their behaviour in the first billion years of
galaxy lives. Similarly, little is known about molecular mantles in
strong starbursts and at high redshift. More generally, all processes of
dust surface chemistry, including H2 formation, remain
challenging. The extragalactic chemistry of the deuterium is still in
its infancy, together with the information it can bring about the
dust-gas cycle in various galactic environments. If the current results
about extragalactic interstellar chemistry appear a bit disapointing as
too similar to that of the Milky Way or less rich, it could be due to
our lack of sensitivity, and a reassessment with ALMA is obviously
necessary. In any case, the progress expected in isotopic ratio
determination will be important for a better appreciation of
nucleosynthesis and the chemical evolution of various types of
galaxies. Tracing molecules in AGN, in the vicinity of the nucleus, up
to the molecular torus, is one of the fields which will most benefit
from the gains in sensitivity and angular resolution from space or with
millimetre and radio interferometry and adaptive optics. This will
provide unique opportunities to probe X-ray dominated chemistry and the
special properties of the molecular gas in the `molecular torus'. VLBI
detections of H2O mega-masers already probe the even inner
region of the accretion disk, offering a unique way of accurate
determination of the mass of the super-massive black hole. Their
expected extension at high redshift is very promising for a direct
accurate calibration of the cosmic distance scale, and in particular the
precise measurement of H0. More generally extragalactic OH
and H2O mega-masers provide
unique examples of radiation amplification with the most extreme cosmic
scales and energies. Fully appreciating the properties of mega-masers
remains challenging, as well as extending their systematic detection in
the most powerful starbursts at high redshift. Molecules, such as
H2 and OH, also participate in high precision spectroscopy of
absorption lines at high redshift, allowing one to probe possible
variations of fundamental constants such as me /
mp and 
.
The multiplication of high-z background sources, especially gamma-ray
bursts, will provide new opportunities, as well as probing molecules at
the interface between (proto-) galaxies and the intergalactic medium.
It is clear that the vast and rich field of molecules in galaxies will see many exciting developments in the next years and importantly contribute to the progress of the exploration of the world of galaxies and their evolution.
Acknowledgements
I owe a great debt of gratitude to Al Glassgold and Pierre Cox for their past and present close collaboration, their careful reading of the whole manuscript and numerous helpful comments and suggestions. I specially thank James Lequeux and Françoise Combes, as well as two anonymous referees, for numerous helpful comments and suggestions. I also want to thank François Boulanger, Asunciòn Fuente, George Helou, Michèle Leduc, Gary Mamon, David Merritt, Padelis Papadopoulos, Patrick Petitjean, Hélène Roussel and Phil Solomon for various help and comments.