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The morphologies of
galaxies are not unchanging. They have evolved since they were formed
and are continuing to evolve today. The presence in certain galaxies of
characteristic structures known as "bars" would suggest that
galaxies have not evolved as isolated systems, but rather they have interacted
strongly with their environment. Researchers from the Laboratoire d'étude
du rayonnement et de la matière en astrophysique (Laboratory for
research into radiation and matter in astrophysics, Observatoire de Paris
- CNRS) have just shown that bars observed in two-thirds of galaxies result
from large quantities of gas being accreted by the galaxies all through
their life, the gas coming from the intergalactic medium.
Most galaxies are spiral galaxies
made up of a rotating thin disk over which spiral arms extend, and a central
spheroid bulge. We distinguish between two main categories of spiral galaxies
depending on whether or not they have bars. A galactic bar is an elongate
dense structure which lies at the center of the disk and which is made
up of stars and of interstellar gas. Two-thirds of spiral galaxies are
barred to various extents, and one third have no bars.
Bars are instabilities that appear naturally in a rotating self-gravitating
disk such as a spiral galaxy disk. The presence of a bar then disturbs
the dynamic evolution of the disk. In particular, its symmetry is broken
by the bar. The resulting torsion torque pushes the gas initially situated
at a major radius towards the outside of the disk, whereas the gas initially
situated at a minor radius falls into the center of the disk. The build-up
of gas at the center of the disk triggers a surge of star formation, and
above all progressively weakens the bar. The increase in the central mass
leads to enrichment of the bulge. The disk is then less subjected to its
own gravity than to the gravity of the bulge, which stabilizes it and
ultimately makes it non-barred. Bars thus self-destruct, and they disappear
after about 2 to 6 billion years.
It is then surprising that two-thirds of the galaxies close to our own,
and at least about ten billion years old, are barred galaxies. We would
expect that most of the bars would already have been destroyed, and thus
that we would observe a large majority of spiral galaxies that are not
barred. One possible explanation is that, after self-destructing, the
bars have formed again.
If galaxies are considered to be isolated, with no interaction with their
outside environment, no mechanism could explain how the bars could form
again. The disk remains dominated by the bulge, and is therefore stable.
In addition, the central movements are very disorderly after the first
bar is destroyed, which contributes to stabilizing the disk and to preventing
the appearance of a new bar.
Assuming that galaxies accrete large quantities of gas throughout their
lives, it is possible to explain how new galaxy bars are formed. The accreted
gas increases the mass of the disk relative to the bulge, and makes the
movements at the center of the disk more orderly. The two conditions required
for the disk to become unstable again and for a new bar to form are thus
satisfied. Numerical simulations have been used to show that the disk
then develops a second bar. In turn, the second bar self-destructs, and
then a third bar forms. The large number of bars observed today would
thus be the result of bars forming again after the original bars have
self-destructed.
It is possible to measure the strength of the bars in the galaxies observed
in the infrared domain. The statistical distribution of the bar strengths
was determined over a sample of 163 galaxies. The resulting histogram
shows that very weak bars are rare; most of the galaxies have bars that
are quite strong. The precise shape of this histogram constitutes a test
for the scenario whereby new bars are formed by gas accretion. We already
known that this mechanism makes it possible to form bars, but is that
really how bars have evolved in the Universe? Numerical simulations of
gas accretion have made it possible to synthesize a new histogram representing
bar strength distribution in galaxies. In the presence of gas accretion,
the observational result is indeed reproduced by these simulations, provided
that the right rate of accretion is used. This shows that this scenario
for the evolution of galaxies corresponds to reality. Interpretation of
these observations also indicates that the galaxies double their mass
by gas accretion within ten billion years, i.e. less than the age of the
Universe. Conversely, numerical simulations that do not take account of
gas accretion by galaxies do not reproduce observations of bars.
Bars are thus tracers of the evolution of galaxies. The fact that most
galaxies are barred is a visible result of non-observed continual gas
accretion which leads galaxies to double their mass in ten billion years.
Galaxies are thus far from being isolated systems. On the contrary, they
evolve according to their environments. We have known for a long time
that they sometimes interact with one another, and they now appear to
be systems that are continuing to form today by continually accreting
the gas that surrounds them.
References
F. Bournaud, F. Combes,A&A 392 (September II 2002), 83-102
D. L. Block, F. Bournaud, F. Combes, I. Puerari and R. Buta, A&A 394
(November II 2002), L35-L38 (Section 'Letters')
Researcher contact:
Françoise Combes
Laboratoire d’étude du rayonnement et de la matière
en astrophysique ( Observatoire de Paris – CNRS)
Tel: +33 1 40 51 20 77
e-mail: Francoise.Combes@obspm.fr
Frédéric Bournaud
Laboratoire d’étude du rayonnement et de la matière
en astrophysique ( Observatoire de Paris – CNRS)
Tel: +33 1 40 51 20 61
e-mail: Frederic.Bournaud@obspm.fr
CNRS-INSU
contact:
Philippe Chauvin
Tel: +33 1 44 96 43 36 ;
e-mail: Philippe.Chauvin@cnrs-dir.fr
Press contact
:
Carine Noël
Tel: +33 1 44 96 49 88
Fax: +33 1 44 96 49 93
e-mail: carine.noel@cnrs-dir.fr
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