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The Milky Way: an unusual galaxy and an exceptionally calm formation (Forwarded)



 
 
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Old June 21st 07, 04:20 AM posted to sci.space.news
Andrew Yee[_1_]
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Default The Milky Way: an unusual galaxy and an exceptionally calm formation (Forwarded)

Observatoire de Paris
Paris, France

Contact:
Francois Hammer, Observatoire de Paris, GEPI, et CNRS
Tel: 33 1 45 07 74 08
Fax: 33 1 45 07 77 09

15 June 2007

The Milky Way: an unusual galaxy and an exceptionally calm formation

Astrophysicists generally consider our Galaxy as an ordinary spiral galaxy,
and even use its properties to characterize models of numerical simulation
to trace the formation of galaxies at the first epochs of the Universe. This
assumption is today called into question by the last results of a team of
astrophysicists of the GEPI laboratory from the Paris Observatory.

The team of astronomers [1] has just compared our Galaxy with other spiral
galaxies (or galaxies with discs). This shows that for the Milky Way, the
star content and the disc radius are quite lower than than is observed for
the other galaxies. The halo of our Galaxy is also exceptional: its stars
are particularly poor in heavy elements. The team explains these
characteristics by the fact that our Galaxy underwent very few encounters or
mergers with other galaxies since the last 10-11 billion years.

Today, the majority of large galaxies are spiral galaxies, i.e. large discs
in rotation around a bulge of relative small size. For example, our Sun is a
star of the disc of our Galaxy, the Milky Way, orbiting at a speed of 220
km/s around the galactic center.

The galaxies are characterized by fundamental quantities like the rotation
velocity of disc stars around the center, the disc radius and finally the
star content, commonly called "stellar mass". It is rather easy to measure
these quantities for external galaxies, since we have a global vision of
them. The difficulty to make these measurements for the Milky Way is due to
the fact that we live in it: for example, the extinction by interstellar
dust can hide part of the Galaxy and thus distort measurements. Enormous
progress was made these last years, with in particular very detailed
measurements in near and far infrared which are not affected by dust.

The Milky Way has an unusual mass and radius

By comparing these measurements with those done on nearby galaxies, the
astrophysicists of Paris Observatory realized with great surprise that our
Galaxy was rather particular. Indeed, for a given rotational velocity of the
disc, its radius and its stellar mass are twice smaller than the average for
other galaxies. Thus, only 7% of nearby spiral galaxies have properties
similar to the Milky Way. On the other hand, the large Andromeda galaxy has
average properties.

The particularly intact environment of our Galaxy

Spiral galaxies do not contain only a disc and a bulge: they are also
surrounded by a halo, generally known to contain the invisible matter
necessary to their stability. This halo, contains visible matter, including
stars, whose properties are particularly affected by mergers between
galaxies. For example, when a galaxy absorbs another one, the effects of the
collision are so violent that the surroundings are considerably affected by
the tidal debris, and moreover, enriched by the new stars formed in the
merger. Again, the Milky Way is very particular: its environment contains
only old stars, poor in heavy elements, contrary to other galaxies, in
particular Andromeda.

Is the Milky Way particularly favorable to the emergence of life?

We know in addition that the Milky Way did not undergo major mergers with
other galaxies since nearly 11 billion years. We also know that Andromeda
underwent many mergers in a recent past (a few billion years). These recent
mergers "pollute" the environment of the galaxies by dispersed material or
lately formed stars. By generating new stars, the stellar mass and the
radius of galaxies increase. The particular properties of the Milky Way --
small disc radius, low stellar mass, stars little enriched in the halo --
are thus explained by the exceptionally calm history of our Galaxy.

The absence of galaxy encounters during more than ten billion years could be
a condition particularly favorable to the emergence of life. Indeed,
collisions between galaxies are in general very violent, and imply very
powerful emissions of energy (for example supernovae) which are very harmful
to complex molecules at the origin of the living bodies.

[1] The team is composed of: Francois Hammer, Mathieu Puech, Laurent Chemin,
Hector Flores and Matthew Lehnert, GEPI-Observatoire de Paris, unite mixte de
recherche du CNRS, associe a l'Universite Paris Diderot.

Reference

The Milky Way: An Exceptionally Quiet Galaxy; Implications for the formation
of spiral galaxies
Astrophysical Journal 662, p. 322,
http://arxiv.org/abs/astro-ph/0702585

IMAGE CAPTION:
[http://www.obspm.fr/actual/nouvelle/...tomw-f2-en.jpg (106KB)]
Distribution of stellar mass (in billion solar mass) and disc radius (in
units of kilo-parsecs) for large spiral galaxies similar to the Milky Way.
The arrows indicate the position of our Galaxy in these diagrams. It shows
that the star content and the radius of our Galaxy are approximately half of
the average for large galaxies.
 




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