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SCIENCES

Universe , Field Director – Fabienne Casoli

Galaxies , Subject Head – Françoise Combes

Coordinated by

Françoise Combes

First published 2020 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

ISTE Ltd

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John Wiley & Sons, Inc.

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© ISTE Ltd 2020

The rights of Françoise Combes to be identified as the author of this work have been asserted by her in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2020942773

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN 978-1-78945-012-5

ERC code:

PE9 Universe Sciences

PE9_8 Formation and evolution of galaxies

PE9_9 Clusters of galaxies and large scale structures

The galaxies were not identified as worlds apart, as the philosopher Immanuel Kant foresaw, until the 1920s, just over a century ago. Before then, astronomers had observed nebulae, but had not distinguished between a cloud of emitting gas, such as the Orion Nebula, and outer galaxies, such as the Andromeda Nebula. At the beginning of the 20th Century, a great debate took place to know the size of our world (the Milky Way), and the distance of the various stars in the sky. In 1924, Edwin Hubble observed the variable stars in Andromeda of the Cepheids, which Henrietta Leavitt in 1909 had shown to be a good indicator of distance. Thus, it was shown that Andromeda was a galaxy “outside” ours, located about 2 million light-years away.

Progress since then has been meteoric. We now know millions of galaxies, and determine their distance because of the expansion of the Universe and the corresponding redshift. Due to the finite speed of light, we can go back in time, observing distant galaxies in their youth. We thus observe galaxies as far as the edge of our Universe, at the limit of our horizon, which allows us to reconstruct their history.

In this book, we will first describe the various morphologies and categories of galaxies, which are essential for a better understanding of their formation and evolution. There are several classifications, depending on whether we consider the various stellar components: bulge, disc, spheroid, photometry and more or less blue colors according to the rate of star formation, or the kinematics of stars, that is to say a kinetic energy either dominated by rotation, or by the velocity of dispersion, or the fraction of gas. Disc galaxies generally contain spiral or barred structures, which are the engine of evolution. These structures give rise to resonances which will be the source of rings, or pseudo-rings, very useful to know the speed of spirals and bars.

Our Galaxy, the Milky Way, the best known and most familiar, has a barred spiral structure, yet it took a long time to be identified, because we are in its plane: it appears edge-on, obscured by dust lanes. Its structure appears more clearly in the near infrared. It consists of a thin disc, where the gas and young stars are located, and a thick disc, which dates back billions of years. It has a pseudobulge, mainly due to the bar, and its vertical resonance. There is also a more or less spherical, diffuse halo of stars, whose formation would be essentially due to the accretion of small satellite galaxies. These are destroyed by tidal interaction, and deploy in a multitude of stellar streams. We can go back to the history of the formation of the different components, by galactic archaeology, by determining the age and metallicity of stars. The GAIA astrometric satellite has recently made enormous progress in specifying the distances and proper motions of a large number of webs.