Galaxies. Группа авторов

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      SCIENCES

      Universe, Field Director – Fabienne Casoli

      Galaxies, Subject Head – Françoise Combes

      Galaxies

       Formation and Evolution

       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

      27-37 St George’s Road

      London SW19 4EU

      UK

       www.iste.co.uk

      John Wiley & Sons, Inc.

      111 River Street

      Hoboken, NJ 07030

      USA

       www.wiley.com

      © 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

      Introduction

      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.

      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.

      Galaxies can be divided into two main categories: disc galaxies or “late-type” and elliptical galaxies or “early-type”. In our Local Universe, the majority of galaxies are disc galaxies and dominate in number, but since elliptical galaxies are more massive, they dominate in mass in the Universe. Early-type galaxies have a spheroidal morphology, and even if they sometimes have a stellar disc, it is subdominant in front of the massive bulge. Their stellar populations are old, and they have little or no gas. For a very long time, astronomers thought that the flattening of stellar spheroids was due to rotation, but when it became possible to measure their velocities, it was realized that the shapes of these spheroids were instead due to the anisotropy of the velocity dispersion: ellipticals have no or very little rotation. It is then difficult to deproject these systems, which can be triaxial. They are classified as slow or fast rotators. The origin of these differences comes from their history of formation by merger of spiral galaxies.

      Interactions between galaxies contribute to the enrichment of galaxies with gas, which can create a starburst. Interactions are visible even at large distances because of tidal arms and morphological perturbations of galaxies. Of course, these effects are all the more visible in groups and clusters of galaxies. In the latter, galaxies are swept away from their gas by the ram pressure. Looking back in time, we can see that the number of interacting galaxies was greater


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