Astrophysics and Space Science Library 418 Eija Laurikainen Reynier Peletier Dimitri Gadotti Editors Galactic Bulges Astrophysics and Space Science Library Volume 418 Editorial Board Chairman W.B. Burton, National Radio Astronomy Observatory, Charlottesville, VA, USA ([email protected]); University of Leiden, The Netherlands ([email protected]) F. Bertola, University of Padua, Italy C.J. Cesarsky, Commission for Atomic Energy, Saclay, France P. Ehrenfreund, Leiden University, The Netherlands O. Engvold, University of Oslo, Norway A. Heck, Strasbourg Astronomical Observatory, France E.P.J. Van Den Heuvel, University of Amsterdam, The Netherlands V. M . K a sp i , McGill University, Montreal, Canada J.M.E. Kuijpers, University of Nijmegen, The Netherlands H. Van Der Laan, University of Utrecht, The Netherlands P.G. Murdin, Institute of Astronomy, Cambridge, UK B.V. Somov, Astronomical Institute, Moscow State University, Russia R.A. Sunyaev, Space Research Institute, Moscow, Russia More information about this series at http://www.springer.com/series/5664 Eija Laurikainen • Reynier Peletier • Dimitri Gadotti Editors Galactic Bulges 123 Editors Eija Laurikainen Reynier Peletier Astronomy and Space Physics Kapteyn Institute University of Oulu University of Groningen Oulu, Finland Groningen, The Netherlands Dimitri Gadotti European Southern Observatory (ESO) Santiago, Chile ISSN 0067-0057 ISSN 2214-7985 (electronic) Astrophysics and Space Science Library ISBN 978-3-319-19377-9 ISBN 978-3-319-19378-6 (eBook) DOI 10.1007/978-3-319-19378-6 Library of Congress Control Number: 2015949217 Springer Cham Heidelberg New York Dordrecht London © Springer International Publishing Switzerland 2016 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. Cover Illustration: In the cover the 3.6 images of one nearby galaxy, NGC 4565, is shown. The images are from the Spitzer Survey of Stellar Structure in Galaxies (PI Kartik Sheth). Printed on acid-free paper Springer International Publishing AG Switzerland is part of Springer Science+Business Media (www.springer.com) Preface The main motivation for this book was our impression that when observers talk about bulges in galaxies, they do not necessarily mean the same thing as people making theoretical models. Even among the observers, it is not always clear what they mean by classical and pseudobulges. According to most researchers, classical bulges are highly relaxed systems typically formed in galaxy mergers or by coalescence of massive gas clumps at high redshifts, whereas pseudobulges have more disc-like properties. Some people divide pseudobulges further into discy pseudobulges, defined as small central discs in the plane of the galactic disc, and boxy/peanut bulges which are actually dynamically heated, vertically thick inner parts of bars. Galaxies can also have central star clusters, usually called nuclear clusters, which structures are not called bulges in this book (see the review by Cole and Debattista). In recent years significant progress has been made both on the theoretical and observational sides, including galactic and extragalactic research. In this book we try to bring all these communities closer to each other. Most probably many of us share the Copernican approach trying to understand the universe with some simple beautiful theory. Such an attempt is the current paradigm of structure formation, i.e., the hierarchical growth of dark matter halos (CDM), and the subsequent formation of bulges and discs from baryonic matter. However, there is always lurking a possibility that something important is hidden in the myriad of details in the observations, which might eventually lead to something unexpected. This book is an attempt to review our current understanding of galactic bulges both from the observational and theoretical points of view, written by specialists in the field. An inspiring historical and philosophical review about the concept of the bulge is given by Barry Madore, who encourages us to refresh our minds to see galactic bulges with open eyes. The main achievements in the field and the outstanding yet unresolved problems are discussed by John Kormendy. One of the problems in the current paradigm is why hierarchical clustering produces so many giant galaxies which have only small bulges or even no bulges at all, and also, why so few of the bulges are classical. Galaxy simulations can convincingly make bulge-less dwarf galaxies, but it is still a puzzle what makes also so many bright disc galaxies almost bulge-less. Echoing the morphology-density v vi Preface relation discovered in the early 1980s, the fraction of classical bulges is a strong function of galaxy environment, i.e., massive bulges appear mainly in galaxy clusters or in small groups of galaxies. Major merger models convincingly explain bulges in the brightest galaxies in the nearby universe, but problems appear when Milky Way mass galaxies, or galaxies having masses lower than that, are considered. There is observational evidence showing that even half of the galaxies at high and moderate galaxy inclinations have boxy/peanut- or X-shape inner morphology, generally associated to bars. This is the case also with the Milky Way (see the reviews by Shen and Li and Gonzalez and Gadotti), where no clear evidence of a classical bulge is seen, or at least its presence is debated. We may even ask whether the central mass concentrations in most of the Milky Way mass galaxies actually form part of a bar? Interestingly, the average surface brightness profiles of field galaxies, up to z D 2, seem to show the astonishing fact that mass is gradually accumulated to the discs at all radial distances from the galaxy center (van Dokkum et al. 2013, ApJL, 771, 35). In this process the Sérsic index of the total surface brightness profile increases, which makes it challenging to distinguish possible merger-built bulges (or galaxies) from those formed by disc instability (or slow accretion events) during the last few gigayears. Needless to say, it would be important to integrate secular evolution into the paradigm of galaxy evolution. In the theoretical models, strong feedback from supernovae is generally used to delay star formation, thus inhibiting the formation of massive bulges. Intriguing reviews in this book are given, by Brooks and Christensen for merger models and by Bournaud for models where bulges form at high redshifts from gas-rich giant clumps, which gradually drift to the central regions of galaxies. A promising way of explaining the observed low mass bulges in the clumpy universe is obtained in models where a balance appears between feedback and gas accretion, occurred via cold flows or wet minor mergers. Such a balance seems to explain well also the observed cosmic star formation rate at different redshifts (Almeida et al. 2014, A&ARv, 22, 71). In the merger models, a promising channel for making low mass bulges is to add a new superbubble feedback mechanism to the models. A manifestation of strong feedback in galaxies is the observed large metal enrichment of the interstellar medium, which is significant not only in galaxies, but quite unexpectedly also in their environments at large distances from the galaxies (Peeples 2015, Nature, 517, 444). Does this mean that feedback is indeed extremely efficient in massive galaxies? Or alternatively, does there exist a large number of unseen, gas depleted dwarfs hidden in the darkness, galaxies which have expelled their metals in supernova explosions to the interstellar medium? Or, should we rather think that the potential bulge mass actually resides in thick discs or small stellar halos? In any case, the puzzle of the observed low mass bulges in the nearby galaxies still remains, in particular because the many processes adding mass to the bulge accumulate during the Hubble time. In the review by Combes, this problem has been approached from the MOND (Modified Newtonian Dynamics with no need for dark matter) point of view, which reduces relaxation and thus also suppresses the formation of classical bulges. As also the dynamical timescale for the giant clumps Preface vii to drift into the central regions of galaxies is increased, the bulges that form are less massive. Although the modelers are handling the issue of structure formation of galaxies in a clever manner, sometimes it remains a problem that the observers are not capable of giving an unambiguous picture of the properties of bulges and discs in the actual universe. In this book we try to take one step forward trying to make order in the wild way of using the different concepts of bulges. Bulges are flux concentrations in the central regions of galaxies, but as they are often divided into classical and pseudobulges, we try to systematize the way of using these concepts. However, a word of warning should be in place here. As discussed by Madore in this book, at the early days of extragalactic research, colorful expressions were used to describe the central flux concentrations in galaxies, without attempting to associate them to any specific physical processes.