Extragalactic astronomy and cosmology

Terry D. Oswalt
Editor-in-Chief
William C. Keel
Volume Editor

Planets, Stars and
Stellar Systems
volume 6

Extragalactic Astronomy
and Cosmology


Planets, Stars and Stellar Systems
Extragalactic Astronomy and Cosmology



Terry D. Oswalt (Editor-in-Chief )
William C. Keel (Volume Editor)

Planets, Stars and
Stellar Systems
Volume 6:
Extragalactic Astronomy and
Cosmology
With 314 Figures and 12 Tables


Editor-in-Chief
Terry D. Oswalt

Department of Physics & Space Sciences
Florida Institute of Technology
University Boulevard
Melbourne, FL, USA
Volume Editor
William C. Keel
Department of Physics and Astronomy
University of Alabama
Tuscaloosa, AL, USA

ISBN 978-94-007-5608-3
ISBN 978-94-007-5609-0 (eBook)
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DOI 10.1007/978-94-007-5609-0
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Series Preface
It is my great pleasure to introduce “Planets, Stars, and Stellar Systems” (PSSS). As a “Springer
Reference”, PSSS is intended for graduate students to professionals in astronomy, astrophysics
and planetary science, but it will also be useful to scientists in other fields whose research interests overlap with astronomy. Our aim is to capture the spirit of 21st century astronomy – an
empirical physical science whose almost explosive progress is enabled by new instrumentation,
observational discoveries, guided by theory and simulation.
Each volume, edited by internationally recognized expert(s), introduces the reader to a
well-defined area within astronomy and can be used as a text or recommended reading for
an advanced undergraduate or postgraduate course. Volume 1, edited by Ian McLean, is an
essential primer on the tools of an astronomer, i.e., the telescopes, instrumentation and detectors used to query the entire electromagnetic spectrum. Volume 2, edited by Howard Bond, is a
compendium of the techniques and analysis methods that enable the interpretation of data collected with these tools. Volume 3, co-edited by Linda French and Paul Kalas, provides a crash
course in the rapidly converging fields of stellar, solar system and extrasolar planetary science.
Volume 4, edited by Martin Barstow, is one of the most complete references on stellar structure
and evolution available today. Volume 5, edited by Gerard Gilmore, bridges the gap between
our understanding of stellar systems and populations seen in great detail within the Galaxy
and those seen in distant galaxies. Volume 6, edited by Bill Keel, nicely captures our current
understanding of the origin and evolution of local galaxies to the large scale structure of the
universe.

The chapters have been written by practicing professionals within the appropriate subdisciplines. Available in both traditional paper and electronic form, they include extensive
bibliographic and hyperlink references to the current literature that will help readers to acquire a
solid historical and technical foundation in that area. Each can also serve as a valuable reference
for a course or refresher for practicing professional astronomers. Those familiar with the “Stars
and Stellar Systems” series from several decades ago will recognize some of the inspiration for
the approach we have taken.
Very many people have contributed to this project. I would like to thank Harry Blom and
Sonja Guerts (Sonja Japenga at the time) of Springer, who originally encouraged me to pursue this project several years ago. Special thanks to our outstanding Springer editors Ramon
Khanna (Astronomy) and Lydia Mueller (Major Reference Works) and their hard-working editorial team Jennifer Carlson, Elizabeth Ferrell, Jutta Jaeger-Hamers, Julia Koerting, and Tamara
Schineller. Their continuous enthusiasm, friendly prodding and unwavering support made this
series possible. Needless to say (but I’m saying it anyway), it was not an easy task shepherding
a project this big through to completion!
Most of all, it has been a privilege to work with each of the volume Editors listed above and
over 100 contributing authors on this project. I’ve learned a lot of astronomy from them, and I
hope you will, too!

January 2013

Terry D. Oswalt
General Editor



Preface to Volume 6
Intentionally, this compendium invites contrast with the similarly titled volume from 1975,
edited by Alan and Mary Sandage and Jerome Kristian. They describe its view as largely
a product of the late 1960s. Our understanding of galaxies then, despite the already enormous observational and theoretical effort summarized, now seems woefully incomplete, largely
because we stand on the shoulders of technological innovators. We were then at the very dawn
of X-ray observations of galaxies and clusters, and likewise our knowledge of the radio structures of galaxies and AGN was poised for dramatic improvements in quality and quantity. AGN
were still a novelty whose connection to “ordinary” galaxies was almost unknown; the model

of energy release during accretion into massive black holes cold not yet be clearly formulated.
As far as we knew then galaxy masses were baryonic, and there was something suspicious about virial mass estimates for galaxy clusters. Photometry of galaxies remained a tedious
project, either using photomultipliers or the black art of calibrated photographic photometry.
Digital instrumentation for optical astronomy was just beginning to appear; the basic techniques of observation had advanced only incrementally for many years. One catalog volume
could still contain essentially all the photometric and redshift data ever obtained for galaxies.
We knew almost nothing about the evolution of galaxies; the relevant observations remained
closely mixed with the quest for the fundamental parameters of cosmology.
Since that volume appeared (during my undergraduate years), our view of galaxies and their
context has broadened dramatically, sometimes in ways scarcely conceivable then.
It seems appropriate to contrast many of these views to our current picture. Not only can we
trace the history of star formation (galaxy evolution seen in the act) across cosmic time, but we
can address this question with constraints all the way from the X-ray to radio regimes, coupling
direct detection of young stellar populations with the secondhand emission from dust grains
and supernova products.
We continue to find value in the galaxy categories bequeathed by Edwin Hubble, Alan
Sandage, and Sidney van den Bergh, but now extend the study of galaxy structure across cosmic time and wavelength. Connections emerge (some hotly debated) between details of galaxy
structure and events in galaxy history. Again and again, crucial observable properties of galaxies
are seen to be driven by dark matter, whose properties are being narrowed by such techniques
as gravitational lensing.
With the finding of supermassive black holes (and thus potentially “dead quasars”) in most
luminous galaxies, we seem to have the answer to a question posed by Joe Miller decades ago-are
quasars important or merely interesting? If galaxies are important, so are quasars.
Our understanding of clusters and larger-scale structures has been revolutionized, both with
the finding that the hot intracluster medium carries more mass than the stars, and with broad
surveys showing statistical properties of superclusters, voids, and filaments (it is gratifying to
note that one figure in this volume shows a reprocessing of the same galaxy counts shown in
contour form by C.D. Shane in 1975). Further, we are starting to fill in physical detail as to the
ways galaxies are affected by their environments. At the level of detail we can now reach, no
galaxy is really an island Universe.

viii

Preface to Volume 6

The cosmic distance scale remains important, but we are in a very different stage. A web of
interlocking as well as independent measurements has narrowed the value of the local Hubble
constant to a few per cent, so that local motions superimposed on the expansion are measurable.
We are at a key juncture in cosmology. Recent results have dramatically narrowed the values
of the Hubble constant, mass density, and fluctuation amplitude in the early Universe, with
further refinements expected soon from the Planck mission. Fine structure in the microwave
background radiation encodes a rich range of both physical and astrophysical processes. Yet
we still only anticipate a physical understanding of whatever causes the acceleration of cosmic
expansion indicated by multiple techniques. Such terms as dark energy, cosmological constant,
and quintessence at this point serve mostly to organize our ignorance.
Likewise, with new facilities capable of narrowing the observational Dark Ages, we see
promise of adding new bodies of data to underpin our understanding of galaxy formation, and
the first stars, and the growth of black holes in the early Universe.
All readers of this collection owe a debt to the community spirit of the authors, who have
invested so much time and effort into making their contributions. I hope that this collection
shares with its predecessor a long useful life for many chapters, but also the scientific joy of
being overtaken in some aspects by utterly unexpected discoveries.

William C. Keel
Tuscaloosa, Alabama
USA


Editor-in-Chief
Dr. Terry D. Oswalt

Department Physics & Space Sciences
Florida Institute of Technology
150 W. University Boulevard
Melbourne, Florida 32901
USA
E-mail:

Dr. Oswalt has been a member of the Florida Tech faculty since 1982 and was the first professional astronomer in the Department of Physics and Space Sciences. He serves on a number of
professional society and advisory committees each year. From 1998 to 2000, Dr. Oswalt served
as Program Director for Stellar Astronomy and Astrophysics at the National Science Foundation. After returning to Florida Tech in 2000, he served as Associate Dean for Research for the
College of Science (2000–2005) and interim Vice Provost for Research (2005–2006). He is now
Head of the Department of Physics & Space Sciences. Dr. Oswalt has written over 200 scientific
articles and has edited three astronomy books, in addition to serving as Editor-in-Chief for the
six-volume Planets, Stars, and Stellar Systems series.
Dr. Oswalt is the founding chairman of the Southeast Association for Research in Astronomy (SARA), a consortium of ten southeastern universities that operates automated 1-meter
class telescopes at Kitt Peak National Observatory in Arizona and Cerro Tololo Interamerican
Observatory in Chile (see the website www.saraobservatory.org for details). These facilities,
which are remotely accessible on the Internet, are used for a variety of research projects by
faculty and students. They also support the SARA Research Experiences for Undergraduates
(REU) program, which brings students from all over the U.S. each summer to participate oneon-one with SARA faculty mentors in astronomical research projects. In addition, Dr. Oswalt
secured funding for the 0.8-meter Ortega telescope on the Florida Tech campus. It is the largest
research telescope in the State of Florida.
Dr. Oswalt’s primary research focuses on spectroscopic and photometric investigations of
very wide binaries that contain known or suspected white dwarf stars. These pairs of stars, whose
separations are so large that orbital motion is undetectable, provide a unique opportunity to
explore the low luminosity ends of both the white dwarf cooling track and the main sequence;
to test competing models of white dwarf spectral evolution; to determine the space motions,
masses, and luminosities for the largest single sample of white dwarfs known; and to set a lower
limit to the age and dark matter content of the Galactic disk.

Volume Editor
Dr. William C. Keel
Department of Physics and Astronomy
University of Alabama
Box 870324
206 Gallalee Hall
Tuscaloosa, AL 35487
USA

William C. Keel is Professor of Astronomy at the University of Alabama in Tuscaloosa. His
astronomical interests began as a youngster using a secondhand reflector in the back yard, and
he remains active as an amateur as well as professional astronomer. His undergraduate work
was at Vanderbilt University, followed by a Ph.D. from the University of California, Santa Cruz.
Dr. Keel had postdoctoral positions at Kitt Peak National Observatory and at Leiden, before
taking up a faculty position in Alabama. His research interests span the galaxies - active galactic
nuclei, galaxy interactions and evolution, dust in galaxies. Observationally oriented, his work
has used spectral bands from the radio to the X-ray regimes, with the strongest emphasis in the
optical and ultraviolet. These results have been reported in 150 refereed papers.
In recent years, much of his work has been tied to the enormously successful Galaxy Zoo
citizen-science project. He continues to have scheduling responsibilities for the two telescopes
of the SARA consortium, and has served on numerous NASA review panels including two
Senior Reviews.
Dr. Keel has been active in outreach beyond the formal classroom, through magazine
articles, an online presence in several discussion forums, and webcomics explaining ongoing
Hubble Space Telescope programs.
He has written a technical monograph, The Road to Galaxy Formation, and the nontechnical volume The Sky at Einstein’s Feet tracing the impact of relativity throughout astronomy.

Table of Contents
Series Preface..........................................................................................
Preface to Volume 6..................................................................................
Editor-in-Chief.........................................................................................
Volume Editor .........................................................................................
List of Contributors ..................................................................................

v
vii
ix
xi
xv

Volume 6
1 Galaxy Morphology .......................................................................

1

Ronald J. Buta

2 Elliptical and Disk Galaxy Structure and Modern Scaling Laws ..............

91

Alister W. Graham

3 Star Formation in Galaxies .............................................................. 141
Samuel Boissier

4 The Cool ISM in Galaxies ................................................................. 183
Jan M. van der Hulst ⋅ W. J. G. de Blok

5 The Influence of Environment on Galaxy Evolution.............................. 207
Bernd Vollmer

6 Clusters of Galaxies........................................................................ 265
Richard Bower

7 Active Galactic Nuclei..................................................................... 305
Eric S. Perlman

8 The Large-Scale Structure of the Universe ......................................... 387
Alison L. Coil

9 The Distance Scale of the Universe ................................................... 423
Wendy L. Freedman ⋅ Barry F. Madore

10 Galaxies in the Cosmological Context................................................ 451
Gabriella De Lucia


xiv

Table of Contents

11 Evolution of Active Galactic Nuclei ................................................... 503
Andrea Merloni ⋅ Sebastian Heinz

12 The Intergalactic Medium ............................................................... 567

Renyue Cen

13 Cosmic Microwave Background........................................................ 609
John Mather ⋅ Gary Hinshaw ⋅ Lyman Page

Index ..................................................................................................... 685


List of Contributors
W. J. G. de Blok
Astronomy Group
ASTRON
Netherlands Foundation for Radio
Astronomy
Dwingeloo
The Netherlands

Samuel Boissier
Laboratoire d’Astrophysique de Marseille
Université Aix-Marseille & CNRS
UMR7326
Marseille cedex 13
France

Richard Bower
Department of Physics
University of Durham
Durham
UK

Ronald J. Buta
Department of Physics and Astronomy
University of Alabama
Tuscaloosa, AL
USA

Renyue Cen
Department of Astrophysical Sciences
Princeton University Observatory
Peyton Hall
Princeton, NJ
USA

Alison L. Coil
Department of Physics
University of California
San Diego, CA
USA

Wendy L. Freedman
Carnegie Observatories
Pasadena
CA
USA

Alister W. Graham
Centre for Astrophysics and
Supercomputing
Swinburne University of Technology
Hawthorn

Australia

Sebastian Heinz
Astronomy Department
University of Wisconsin-Madison
Madison, WI
USA

Gary Hinshaw
Department of Physics and Astronomy
University of British Columbia
Vancouver, BC
Canada

Jan M. van der Hulst
Radio Astronomy
Kapteyn Astronomical Institute
University of Groningen
Groningen
The Netherlands


xvi

List of Contributors

Gabriella De Lucia
INAF – Astronomical Observatory of Trieste
Trieste
Italy

Barry F. Madore
Carnegie Observatories
Pasadena, CA
USA

John Mather
Astrophysics Science Division
NASA/GSFC Code 443
Observational Cosmology
Greenbelt, MD
USA

Andrea Merloni
Max-Planck-Institut für Extraterrestrische
Physik
Garching
Germany

Lyman Page
Department of Physics
Princeton University
Princeton, NJ
USA

Eric S. Perlman
Department of Physics and Space Sciences
Florida Institute of Technology
Melbourne, FL
USA

Bernd Vollmer
Observatoire astronomique de Strasbourg
Strasbourg
France


1

Galaxy Morphology
Ronald J. Buta
Department of Physics and Astronomy, University of Alabama,
Tuscaloosa, AL, USA

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

2

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

3

Galaxy Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

4

A Continuum of Galactic Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

5
5.1
5.2
5.3

Galaxy Types: Stage, Family, and Variety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Elliptical and Spheroidal Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
S0 and Spiral Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Irregular Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10
10
14
20

6
6.1
6.2
6.3
6.4
6.5

Other Dimensions to Galaxy Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Outer Rings and Pseudorings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inner and Outer Lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nuclear Rings and Bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spiral Arm Morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Luminosity Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

21
21
24
24
27
30

7

The Morphology of Galactic Bars and Ovals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

8

Dust Morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

9

The Morphologies of Galactic Bulges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

10
10.1
10.2
10.3

10.3.1
10.3.2
10.3.3
10.3.4
10.4
10.5
10.6

Effects of Interactions and Mergers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Normal Versus Catastrophic Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Environmental Effects on Star-Forming Disks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interacting and Peculiar Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tidal Tails, Arms, Bridges, and Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dust-Lane Ellipticals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shell/Ripple Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ultraluminous Infrared Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Warps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Morphology of Active Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Morphology of Brightest Cluster Members . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

T.D. Oswalt, W.C Keel (eds.), Planets, Stars and Stellar Systems. Volume 6: Extragalactic Astronomy and
Cosmology, DOI 10.1007/978-94-007-5609-0_1, © Springer Science+Business Media Dordrecht 2013

39
39
42
44
44
46
46

47
48
49
52


2

1

Galaxy Morphology

11
11.1
11.2
11.3

Star Formation Morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hα Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ultraviolet Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Atomic and Molecular Gas Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53
53
54
55

12

Infrared Observations: Galactic Stellar Mass Morphology . . . . . . . . . . . . . . . . . . 59

13

Intermediate and High Redshift Galaxy Morphology . . . . . . . . . . . . . . . . . . . . . . 65

14

Giant Low-Surface-Brightness Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

15
15.1
15.2
15.2.1
15.2.2
15.2.3
15.3
15.4
15.5

Galaxy Morphology in Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Normal Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dwarf Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
dE, dS0, BCD, and cE Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Local Group Dwarf Spheroidals and Irregulars . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dwarf Spirals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galaxy Zoo Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Isolated Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Deep Field Color Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16

Large-Scale Automated Galaxy Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

17

The Status and Future of Morphological Studies . . . . . . . . . . . . . . . . . . . . . . . . . . 81

71
71
72
73
74
75
76
77
79

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84


Galaxy Morphology

1

Abstract: Hidden in the bewildering details of galaxy morphology are clues to how galaxies
formed and have evolved over a Hubble time. This article reviews the phenomenology of galaxy
morphology and classification using an extensive set of illustrations to delineate as many types
as possible and to show how different types connect to various physical processes and characteristics. The old classification systems are refined, and new types introduced, as the explosion in
available morphological data has modified our views on the structure and evolution of galaxies.
Keywords: Galaxies: Active, Galaxies: Classification, Galaxies: Clusters, Galaxies: Dwarfs,

Galaxies: Elliptical, Galaxies: Galaxy Zoo project, Galaxies: High redshift, Galaxies: Isolated,
Galaxies: Peculiar, Galaxies: S0s, Galaxies: Spiral, Galaxies: Structure

1 Introduction
In the nearly 100 years since galaxy morphology became a topic of research, much has been
learned about galactic structure and dynamics. Known only as “nebulae” a century ago, galaxies were found to have a wide range of largely inexplicable forms whose relations to one another
were a mystery. As data accumulated, it became clear that galaxies are fundamental units of matter in space, and an understanding of how they formed and evolved became one of the major
goals of extragalactic studies. Even in the era of space observations, galaxy morphology continues to be the backbone of extragalactic research as modern instruments provide information
on galactic structure across a wide range of distances and look-back times.
In spite of the advances in instrumentation and the explosion of data, classical galaxy morphology (i.e., the visual morphological classification in the style of Hubble and others) has not
lost its relevance. The reasons for this are as follows:
1. Morphology is still a logical starting point for understanding galaxies. Sorting galaxies into
their morphological categories is similar to sorting stars into spectral types and can lead to
important astrophysical insights. Any theory of galaxy formation and evolution will have
to, at some point, account for the vast array of galactic forms.
2. Galaxy morphology is strongly correlated with galactic star formation history. Galaxies
where star formation ceased gigayears ago tend to look very different from those where star
formation continues at the present time. Classical morphology recognizes these differences
in an ordered way.
3. Information on galaxy morphology, in the form of new types of galaxies, multiwavelength
views of previously known galaxy types, and higher resolution views of all or part of some
galaxies, has exploded as modern instrumentation has superceded the old photographic
plates that were once used exclusively for galaxy classification.
4. Galaxy classification has gone beyond the realm of a few thousand galaxies to that of a
million galaxies through the Galaxy Zoo project. Not only this, but Galaxy Zoo has taken
morphology from the exclusive practice of a few experts to the public at large, thus facilitating citizen science at its best. Galaxy Zoo images are also in color, thus allowing the
recognition of special galaxy types and features based on stellar populations or gaseous
emission.
5. Finally, deep surveys with the Hubble Space Telescope have extended morphological studies well beyond the realm of the nearby galaxies that dominated early catalogues, allowing
detailed morphology to be distinguished at unprecedented redshifts.

3


4

1

Galaxy Morphology

Now, more than ever, galaxy morphology is a vibrant subject that continues to provide surprises
as more galaxies are studied for their morphological characteristics across the electromagnetic
spectrum. It is clear that a variety of effects are behind observed morphologies, including initial
protogalactic cloud conditions, environmental density and merger/interaction history, internal
perturbations, gas accretions, nuclear activity, properties of the dark matter halo, secular evolution, as well as the diversity in star formation histories, and that a global perspective based
on large numbers of galaxies will improve theoretical models and give a more reliable picture
of galactic evolution.
The goal of this chapter is to present the phenomenology of galaxy morphology in an organized way and highlight recent advances in understanding what factors influence morphology
and how various galaxy types are interpreted. This chapter is a natural follow-up to the excellent
review of galaxy morphology and classification by Sandage (1975) in Volume IX of the classic
Stars and Stellar Systems series. It also complements the recently published de Vaucouleurs Atlas
of Galaxies (Buta et al. 2007, hereafter the dVA), which provided a detailed review of the state
and technique of galaxy classification up to about the year 2005. Illustrations are very important in a review of this nature, and this chapter draws on a large number of sources of images.
For this purpose, the Sloan Digital Sky Survey (SDSS), the NASA/IPAC Extragalactic Database
(NED), and the dVA have been most useful.

2 Overview
As extended objects rather than point sources, galaxies show a wide variety of forms, some due
to intrinsic structures, others due to the way the galaxy is oriented to the line of sight. The random orientations and the wide spread of distances are the principal factors that can complicate
interpretations of galaxy morphology. If we could view every galaxy along its principal axis of

rotation, and from the same distance, then fairer comparisons would be possible. Nevertheless,
morphologies seen in face-on galaxies can also often be recognized in more inclined galaxies
(> Fig. 1-1). It is only for the highest inclinations that morphology switches from face-on radial
structure to vertical structure. In general, we either know the planar structure in a galaxy or we
know its vertical structure, but we usually cannot know both well from analysis of images alone.

33°

49°

71°

81°

⊡ Fig. 1-1
Four galaxies of likely similar face-on morphology viewed at different inclinations (number below
each image). The galaxies are (left to right): NGC 1433, NGC 3351, NGC 4274, and NGC 5792. Images
are from the dVA (filters B and g)


Galaxy Morphology

1

Galaxy morphology began to get interesting when the “Leviathan of Parsonstown,” the
72-in. meridian-based telescope built in the 1840s by William Parsons, Third Earl of Rosse, on
the grounds of Birr Castle in Ireland, revealed spiral patterns in many of the brighter Herschel
and Messier “nebulae.” The nature of these nebulae as galaxies was not fully known at the time,
but the general suspicion was that they were star systems (“island universes”) like the Milky
Way, only too distant to be easily resolved into their individual stars. In fact, one of Parsons’

motivations for building the “Leviathan” was to try and resolve the nebulae to prove this idea.
The telescope did not convincingly do this, but the discovery of spiral structure itself was very
important because such structure added to the mystique of the nebulae. The spiral form was
not a random pattern and had to be significant in what it meant. The telescope was not capable
of photography, and observers were only able to render what they saw with it in the form of
sketches. The most famous sketch, that of M51 and its companion NGC 5195, has been widely
reproduced in introductory astronomy textbooks.
While visual observations could reveal some important aspects of galaxy morphology, early
galaxy classification was based on photographic plates taken in the blue region of the spectrum.
Silver bromide dry emulsion plates were the staple of astronomy beginning in the 1870s and
were relatively more sensitive to blue light than to red light. Later, photographs taken with
Kodak 103a-O and IIa-O plates became the standard for galaxy classification. In this part of
the spectrum, massive star clusters, dominated by spectral class O and B stars, are prominent
and often seen to line the spiral arms of galaxies. These clusters can give blue-light photographs
a great deal of detailed structure for classification. It is these types of photographs which led to
the galaxy classification systems in use today.
In such photographs, we see many galaxies as a mix of structures. Inclined galaxies reveal the
ubiquitous disk shape, the most highly flattened component of any galaxy. Studies of Doppler
wavelength shifts in the spectra of disk objects (like HII regions and integrated star light) reveal
that disks rotate differentially. If a galaxy is spiral, the disk is usually where the arms are found,
and also where the bulk of interstellar matter is found. The radial luminosity profile of a disk
is usually exponential, with departures from an exponential being due to the presence of other
structures.
In the central area of a disk-shaped galaxy, there is also often a bright and sometimes less
flattened mass concentration in the form of a bulge. The nature of bulges and how they form have
been a topic of much recent research and are discussed further in > Sect. 9. Disk galaxies range
from virtually bulgeless to bulge-dominated. In the center, there may also be a conspicuous
nucleus, a bright central concentration that was usually lost to overexposure in photographs.
Nuclei may be dominated by ordinary star light, or may be active, meaning their spectra show
evidence of violent gas motions.

Bars are the most important internal perturbations seen in disk-shaped galaxies. A bar is
an elongated mass often made of old stars crossing the center. If spiral structure is present, the
arms usually begin near the ends of the bar. Although most easily recognized in the face-on
view, bars have generated great interest recently in the unique ways they can also be detected in
the edge-on view. Not all bars are made exclusively of old stars. In some bulgeless galaxies, the
bar has considerable gas and recent star formation.
Related to bars are elongated disk features known as ovals. Ovals usually differ from bars in
lacking higher order Fourier components (i.e., have azimuthal intensity distributions that vary
mainly as 2θ), but nevertheless can be major perturbations in a galactic disk. The entire disk of
a galaxy may be oval, or a part of it may be oval. Oval disks are most easily detected if there is
considerable light or structure at larger radii.

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6

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Galaxy Morphology

Rings are prominent features in some galaxies. Often defined by recent star formation, rings
may be completely closed features or may be partial or open, the latter called “pseudorings.”
Rings can be narrow and sharp or broad and diffuse. It is particularly interesting that several
kinds of rings are seen and that some galaxies can have as many as four recognizable ring features. Nuclear rings are the smallest rings and are typically seen in the centers of barred galaxies.
Inner rings are intermediate-scale features that often envelop the bar in a barred galaxy. Outer
rings are large, low-surface-brightness features that typically lie at about twice the radius of a bar.
Other kinds of rings, called accretion rings, polar rings, and collisional rings, are also known
but are much rarer than the inner, outer, and nuclear rings of barred galaxies. The latter kinds
of rings are also not exclusive to barred galaxies but may be found also in nonbarred galaxies.

Lenses are features, made usually of old stars, that have a shallow brightness gradient interior
to a sharp edge. They are commonly seen in Hubble’s disk-shaped S0 class ( > Sect. 5.2). If a bar
is present, the bar may fill a lens in one dimension. Lenses may be round or slightly elliptical in
shape. If elliptical in shape, they would also be considered ovals.
Nuclear bars are the small bars occasionally seen in the centers of barred galaxies, often lying
within a nuclear ring. When present in a barred galaxy, the main bar is called the “primary bar”
and the nuclear bar is called the “secondary bar.” It is possible for a nuclear bar to exist in the
absence of a primary bar.
Dust lanes are often seen in optical images of spiral galaxies and may appear extremely regular and organized. They are most readily evident in edge-on or highly inclined disk galaxies
but are still detectable in the face-on view, often on the leading edges of bars or the concave
sides of strong inner spiral arms.
Spiral arms may also show considerable morphological variation. Spirals may be regular
1-, 2-, 3-, or 4-armed patterns and may also be higher order multiarmed patterns. Spirals may
be tightly wrapped (low pitch angle) or very open (high pitch angle). A grand-design spiral is
a well-defined global pattern, often detectable as smooth variations in the stellar density of old
disk stars. A flocculent spiral is made of small pieces of spiral structure that appear sheared by
differential rotation. The appearance of these features can be strongly affected by dust, such that
at longer wavelengths a flocculent spiral may appear more grand design. Pseudorings can be
thought of as variable pitch angle spirals which close on themselves, as opposed to continuously
opening, constant pitch angle, logarithmic spirals.
There are also numerous structures outside the scope of traditional galaxy classification,
often connected with strong interactions between galaxies. Plus, the above described features
are not necessarily applicable or relevant to what we see in very distant galaxies. Accounting for
all of the observed features of nearby galaxies, and attempting to connect what we see in nearby
to what is seen at high redshift, is a major goal of morphological studies.

3 Galaxy Classification
As noted by Sandage (1975), the first step in studying any class of objects is a classification of
those objects. Classification built around small numbers of shared characteristics can be used for
sorting galaxies into fundamental categories, which can then be the basis for further research.

From such research, physical relationships between identified classes may emerge, and these
relationships may foster a theoretical interpretation that places the whole class of objects into a
broader context.


Galaxy Morphology

1

The basic idea of galaxy classification is to take the complex combinations of structures
described in the previous section and summarize them with a few type symbols. Sandage (1975)
describes the earlier classification systems of Wolf, Reynolds, Lundmark, and Shapley that fell
into disuse more than 50 years ago. The Morgan (1958) spectral type/concentration classification system, which was based on a connection between morphology (specifically central
concentration) and the stellar content of the central regions, was used recently by Bershady et al.
(2000) in a mostly quantitative manner (see also Abraham et al. 2003). Thus, Morgan’s system
has in a way survived into the modern era but not in the purely visual form that he proposed.
Only one Morgan galaxy type, the supergiant cD type, is still used extensively (>Sect. 10.6). Van
den Bergh’s luminosity/arm morphology classification system is described by van den Bergh
(1998; see > Sect. 6.5).
The big survivor of the early visual classification systems was that of Hubble (1926, 1936),
as later revised and expanded upon by Sandage (1961) and de Vaucouleurs (1959). Sandage
(1975) has argued that one reason Hubble’s view prevailed is that he did not try and account
for every superficial detail but kept his classes broad enough that the vast majority of galaxies
could be sorted into one of his proposed bins. These bins were schematically illustrated in Hubble’s famous “tuning fork”1 (Hubble 1936; reproduced in > Fig. 1-2), recognizing a sequence of
progressive flattenings from ellipticals to spirals. Ellipticals had only two classification details:
the smoothly declining brightness distribution with no inflections and no evidence for a disk
and the ellipticity of the isophotes, indicated by a number after the “E” symbol. (For example,
E3 means the ellipticity is 0.3.) Spirals were systems more flattened than an E7 galaxy that could
be subdivided according to the degree of central concentration, the degree of openness of the
arms, the degree of resolution of the arms into complexes of star formation (all three criteria determine position along the fork), and the presence or absence of a bar (determining the

appropriate prong of the fork).

ALS

AL
ORM

SPIR

N

Sb
Sc

Sa

ELLIPTICAL NEBULAE
So
Eo

E3

E7

SBa

BAR

SBb

RED

SBc

SPIR

ALS

⊡ Fig. 1-2
Hubble’s (1936) “tuning fork” of galaxy morphologies is the basis for modern galaxy classification

1

As recently noted by D. L. Block (Block et al. 2004a), this diagram may have been inspired by a similar
schematic by Jeans (1929).

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Galaxy Morphology

The S0 class at the juncture of the prongs of the fork was still hypothetical in 1936. As “armless disk galaxies,” S0s were mysterious because all examples known in 1936 were barred. These
were classified as SBa, but this was a troubling inconsistency because nonbarred Sa galaxies had
full spiral patterns. Hubble predicted the existence of nonbarred S0s to fill the gap between type
E72 and Sa and cure what he felt was a “cataclysmic” transition.
It was not long before Hubble himself realized that the tuning fork could not adequately

represent the full diversity of galaxy morphologies, and after 1936, he worked on a revision that
included real examples of the sought group: nonbarred S0 galaxies. Based on fragmentary notes
he left behind, Sandage (1961) prepared the Hubble Atlas of Galaxies to illustrate Hubble’s revision and also added a third dimension: the presence or absence of a ring. This was the first major
galaxy atlas illustrating a classification system in a detailed, sophisticated way with beautifully
produced photographs. Hubble’s revision, with van den Bergh luminosity classes (Sandage and
Tammann 1981), was updated and extended to types later than Sc by Sandage and Bedke (1994).
Because Sandage (1961) and Sandage and Bedke (1994) describe the Hubble–Sandage revision so thoroughly, the details will not be repeated here. Instead, the focus of the next section
will be on the de Vaucouleurs revision as outlined in the dVA. The reasons for this are (1) the de
Vaucouleurs classification provides the most familiar galaxy types to extragalactic researchers,
mostly because of extensive continuing use of the Third Reference Catalogue of Bright Galaxies
(RC3, de Vaucouleurs et al. 1991) and (2) the de Vaucouleurs classification is still evolving to
cover more details of galaxy morphology considered significant at this time. It should be noted
that both the de Vaucouleurs and Hubble–Sandage revisions are strictly applicable only to z ≈ 0
galaxies and that it is often difficult to fit objects having z > 0.5 neatly into the categories defining
these classification systems. High redshift galaxy morphology is described in > Sect. 13.

4 A Continuum of Galactic Forms
The Hubble tuning fork is useful because it provides a visual representation of information Hubble (1926) had only stated in words. The fork contains an implication of continuity. For example,
it does not rule out that there might be galaxies intermediate in characteristics between an “Sa”
or “Sb” spiral or between a normal “S” spiral and a barred “SB” spiral. Continuity along the elliptical galaxy sequence was always implied as a smooth variation from round ellipticals (E0) to the
most flattened ellipticals (E7). Sandage (1961) describes the modification that made the Hubble
system more three-dimensional: the introduction of the (r) (inner ring) and (s) (pure spiral)
subtypes. Continuity between these characteristics was possible using the combined subtype
(rs). Thus, already by 1961, the Hubble classification system had become much more complicated than it was in 1926 or 1936. The addition of the S0 class was one reason for this, but the
(r) and (s) subtypes were another.
In the Hubble–Sandage classification, it became common to denote galaxies on the left part
of the Hubble sequence as “early-type” galaxies and those on the right part as “late-type” galaxies. By the same token, Sa and SBa spirals became “early-type spirals” while Sc and SBc spirals
became “late-type spirals.” Sb and SBb types became known as “intermediate-type spirals.” The
reason for these terminologies was convenience and borrows terminology often used for stars.
2

Van den Bergh (2009a) shows that E0–E4 galaxies are more luminous on average than are E5–E7 galaxies,
suggesting that all E7 galaxies (and not many have been recognized) are actually S0 galaxies. Van den Bergh
argues that genuine E galaxies may be no more flattened than E6.