Galaxies

Galaxies are vast ensembles of stars, gas and dust, embedded in dark
matter halos. They are the basic building blocks of the Universe,
gathered in groups, clusters and super-clusters. They exist in many
forms, either as spheroids or disks. Classifications, such as the Hubble
sequence (based on mass concentration and gas fraction) and the colormagnitude diagram (which separates a blue cloud from a red sequence)
help to understand their formation and evolution. Galaxies spend a large
part of their lives in the blue cloud, forming stars as spiral or dwarf
galaxies. Then, via a mechanism that is still unclear, they stop forming
stars and quietly end in the red sequence, as spheroids. This
transformation may be due to galaxy interactions, or because of the
feedback of active nuclei, through the energy released by their central
super-massive black holes. These mechanisms could explain the history
of cosmic star formation, the rate of which was far greater in the first half
of the Universe s life.

Galaxies delves into all of these surrounding subjects in six chapters
written by dedicated, specialist astronomers and researchers in the field,
from their numerical simulations to their evolutions.

Autorentext

Francoise Combes is a Professor at College de France on the Chair of Galaxies and Cosmology. She is a specialist of galaxy evolution through bars and spiral waves, and has uncovered dynamical mechanisms to fuel central black holes in active galaxy nuclei.

Zusammenfassung
Galaxies are vast ensembles of stars, gas and dust, embedded in dark matter halos. They are the basic building blocks of the Universe, gathered in groups, clusters and super-clusters. They exist in many forms, either as spheroids or disks. Classifications, such as the Hubble sequence (based on mass concentration and gas fraction) and the colormagnitude diagram (which separates a blue cloud from a red sequence) help to understand their formation and evolution. Galaxies spend a large part of their lives in the blue cloud, forming stars as spiral or dwarf galaxies. Then, via a mechanism that is still unclear, they stop forming stars and quietly end in the red sequence, as spheroids. This transformation may be due to galaxy interactions, or because of the feedback of active nuclei, through the energy released by their central super-massive black holes. These mechanisms could explain the history of cosmic star formation, the rate of which was far greater in the first half of the Universe's life. Galaxies delves into all of these surrounding subjects in six chapters written by dedicated, specialist astronomers and researchers in the field, from their numerical simulations to their evolutions.

Inhalt
Introduction xi

Chapter 1. The Classification of Galaxies **1
**Ronald BUTA

1.1. Introduction 1

1.2. Classes of galaxies 3

1.3. Elliptical galaxies 7

1.4. Spiral galaxies 12

1.5. S0 galaxies 22

1.6. Magellanic spiral and irregular galaxies 25

1.7. Dwarf elliptical, S0, and spheroidal galaxies 26

1.8. Edge-on galaxies 27

1.9. Morphology of interacting and merging galaxies 30

1.10. General properties along the CVRHS sequence 31

1.10.1. Morphological systematics 31

1.10.2. Astrophysical systematics 33

1.11. Other approaches to galaxy classification 34

1.12. Interpretations of morphology 35

1.13. Artificial galaxies and the future of galaxy classification 42

1.14. References 44

Chapter 2. Our Galaxy, the Milky Way **49
**Paola Di MATTEO

2.1. Introduction 49

2.2. Baryonic discs and their spiral structure 52

2.2.1. Neutral, ionized and molecular gas 52

2.2.2. Thin and thick stellar discs 54

2.2.3. Spiral structure from gaseous and stellar tracers 59

2.3. The central kiloparsecs: the bar and the bulge 60

2.4. The stellar halo 63

2.5. On the dark matter content and shape, as inferred from rotation curves and stellar streams 67

2.6. Dissecting the global structure: stellar kinematics, abundances and ages 69

2.6.1. Setting the scene: the solar vicinity 70

2.6.2. Zooming out on a several kpc scale 76

2.6.3. Digging into the bulge 80

2.7. Reconstructing the Milky Way evolution 84

2.8. Perspectives 86

2.9. References 86

Chapter 3. Early-type Galaxies **93
**Eric EMSELLEM

3.1. Introduction 93

3.2. General properties: components and morphology 95

3.2.1. Discs and bars 96

3.2.2. Gas and dust content 96

3.2.3. Dark matter and halo 97

3.2.4. Globular clusters 99

3.2.5. Light and mass profiles 99

3.2.6. Extreme cases: brightest cluster galaxies and ultra-diffuse galaxies 100

3.3. Zoom on the stellar component 101

3.3.1. Scaling relations: FaberJackson, fundamental plane and virial plane 102

3.3.2. Age and metallicity 104

3.3.3. Initial mass function 105

3.4. Dynamics of ETGs 108

3.4.1. Observations 109

3.4.2. Toward a kinematic classification of ETGs 110

3.4.3. Modeling 113

3.4.4. Supermassive black holes 119

3.5. Formation and evolution processes 119

3.5.1. Perspective at z = 0: the massradius plane 119

3.5.2. Growth, mergers and transformations 121

3.5.3. Ex situ versus in situ 123

3.5.4. Environment 124

3.6. Conclusion 126

3.7. References 127

Chapter 4. Spiral Galaxies **137
**Françoise COMBES

4.1. Introduction 137

4.2. Blue and red galaxies: quenching star formation 142

4.2.1. Definition of bimodality 142

4.2.2. The parameters that determine the red sequence 146

4.2.3. Mechanisms for quenching star formation 148

4.3. Spiral galaxies: density waves or not? 152

4.3.1. The winding problem 152

4.3.2. The theory of density waves 155

4.3.3. Role of gas and star formation 159

4.4. Bars: drivers of evolution 160

4.4.1. Formation of bars 162

4.4.2. Orbits in a barred galaxy 164

4.4.3. Response of gas to a barred potential 166

4.4.4. Vertical resonances and peanuts 168

4.4.5. Dark matter and bars 171

4.5. Environment of spiral galaxies 173

4.5.1. Morphological segregation 173

4.5.2. The problem of bulgeless galaxies 174

4.6. Conclusion 176 <p&gt...