Table Of ContentSaas-Fee Advanced Course 37
Swiss Society for Astrophysics and Astronomy
Joss Bland-Hawthorn
Kenneth Freeman
Francesca Matteucci
The Origin of
the Galaxy and
Local Group
Saas-Fee Advanced Course 37
For furthervolumes:
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Joss Bland-Hawthorn Kenneth Freeman
•
Francesca Matteucci Ben Moore
•
The Origin of the Galaxy
and Local Group
Saas-Fee Advanced Course 37
Swiss Society for Astrophysics and Astronomy
Edited by B. Moore
123
Joss Bland-Hawthorn Francesca Matteucci
SydneyInstitutefor Astronomy AstronomyDepartment
Universityof Sydney Trieste University Osservatorio
Sydney, NSW Astronomico(INAF)
Australia Trieste
Italy
Kenneth Freeman
Australian National University Volume Editor
Australian CapitalTerritory, ACT BenMoore
Australia InstituteforTheoretical Physics
Universityof Zürich
Zürich
Switzerland
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Andromeda_Galaxy_(with_h-alpha).jpg, by Adam Evans [CC-BY-2.0 (http://creativecommons.
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ISSN 1861-7980 ISSN 1861-8227 (electronic)
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Preface
Understanding the origin of galaxies is one of the major research goals of astro-
physicists. Our own Milky Way and its neighbouring galaxies provide the ideal
laboratorytofacilitateadeeperunderstandingofhowgalaxiesform.Wecanlook
in detail at the different components of our Milky Way and try to reconstruct
events in the distant past through present-day clues—in essence we are carrying
out archaeology on a galactic scale.
Our Galaxy resides in The Local Group, an overdense region out to about ten
megaparsecs from the Milky Way. This region includes Andromeda (M31) a
similar galaxy as our own, as well as a few dozen smaller galaxies and satellites.
This environment is typical of most galaxies in our Universe. In a few billion
years,theLocalGroupwillhaveevolvedintoasinglelargeellipticalgalaxyasits
most massive members merge together.
TheoriginoftheGalaxyandLocalGroupisplacedwithintheframeworkofthe
standardLCDMbigbangcosmology.TheMilkyWayanditssatellitescontinueto
provide tests and constraints on theories of galaxy formation and on the standard
cosmologicalmodel—namelyahierarchicaluniverseinwhichstructureformation
is driven by an underlying dominant component of cold dark matter.
The following chapters contain a broad and detailed overview of our current
understandingoftheorigin ofourMilky Way Galaxy andthe LocalGroup.They
representthecurrentstateoftheartintheexcitingtopicofNearFieldCosmology.
Theseup-to-datereviewsarebasedonlecturesgivenattheinspiring37thSaas
Fee School held in Muerren, Switzerland. Muerren is a car-free mountain village
high above the spectacular Lauterbrunnen valley in the Bernese Oberland. The
schoolwasentitled‘‘TheOriginoftheGalaxyandLocalGroup’’anditwasattended
by over 100 young astronomy students. The School was organised by Prof. Ben
Moore(UniversityofZurich)andProf.EvaGrebel(UniversityofHeidelberg)and
thelecturesweregivenbyworldexpertsonthesetopics:Prof.JossBland-Hawthorn
(Sydney Institute for Astronomy), Prof. Kenneth Freeman (Mount Stromlo
Observatory)andProf.FrancescaMatteucci(TriesteUniversity).
Zu¨rich, June 2013 Ben Moore
v
TheOriginOfTheGalaxyandLocalGroup
37thSaasFeeAdvancedCourse
Contents
1 Near Field Cosmology: The Origin of the Galaxy
and the Local Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Joss Bland-Hawthorn and Kenneth Freeman
1.1 Prologue. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Far Field Cosmology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 The Cosmic Microwave Background . . . . . . . . . . . . . 4
1.2.2 The First Stars. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.3 The First Black Holes. . . . . . . . . . . . . . . . . . . . . . . . 10
1.2.4 The First Dark Haloes . . . . . . . . . . . . . . . . . . . . . . . 12
1.2.5 Reionization and the First Galaxies . . . . . . . . . . . . . . 15
1.3 Lessons from Galaxy Redshift Surveys. . . . . . . . . . . . . . . . . . 16
1.3.1 Evolution and Environment. . . . . . . . . . . . . . . . . . . . 19
1.3.2 Accretion and Feedback . . . . . . . . . . . . . . . . . . . . . . 20
1.3.3 Baryon Inventory and Metal Enrichment. . . . . . . . . . . 22
1.3.4 Chemical Evolution in Galaxies. . . . . . . . . . . . . . . . . 25
1.3.5 Milky Way and Local Group Analogues
in the Real Universe. . . . . . . . . . . . . . . . . . . . . . . . . 25
1.3.6 Milky Way and Local Group Analogues
in Simulated Universes. . . . . . . . . . . . . . . . . . . . . . . 26
1.4 Gas Accretion onto Galaxies. . . . . . . . . . . . . . . . . . . . . . . . . 27
1.4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
1.4.2 Earliest Epoch of Gas Accretion . . . . . . . . . . . . . . . . 28
1.4.3 Early Ideas on Galaxy Accretion. . . . . . . . . . . . . . . . 30
1.4.4 Accretion Shocks. . . . . . . . . . . . . . . . . . . . . . . . . . . 32
1.4.5 Cooling Flows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
1.4.6 Cold Flows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
1.4.7 Warm Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
1.4.8 Accretion via Major and Minor Mergers. . . . . . . . . . . 38
1.4.9 Accretion of High Velocity Clouds . . . . . . . . . . . . . . 39
1.5 Near Field Cosmology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
1.5.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
1.5.2 A Working Model of How the Galaxy Formed . . . . . . 47
vii
viii Contents
1.5.3 Timescales and Fossils. . . . . . . . . . . . . . . . . . . . . . . 49
1.5.4 Stellar Age Dating. . . . . . . . . . . . . . . . . . . . . . . . . . 51
1.5.5 Goals of Near Field Cosmology. . . . . . . . . . . . . . . . . 54
1.6 Structure of the Galaxy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
1.6.1 The Bulge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
1.6.2 The Disk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
1.6.3 The Stellar Halo . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
1.6.4 The Dark Halo . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
1.7 Signatures of Galaxy Formation. . . . . . . . . . . . . . . . . . . . . . . 63
1.7.1 Zero Order Signatures: Information Preserved Since
Dark Matter Virialized . . . . . . . . . . . . . . . . . . . . . . . 63
1.7.2 First Order Signatures: Information Preserved Since
the Main Epoch of Baryon Dissipation. . . . . . . . . . . . 68
1.7.3 Second Order Signatures: Major Processes
Involved in Subsequent Evolution . . . . . . . . . . . . . . . 74
1.8 Reconstructing the Past Through Chemical Tagging. . . . . . . . . 86
1.8.1 Unravelling a Dissipative Process . . . . . . . . . . . . . . . 86
1.8.2 How Many Star Clusters?. . . . . . . . . . . . . . . . . . . . . 88
1.8.3 Cluster Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . 89
1.8.4 Chemical Homogeneity. . . . . . . . . . . . . . . . . . . . . . . 90
1.8.5 Unique Chemical Signatures . . . . . . . . . . . . . . . . . . . 92
1.8.6 Primary Requirements of Chemical Tagging. . . . . . . . 92
1.8.7 Candidates for Chemical Tagging . . . . . . . . . . . . . . . 97
1.8.8 Short-Term Goal: Size and Structure
in a Multi-Dimensional C-Space . . . . . . . . . . . . . . . . 100
1.8.9 Long-Term Goal: Reconstructing Ancient Star
Groups from Unique Chemical Signatures . . . . . . . . . 101
1.9 Epilogue: Challenges for the Future. . . . . . . . . . . . . . . . . . . . 104
1.9.1 The Limitations of Near Field Cosmology:
Are We Really Putting KCDM to the Test? . . . . . . . . 104
1.9.2 Future Surveys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Appendix A: The Discovery of Dark Matter in Galaxies . . . . . . . . . . 109
Appendix B: Stellar Data: Sources and Techniques. . . . . . . . . . . . . . 112
B.1 Data Needed for Galactic Archaeology. . . . . . . . . . . . 112
B.2 Sources of Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
B.3 Sources of Models. . . . . . . . . . . . . . . . . . . . . . . . . . 125
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
2 Chemical Evolution of the Milky Way and Its Satellites . . . . . . . . 145
Francesca Matteucci
2.1 How to Model Galactic Chemical Evolution. . . . . . . . . . . . . . 145
2.1.1 The Initial Conditions. . . . . . . . . . . . . . . . . . . . . . . . 146
Contents ix
2.1.2 Birthrate Function . . . . . . . . . . . . . . . . . . . . . . . . . . 146
2.1.3 Stellar Yields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
2.1.4 Gas Flows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
2.2 Basic Equations for Chemical Evolution. . . . . . . . . . . . . . . . . 158
2.2.1 Yields per Stellar Generation. . . . . . . . . . . . . . . . . . . 158
2.2.2 Analytical Models . . . . . . . . . . . . . . . . . . . . . . . . . . 159
2.2.3 Detailed Numerical Models. . . . . . . . . . . . . . . . . . . . 162
2.3 The Milky Way. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
2.3.1 The Formation of the Milky Way . . . . . . . . . . . . . . . 164
2.3.2 The Two-Infall Model . . . . . . . . . . . . . . . . . . . . . . . 166
2.3.3 Detailed Recipes for the Two-Infall Model. . . . . . . . . 167
2.3.4 The Chemical Enrichment History
of the Solar Vicinity. . . . . . . . . . . . . . . . . . . . . . . . . 170
2.3.5 The Galactic Disk . . . . . . . . . . . . . . . . . . . . . . . . . . 181
2.3.6 The Galactic Bulge . . . . . . . . . . . . . . . . . . . . . . . . . 186
2.4 What We Have Learned About the Milky Way. . . . . . . . . . . . 194
2.5 The Time-Delay Model and the Hubble Sequence. . . . . . . . . . 194
2.5.1 Star Formation and Hubble Sequence. . . . . . . . . . . . . 195
2.6 Dwarf Spheroidals of the Local Group. . . . . . . . . . . . . . . . . . 198
2.6.1 How do dSphs Form?. . . . . . . . . . . . . . . . . . . . . . . . 198
2.6.2 Observations of dSphs . . . . . . . . . . . . . . . . . . . . . . . 200
2.6.3 Chemical Evolution of dSphs . . . . . . . . . . . . . . . . . . 200
2.6.4 What Have we Learned About dSphs?. . . . . . . . . . . . 208
2.7 Ultra-Faint Dwarfs in the Local Group. . . . . . . . . . . . . . . . . . 212
2.8 Other Spirals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
2.8.1 Chemical Models for External Spirals . . . . . . . . . . . . 215
2.9 Cosmic Chemical Evolution . . . . . . . . . . . . . . . . . . . . . . . . . 217
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
Chapter 1
Near Field Cosmology: The Origin
of the Galaxy and the Local Group
JossBland-HawthornandKennethFreeman
The Galaxy has built up through a process of accretion and merging over billions
of years which continues to this day. Astronomers are now embarking on a new
eraofmassivestellarsurveysoverthecomingdecade.Thesecampaignswillderive
three-dimensionalspacemotionsandheavyelementabundancesformillionsofstars
throughouttheGalaxyanditsneighbours.Thenewobservationswillrevealsigna-
turesoftheformationandearlyevolutionoftheLocalGroup;thisiswhatwemean
by‘nearfieldcosmology.’Wesetthisnewcourseofstudywithinthecontextoffossil
signaturesfromgalaxysurveysandthehighredshiftuniverse.Wediscussthecom-
plexrelationshipbetweenbaryonsanddarkmatterovercosmictime,andintroduce
asyntheticframeworkthatwillallowbothnumericalsimulationsandtheimpending
datadelugetobecompared.Wealsoincluderelevantsourcematerialsfortheyoung
near-fieldcosmologistandsomehistoricalperspectives.
1.1 Prologue
Inanearlierreview,weoutlinedtheconceptofnearfieldcosmologytoemphasize
thefactthatthereareancientsignaturesallaroundustodayprovidingevidenceof
theformation processes that ledtotheGalaxy and the Local Group (Freeman and
Bland-Hawthorn2002).Weseeancientstarsaroundusintheoldthindisk,thethick
disk,thestellarhalo,theinnerbulge,andinsatellitedwarfgalaxies.Abouthalfof
all stars in the Galaxy today formed before a redshift z ∼ 1. The Hubble Space
Telescope(HST)probestheseearlyformationepochsdirectlyduringatimewhen
B
J.Bland-Hawthorn( )
SydneyInstituteforAstronomy,UniversityofSydney,Sydney,NSW2006,Australia
e-mail:[email protected]
K.Freeman
MountStromloObservatory,AustraliaNationalUniversity,WestonCreek,ACT2611,Australia
e-mail:[email protected]
B.Moore(ed.),TheOriginoftheGalaxyandLocalGroup, 1
Saas-FeeAdvancedCourse37,DOI:10.1007/978-3-642-41720-7_1,
©Springer-VerlagBerlinHeidelberg2014
