Chap.5 Formation of Galactic Structures• Overview of galaxy formation• Classical picture of Galaxy formation• Formation of the stellar halo: after Hipparcos• Formation of the stellar halo: after Gaia
• Formation of the thick disk
• Formation of the thin disk
• Formation of satellite galaxies
11. Overview of galaxy formation
Hierarchical assembly of CDM

gasstars

Bekki & Chiba 2001

Via Lactea simulation(Diemand+07)

Stellar halos in various MW-sized halos～varieties due to different merging histories～
Cooper et al. 2010

Halo morphology: reflecting galaxy formation

Old stellar components
Halo

Globular clusters
Field halo stars
Thick disk
age > 10 Gyr(formation
redshift > 2)

Satellite galaxies
Bulge
age~10Gyr
たThin disk

Fossil records in Galaxy formation
Galactic Archaeology

Haloage > 10 Gyr(formation
redshift > 2)

Fossil record of Galaxy formation
Hierarchical merging
Merger tree

Stellar streams

 Spatial distribution anddynamics of stars
 Galaxy collapse and merging

 Chemical abundance of stars
 Star formation and chemicalevolution

Mergingof GES

Phase space

Remnant ofa building blockAbundance ratios

Thick diskThin disk

Helmi & de Zeeuw 2000

Bensby et al. 2014

Sampling ancient halo stars• Metal-poor sample (metallicity biased)– e.g.： [Fe/H] < -1
– Suitable for kinematic analysis• High-velocity sample (kinematically biased)– e.g.： | Vstar – VLSR | > 180 km/s
– Suitable for metallicity analysis

Fraction: ~ 1/1000 near the Sun
62. Classical picture of Galaxy Formation* Monolithic, free-fall collapse
Eggen, Lynden-Bell, Sandage 1962 (ELS)* Chaotic merging of numerous fragments
Searle, Zinn 1978 (SZ)
ELS

SZ

Free fall
Rapidcollapse

Correlation between kinematics
and metal abundances of stars
Metallicity gradientMerging
Slowcollapse

No metallicity gradientAge spread among
globular clusters

7

Lynden-Bell

Sandage

Eggen8

Physical state of the Protogalaxy(Eggen,Lynden-Bell & Sandage 1962)[Fe/H]
Metallicity-2.4

Free-fallcollapse !?

-1.2
-0.4
Metal-rich stars(disk stars)
0.0

Circular orbit

High-proper motionstars(halo stars)

Orbital eccentricitye = (rapo-rperi) / (rapo+rperi)

Radial orbit
9

If free-fall galactic collapse is the case

Large eOnly large-e stars
are remaining.

Small eLarge eIf mass inside an orbit
is suddenly increased10NoteAction integrals for Kepler motions
rJr r

1 max

2 max

p dr  rrminrmin

J 

1 max

 minL2 p

2sin 
21J p d  p

2

 1

L2 GMGM
E 2 dr  L  L
 1
22r
r
2E 1 ed  L  J 

L=|J|+Jθ : conserved
e ：conserved as well⇒ adiabatically invariant（also nearly invariant
for non-Kepler motions）
11

ELS 1962

Norris et al. 1985

Yoshii & Saio 1979

Ryan & Lambert 1995

Abundance error
Metal-weak thick disk

12

3. Formation of the stellar halo: after Hipparcos(& before Gaia)

SDSS

Hipparcos[Fe/H]
Carollo, Beers, Lee, Chiba,
Norris et al. 2007, Natureorbital eccentricity

13

Monolithic collapse
or chaotic merging?

Comparison with numericalsimulation based on CDM model
Bekki & Chiba (2001)

gasstar14

Comparison with simulation results
Bekki & Chiba (2001)

simulation

observation

cumulative distribution

cumulative distribution

Bekki & Chiba 2001

15

Nearby stellar sample from SDSS
Carollo+2007, 2010

Zmax

Boundary ofan orbit

16

Velocity distribution of nearby stars
Sloan Digital Sky Survey
Carollo+2007, 2010

VＲ (km/s)V (km/s)
Thick disk

prograde+Thick disk

Sun
[Fe/H]=000

Sun[Fe/H]=0

halo

halo
-

[Fe/H]

retrograde[Fe/H]

Velocity distribution of nearby stars
Sloan Digital Sky Survey
Carollo+2007, 2010
prograde+V (km/s)

radially anisotropic0

mildly anisotropic
presence of stars
with largely negative Vφ-retrograde

VＲ (km/s)

2-halos：from dynamics
Global halo distribution based onsuperposition of stellar orbits (Carollo+ 2007)-1.6<[Fe/H]<-1.4
Flattened shape[Fe/H]<-2.2
Round shape• Inner halo -> in situ halo
– Flattened shape, -1.6<[Fe/H]<-1• Outer halo -> ex situ halo
– Round shape, [Fe/H]<-2

2-halos：from chemical abundance
Abundance ratios for high-velocity stars (Nissen & Schuster 2010)| Vstar – VLSR | > 180 km/s

(U2+W2)1/2 (km/s)

Blue: high- stars→ inner (in situ) halo?
Red: low- stars→ outer (ex situ) halo?

Based on VLT/UVES & NOT/FIES spectra
High-precision calibration with = 0.02 ~ 0.04 dex

V (km/s)20

How 2-halos have formed?
Outer halo:
Many small satellites
Metal-poor, young
Retrograde
Inner halo:
Massive satellites
Metal-rich
Prograde
Effect of later disk formation

21

Substructure in the stellar halo
Nearby stars in angular-momentum space（errors：a few 100 kpc km/s）

Helmi Stream
Remnant of a pastmerging event(Lx2+Ly2)1/2

Lz

Chiba & Beers 2000

Stellar streams: remnants of merging small galaxies

DECNorthDECRASouthRA

Grillmair & Carlin 2016

Credit: Rensselaer/Benjamin A. Willett

Substructure in the stellar halo
Clump

Remnant of a pastmerging event

LLz

Building block

Nearby stars in the angular-momentum
space (using Hipparcos data）• measurement errors of a few 100(kpc km/s) smear out any
possible substructures

MC & Beers 00

Simulation result of satellite accretion:
Gaia (precise distance and propermotion) + observation of Vrad & [Fe/H]• distinguish each substructure• SF & Chemical evolution

Helmi & de Zeeuw 00

4. Formation of the stellar halo: after Gaia
Gaia-Enceladus/Sausage

Myeong et al. 2019

Helmi et al. 2018, NatureE

Sequoia

retrograde
retrogradeLz

progradeprograde

Merging debris of an SMC-classdwarf galaxy 10 Gyrs ago?

Another story …

Hipparcos+SDSS-Enceladus?
(This feature was already present in previous samples,but only weakly due to the small number of sample stars.)
Chiba & Beers 2000
Using Hipparcos sample

Chiba 2012 (IAP conf. poster)
Using SDSS DR8 G dwarfs
D<2.5kpc, -2<[Fe/H]<-1D<2.5kpc, [Fe/H]<-1
This feature was thought to be a tidal
remnant of a dSph containing ωCen28

Merging of a dwarf galaxy 10 Gyrs ago?
Gaia-Enceladus

Credit: A. HelmiAbundance ratios of Gaia-Enceladus
SDSS-DR7 Calibration Stars + Gaia DR2
E

Gaia-Enceladus+ in situ stars

Lz

Metal-weal
thick disk

Lz

Carollo & Chiba 2021

Thick disk

Lz
Thin disk[α/Fe]

Gaia-Enceladus
low-α stars[Fe/H]Lz

Lz

Abundance ratios
Myeong et al. 2019

Matsuno et al. 2019using SAGA & LAMOSTACCPhase spacedistribution
retrogradeBA
progradeBDeciphering merging history of the Galaxy
More substructures discovered in the nearby
halo phase space reveal past merging history
Horta+ 2022

Sharpe+ 2022

Deciphering merging history of the Galaxy
More substructures discovered in the nearby
halo phase space reveal past merging history
Horta+ 2022

Kruijssen+2020

[Al/Fe] as an indicator of accreted/in situ halo
Hawkins et al. 2015 for −1.20 < [Fe/H] < −0.55

Das et al. 2020 (APOGEE DR14)

Accretedhalo

Accretedhalo

Al from SNII and is sensitive to initial C+N abundance34

5. Formation of the thick diskStar counts toward the SGP
Gilmore & Reid 1983

Luminosity distributionof NGC4565
Van der Kruit & Seale 1981

hz=300pc
hz=1350pc

ρthick~2%ρthin35

Thick disk(s)(km/s)

Vertical velocity dispersion

 Milky Way thick disk
 distinct kinematics,
chemistry, and age:
independent Galactic
component dynamically hot, large
scale height, [Fe/H]~ -0.6,
old age (~10Gyr)

log Age (Gyr)
Lthick/Lthin vs. Vcirc（in external galaxies）

 Extra-galactic thick disks
 common in disk galaxies relatively old and metalpoorVcirc

36

Formation scenarios of the thick disk
① Dissipative collapse (Burkert+1992)② Direct accretion of thick-disk material (Abadi+200s)③ Multiple mergers (Brook+2004, 2005)④ Dynamical heating of a pre-existing thin disk by satellites or subhalos(Quinn+1993; Veláquez & White 1999; Hayashi & Chiba 2006;Kazantzidis+2009), by merging of Gaia-Enceladus?
⑤ Clumpy disk evolution (Noguchi 2009; Bournarud+2007; 2009)⑥ Radial migration due to local spiral arms (Haywood 2008; Schönrich &Binney 2009)37

②. Direct accretion of thick-disk material

DMgas

Hierarchicalclusteringstars
38

Shredded satellite  thick disk?

But [Fe/H] and [α/Fe] in satellites
are very different from thosein the thick disk.
39

④. Dynamical heating of a thin disk by dark-matter subhalos(Hayashi & Chiba 2006)
Distribution of dark halos in a galactic scale(by Moore)

Pre-existing thin disk

40

Numerical simulation of disk heating(Hayashi & Chiba 2006)
Subhalo: Msub
Scaleheight： Zd

Scale length: Rd
Mass: Md M sub , j Z d 8 Rdj 1  M d 

ΔZd / RdNObservedthin disks∑(Msub,j / Md)22

Observed thin disks: Zd / Rd < 0.2(Kregel et al. 2002)⇒ accreted subhalo mass
< 0.15 Md41

Signature for GES merger on thick disk formation
Ciuca, Kawata,, Baba et al 2022
68360 RGBs and Red Clumps from APOGEE-2 + APOKASC-2
Machine learning

Early-epoch gas-rich merger (GES merger) ⇒dilution [Fe/H]↓⇒SF + chemical evolution ⇒[Fe/H]↑, [Mg/Fe]↓⇒ metal-rich part of the thick disk

⑤. Clumpy disk evolution
Thick disks as relics of clumpy disk evolution?
(Noguchi 1999; Bournaud+2007; 2009)
Bournaud+2007

Symmetric structure along z, metal-poor stars?, d<v>/dz?

43

⑥. Radial migration due to local spiral arms
Radial migration of disk stars
thick disk stars?
(Schönrich & Binney 2009)

Stars getting Lz

Stars losing Lz[/Fe] larger at larger RLee+2010 SDSS sample

Rtransient spiral arms etc.

Thick disk×dV/d[Fe/H]<0

理論モデル

V < 179 km/s blue
179 < V km/s < 244 redV > 244 km/s green

Thin disk○
44

Results of SDSS/APOGEE
Anders+14

Presence ofseparate
high [α/Fe] +low scale-lengthcomponent
Hayden+1545

Orbital eccentricity distributions of several models
Sales+ 2009(by visible satellites)46

Wilson+2011: RAVE sample

Dierickx+ 2010: SDSS sample DR7

Lee+ 2011: SDSS sample DR8

Scenarios of bothHeating by dark satellites
Multiple mergers
are favorable.
47

Score sheet for thick-disk formation models
ModeldV/d[Fe/H] dV/dz

Accretion

N/A

N/A

Gas-rich
N/Amergers
Disk heating ? (initialcondition)

FailedRadialmigration
Clumpy diskevolution

FailedN/A

N/A

N/A

Passed[Fe/H][/Fe]
FailedFailedOrbitaleccentricity
FailedN/A
N/A? (timing)

PassedPassed?
Passed?
N/A
N/A

FailedMore theoretical and observational
studies are needed!

Passed

N/A48

6. Formation of the thin disk
G-dwarfs in the solar neighborhood(model: Sommer-Larsen & Yoshii 1990, MN, 243, 468)
dΣgas/dt∝exp(-t / tinfall)
tinfall~4-5 Gyr is required

Simple model(no gas infall)

The Galactic (thin) disk
formed slowly over 4-5 Gyr.
gas flow

tinfall=4.6Gyrbulge

Galactic disk
The Sun& nearby stars

49

Inside-out formation of the thin disk
Toyouchi & Chiba 2014

[Fe/H]

Metallicity gradientStar formation proceeds
faster in inner radii.

Average orbital radius (kpc)

The thin disk has
formed from inner toouter radii
Inside-out formation

Origin of very metal-rich stars with [Fe/H] > +0.2 near the Sun~ Metallicity distribution of F, G dwarfs near the Sun ~Feltzing & Chiba (2013)

B stars

MD of B-type stars
reflects that of ISM
near the Sun
Very meal-rich stars
with [Fe/H] > + 0.2
cannot be formednear the Sun
These very metal-rich stars(possibly having exo-planets)
are migrated from inner radii

New aspects using Gaia
Chang 2022 (Master Thesis, Tohoku U)
5436 RGBs & RCs (APOKASC-2, APOGEE DR16, Gaia EDR3)
Ages from Kepler seismology data (frequency -> mass -> age)
𝑴 𝟏𝟐
𝑹 −𝟏
∆ν ∝ 𝟑 ∝√ρ ∝𝑹𝒈𝟏

𝑪𝑺𝑴
𝑯
νmax ∝∝∝.
.
𝑻𝒆𝒇𝒇𝟎 𝟓
𝑻𝒆𝒇𝒇𝟎 𝟓 𝑹𝟐
𝑪𝑺−𝟏

Chang

Newly derived AMR
Chang 2022 (Tohoku Univ.)
Pop C (thick + old thin disk stars)
Dip-like feature at ~ 8Gyr ago
but ambiguous[Fe/H][Fe/H]Pop B (thin disk stars)
Peak at ~ 2 Gyr + extended to~ 4 Gyr

Age (Gyr)

Age (Gyr)New AMR - Signature for radial migration event[Fe/H]

Chang 2022 (RGBs+RCs)

Turning pointat around ~ 3 Gyrs ago

Satellite passage ~3 Gyr ago
driving Radial Migration?

Age [Gyr]

Metal-rich,older stars

Metal-poor,
younger stars

The effect of satellite infall & radial migration in AMR
Lu et al. 2021 (simulation)
Cosmological simulation with NIHAO-UHD– Sgr-dwarf like satellite is infalling at z=0.34.
The second passage at z =0.255 yields turning
points and radial migration for low-α stars in AMR.
Before 1st passage z = 0.34
After 2nd passage z = 0.255
７kpc<R<9kpc, 0 < |ｚ| < 1
Before 1st passage

After 2nd passage

Thick diskThin diskall

At current epoch

Thick diskallThin disk

Thick diskallThin disk

Further evidence for radial migration of stars

Chang 2022

Sellwood & Binney 2002, Schoenrich & Binney 2009
Thick diskThin disk

・Epicycle motion
・Angular-momentum
transfer by transientspiral arms

Metal-rich stars

Metal-poor stars

Large Vϕ movingfrom large RThin disk<Vϕ>
Lz=VφR～const.
(km/s)(Metal-rich) star moving from inner R： Vφ is slower(Metal-poor) star moving from outer R： Vφ is faster

Radial migration of stars driven bysatellite infall and associated transient bar/spirals

Thick disk

Small Vϕ movingfrom small R

MDFs of the stars hosting planets
Johnson et al. 2010

These stars may
have been migrated
from inner R57

SFH of disk stars within 2kpc from the Sunusing Gaia DR2
Ruiz-Lara et al. 2020 Nature Astronomy

Peaks atlookback t of・5.7 Gyr(incl. Sun formation)

・1.9 Gyr・1.0 Gyr
These peaks overlap
with the timings ofpericentric passagesof Sgr dwarf

The orbit of Sgr dwarf
Mtot ~ 2.5 x 1010 Msun

Ruiz-Lara et al. 2020

SFHs of thick/thin disks & Sgr dwarf
Ruiz-Lara et al. 2020

Peak at 5.7 Gyrexists for bothThick diskThin disk

Thick disk

Thindisk

SFHs of M54 & Sgr dwarf
Siegel+ 2007

SFH ofSgr dwarf
is alsoaffected