Chap.5 Formation of the Galaxy

• Overview• Classical scenarios of Galaxy Formation

– ELS, SZ• Modern picture of Galaxy Formation

– Formation of the stellar halo• Formation of the thick disk• Formation of the thin disk

1

galaxy formation through hierarchical assembly of Cold Dark Matter

Visible parts are seenonly at a central region

CDM in a galaxy-sized haloHierarchical assembly of CDM

5.1 Overview

2

Old components in the Galaxy

Halo Globular cluster105Msun

Field halo star

Thick disk Bulge

Bulge：～10 Gyr oldHalo, Thick disk：> 10 Gyr old

3

HaloThick diskThin disk

Bulge

Tracing Galactic Past from Old Stars

Galactic ArchaeologyRedshift

4

Thick disk

Thin disk

Fossil record of the Galaxy Spatial distribution and dynamics of

stars Galaxy collapse and merging

Chemical abundance of stars Star formation and chemical evolution

Phase space

Tidal streams

Remnant of a building block

Galactic ArchaeologyNear-field Cosmology

5

Sampling ancient halo stars

Metal-poor sample (metallicity biased)e.g.：[Fe/H] < -1Suitable for kinematic analysis

High-velocity sample (kinematically biased) e.g.：Vlos > 65 km/s (Oort 1922,1926)Suitable for metallicity analysis

Fraction: ~ only one among nearby 1000 stars6

* Monolithic, free-fall collapseEggen, Lynden-Bell, Sandage 1962 (ELS)

* Chaotic merging of numerous fragmentsSearle, Zinn 1978 (SZ)

ELS SZFree fall Merging

Correlation between kinematicsand metal abundances of stars

Metallicity gradient

No metallicity gradientAge spread among

globular clusters

Rapidcollapse

Slowcollapse

5.2 Classical scenarios of Galaxy Formation

7

Eggen

Lynden-Bell Sandage

8

[Fe/H]-2.4

-1.2

-0.4

0.0

Free-fall collapse !?

High-proper motion stars

(halo stars)

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

Metallicity

Metal-rich stars(disk stars)

Physical state of the Protogalaxy(Eggen,Lynden-Bell & Sandage 1962)

Circular orbit Radial orbit9

Free-fall galactic collapse

Large e

Large eSmall eIf mass inside an orbitis suddenly increased

Only large-e starsare remaining.

10

NoteAction integrals for Kepler motions

∫

∫

∫∫

==

−=−=

−

−=−=+−==

φφφ

φ

θ

θ

φθ

φπ

θθπ

ππ

pdpJ

JLdp

LJ

eLL

EGMdr

rGM

rLEdrpJ

r

r

r

rrr

21

sin1

11

122

21

max

min

max

min

max

min

2

22

22

2

L=|Jφ|+Jθ : conservede ：conserved as well⇒ adiabatically invariant

（also nearly invariant for non-Kepler motions） 11

ELS 1962

Yoshii & Saio 1979

Norris et al. 1985

Ryan & Lambert 1995

Abundance errorMetal-weak thick disk 12

orbital eccentricity

[Fe/H]

ELS1962

Latest result

SDSS

13

Carollo+07

Monolithic collapseor chaotic merging?

Comparison with numericalsimulation based on CDM

modelBekki & Chiba (2001)

gas

star 14

simulation observation

Comparison with simulation resultsBekki & Chiba (2001)

cumulative distribution

Bekki & Chiba 2001 15

Nearby stellar sample from SDSSBoundary of

an orbit

5.3.1 Stellar halo

16

Zmax

5.3 Modern Picture of Galaxy Formation

Carollo+07, +10

太陽近傍にある星の銀河動径と回転方向の速度

Sloan Digital Sky Survey

[Fe/H]

Vφ (km/s)

[Fe/H]

VＲ (km/s)

太陽[Fe/H]=0

太陽[Fe/H]=0

0

+

-

0

正回転

逆回転

厚い円盤

ハロー

厚い円盤

ハロー

銀河回転方向速度

銀河動径方向速度

太陽近傍にある星の銀河動径と回転方向の速度

Sloan Digital Sky Survey

[Fe/H]

Vφ (km/s)

[Fe/H]

VＲ (km/s)

0

+

-

0

正回転

逆回転

動径方向に速度幅大

ハローの2重構造

• 内側のハロー inner halo– つぶれた形（伸びた形という説も）,

金属量多い側 -1.6

20

Abundance ratios in the halo2-halos in abundance ratios (Nissen & Schuster 2010)

Blue: high-α stars → inner halo? Red: low-α stars → outer halo?

Based on VLT/UVES & NOT/FIES spectraHigh-precision calibration with ∆= 0.02 ~ 0.04 dex

How 2-halos have formed?

21

Inner halo:Massive satellitesMetal-richProgradeEffect of later disk formation

Outer halo:Many small satellitesMetal-poor, youngRetrograde

銀河系ハローにある小銀河合体の痕跡

北半球

南半球

赤経

赤緯

赤経

赤緯

恒星ストリーム

Grillmair & Carlin 2016

Tidal debris of a merging small galaxy

Stream-like structure

Remnant of halo formation 24

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

ClumpRemnant of a past

merging event

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

Lz

L⊥

Building block

MC & Beers 00

Helmi & de Zeeuw 00

Gaia DR2 resultsGaia-Enceladus

Myeong et al. 2019 Helmi et al. 2018

(Incl. retrogradestars)

SausageSequoia

prograderetrograde

prograderetrograde

Past merging eventof a radially fallingLMC-class galaxy?

A counter-rotatingaccretion event?

Substructures in Gaia DR2

27

Yuan et al. 2019using LAMOST & Gaia Dr2

(see also Myeong et al. 2019)

E (1

05km

2 /s2 )

hz=300pc

hz=1350pc

Star counts toward the SGPGilmore & Reid 1983

ρthick~2%ρthin

Luminosity distributionof NGC4565

Van der Kruit & Seale 1981

5.4 Formation of the thick disk

28

Thick diskVertical velocity dispersion

Lthick/Lthin vs. Vcirc（in external galaxies）

Vcirc

log Age (Gyr)

(km/s)

29

Milky Way thick disk distinct kinematics,

chemistry, and age: independent Galactic component

dynamically hot, large scale height, [Fe/H]~ -0.6, old age (~10Gyr)

Extra-galactic thick disks common in disk galaxies relatively old and metal

poor

Formation scenarios of a 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)

5. Clumpy disk evolution (Noguchi 2009; Bournarud+2007; 2009)

6. Radial migration due to local spiral arms (Haywood 2008; Schönrich & Binney 2009)

30

Hierarchical clustering

DMgas

stars31

2. Direct accretion of thick-disk material

Shredded satellite → thick disk?

32

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

33

Numerical simulation of disk heating(Hayashi & Chiba 2006)

ΔZd / Rd

∑(Msub,j / Md)2

∑=

=

∆ N

j d

jsub

d

d

MM

RZ

1

2,8

Scale length: RdMass: Md

Scaleheight： Zd

Subhalo: Msub

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

< 0.15 Md

Observedthin disks

34

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

Symmetric structure along z, metal-poor stars?, d/dz? 35

5. Clumpy disk evolution

Bournaud+2007

Vφ < 179 km/s blue179 < Vφ km/s < 244 redVφ > 244 km/s green

R

Stars losing LzStars getting Lz

transient spiral arms etc.

理論モデル

Radial migration of disk stars(Schönrich & Binney 2009)

Lee+2010 SDSS sample[α/Fe] larger at larger R

thick disk stars?

dVφ/d[Fe/H]

最近のサーベイ結果 (APOGEE)

37

Anders+14

Hayden+15

独立したhigh [α/Fe]成分が確かに存在している

Orbital eccentricity distributions of several modelsSales+ 2009

(by visible satellites)

38

Wilson+2011: RAVE sample Dierickx+ 2010: SDSS sample DR7

Lee+ 2011: SDSS sample DR8

Scenarios of bothHeating by dark satellitesMultiple mergers

are favorable.

39

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

no infall

tinfall=4.6Gyr

The Galactic (thin) diskformed slowly over 4-5 Gyr.

40

bulge

Galactic disk

gas flow

The Sun& nearby stars

5.5 Formation of the thin disk

Formation of the thin disk

Average orbital radius (kpc)

[Fe/H] Metallicity gradient

Star formation proceedsfaster in inner radii.

The thin disk has formed from inner to outer radiiInside-out formation

Toyouchi & Chiba 2014

42

B stars MD of B-type stars reflects that of ISMnear the Sun

Very meal-rich starswith [Fe/H] > + 0.2cannot be formednear the Sun

Feltzing & Chiba (2013)using Nieva and Przybilla (2012) data

Comparison with metallicity distribution (MD) ofyoung stars (B-type stars)

Radial migration of stars

43

R

角運動量を失った星角運動量を得た星transient spiral arms などの効果により、別の半径で生まれた星が少しずつ移動してくる 太陽近傍

Sellwood & Binney 2002, Schoenrich & Binney 2009

VφR～一定から、内側から移動してきた星： Vφが周囲星より小外側から移動してきた星： Vφが周囲星より大

44

太陽近傍（7 < ｒ < 9kpc）星はどの半径 r0 で生まれたか？

Minchev+ 2013: simulation studies

太陽近傍（7 < ｒ < 9kpc）星の金属量分布はどの半径 r0 で生まれた星の金属量から成るか？

Radial migration of stars

星の動径移動の効果太陽近傍円盤星の元素組成比

Bensby+ 2014

[α元素/Fe]

厚い円盤薄い円盤

Lee et al. 2011[Fe/H]

(km/s)

平均回転速度

薄い円盤

厚い円盤

内側から移動してVϕ小

外側から移動してVϕ大

惑星を持つ星の金属量依存性

46

これらの星はどうやってできたのか？

どこから来たのか？

Johnson et al. 2010

Chap.5 Formation of the Galaxyスライド番号 2スライド番号 3スライド番号 4スライド番号 5Sampling ancient halo starsスライド番号 7スライド番号 8スライド番号 9スライド番号 10スライド番号 11スライド番号 12スライド番号 13スライド番号 14スライド番号 15スライド番号 16太陽近傍にある星の�銀河動径と回転方向の速度�Sloan Digital Sky Survey太陽近傍にある星の�銀河動径と回転方向の速度�Sloan Digital Sky Surveyハローの2重構造スライド番号 20How 2-halos have formed?スライド番号 22スライド番号 23スライド番号 24スライド番号 25Gaia DR2 resultsSubstructures in Gaia DR2スライド番号 28Thick diskFormation scenarios of a thick diskスライド番号 31スライド番号 32スライド番号 33スライド番号 34スライド番号 35スライド番号 36最近のサーベイ結果 (APOGEE)スライド番号 38スライド番号 39スライド番号 40Formation of the thin diskスライド番号 42Radial migration of starsRadial migration of stars星の動径移動の効果惑星を持つ星の金属量依存性