Chap.3 The nature of old Galacticcomponents• Overview of old Galactic components– bulge, thick/thin disks, stellar halo

• Globular clusters– metallicity and age distributions• Galactic satellites– spatial and metallicity distributions1

Old stellar components in the Milky Way
HaloGlobular 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)

Lookback formation time of stellar componentsThe Sun

bulge

Fossil records of galaxy formation⇒ Near-field cosmology

3.1 Galactic bulge

bulge+disk

1Gyrisochrone

Large ISM extinction⇒IR Obs
Bulge stars in the NIR
CM-diagram(using photometric data)

disk@(l,b)=(30,0)

bulge

Zoccali et al. 2003,
A&A, 399, 931
4

Zoccali et al. 2003

HB

10Gyr isochrone
at overall metallicity [M/H]=-1.3, 0.0

3 & 5 Gyr isochrone
at [M/H]=-1.3, 0.05

Distributions of [Fe/H] and [Mg/Fe] in Galactic Bulge
Bensby et al. 2013
Alves-Brito et al. 2010
(for RGBs)

6

Abundance of α-elements for bulge/disk dwarfs(Bensby et al. 2013)
Filled grey: bulge
Red: thick disk
Blue: thin disk
Bulge stars show
highest [α/Fe]
at given [Fe/H]

Age distribution for bulge dwarfs(Bensby et al. 2013)

8

BRAVA (Bulge Radial Velocity Assay)
Howard+ 2009
COBE image

COBE 2mimage
BRAVA survey
fieldsSpheroidBar* Cylindricalrotation* No classicalbulge with hotdynamics
9

3D structure of a boxy/peanut shape bar
Athanassoula (2005)

Orbits in a bar
Edge-onFace-on10

Galactic Bulge: summary• Old age: ~ 10 Gyr• Broad metallicity distribution function (MDF)– -1 < [Fe/H] < 0, metal-rich stars are dominant.

• Alpha enhanced– Enrichment by Type II SNe (time scale < 1 Gyr)

• Boxy/peanut shape with cylindrical rotation– Resembling 3d dynamical structure of a bar (pseudobulge)which formed via disk instability from old disk stars– No or small fraction of a classical bulge or spheroid, which
is formed via major merger?

Bulge formation by disk instability?

113.2 Galactic diskStar counts toward the Galactic pole:discovery of the thick disk component
Gilmore & Reid (1983)

Number density ofstars vs z (pc)1350pcExponential scaleheight hz

300pc

Thin disk
Thick diskhz(kpc) σR(km/s)~0.3
34~1.061σz(km/s)18
39<Vφ>(km/s)
220
200

12

Edvardsson et al. 1993

Thick disk stars
at [Fe/H] < -0.4, age >10Gyr

Quillen & Garnett, astro-ph/0004210

13

Feltzing et al. 2003
High [α/Fe] thick-disk stars

Van der Kruit & Searle 1981
Thick disk in NGC4565

Thick diskThin disk

14

[alpha-elements/Fe]= Type II SNe / Type Ia SNe
Chemical clock diagram
Thick diskThin disk

Bensby+ 201415

[α/Fe] vs. [Fe/H] at different R - SDSS/APOGEE
Anders+2014

Presence ofseparate
high [α/Fe] +low scale-lengthcomponent
Hayden+15

Xiang & Rix 2022

Age-Metallicity Relation (AMR)

Gaia eDR3
LAMOST
247000SG stars

Thick disk
F・G-dwarfs near the Sun(Bensby et al. 2014)
Presence of old but metal-rich stars
gives a large dispersion in AMR
17

Metallicity distribution of the thick disk
(reproduced from Wyse & Gilmore 1995)(Chiba & Beers 2000)

fiducial metallicity range of the thick disk
-1 < [Fe/H] < -0.4
But the rapidly-rotating disk component is extended
toward lower metallicity: Metal-Weak Thick Disk (MWTD)

18

Thin disk vs. Thick disk
<Vϕ>(km/s)

Thin disk
Thick disk

Positivegradient

Negativegradient

Lee et al. 2011

[Fe/H][Fe/H]
SDSS
sampleThin disk

Toyouchi & [Fe/H]
Chiba 2014

Thin disk
Negative gradientInside-out formation

Thick disk
Positive gradient
Thick disk

Mean orbital radius(kpc)?

Evidence that the Galactic disk has been disturbed
Antoja et al. 2018, NatureGaia DR2
Phase-space distribution
for nearby stars
Evidence forrecent disturbance events(e.g. by satellite infall)
and relaxation via
Phase mixing

3.3 Galactic halo
Ryan & Norris 1991, AJ, 101, 1865 (before Gaia)
High velocity stars observed from the Sun(V < -220 km/s) ⇒ 372 stars belonging to the halo

[Fe/H]
Typical metallicity range of halo stars: [Fe/H] < -1
(A strict selection to avoid disk stars is like [Fe/H] < -1.5)

Recent result using Gaia
Bonaca et al. 2017, ApJ, 845, 101~ 160,000 stars with Gaia + RAVE/APOGEE (D<3kpc)⇒ | Vtotstar – VLSR | > 220 km/s for halo and < 220 km/s for disk
halo

disk

Presence of metal-rich halo stars with [Fe/H] > -1
⇒ what is the origin of these stars ?

5 Gyr1015

Unavane, Wyse, Gilmore. (1996)
Halo stars (near the Sun)are old (>10 Gyr).
Later accreted fraction< 10%23

[/Fe] for Galactic stars (Venn et al. 2004)
HaloThick diskThin disk
Retrograde halo
Dwarf satellites

24

Halo and disk stars in abundance-ratio diagramusing Gaia DR2 and APOGEE DR14
Mackereth et al. (2019)25

Duality (2-halos) in [α/Fe] ratios of halo stars(Nissen & Schuster 2010)
High velocity stars =~ halo stars| Vtotstar – 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)
26

[Al/Fe] for accreted (low α）/canonical (high α) haloHawkins et al. 2015 for −1.20 < [Fe/H] < −0.55 (APOGEE DR12)high αlow α

Al from SNII and is sensitive to initial C+N abundance27U(VR)3D velocities of nearby 1203 stars
using Hipparcos Catalog(Chiba & Beers 2000)

V(VΦ-VLSR)
discontinuity

W(VZ)

halo-1.7

diskVelocity distribution of nearby stars
Sloan Digital Sky Survey(Carollo et al. 2007, 2010)V (km/s)

VＲ (km/s)

Thick disk

prograde+Thick disk

Sun
[Fe/H]=000Sun[Fe/H]=0

halo

halo-[Fe/H]

retrograde[Fe/H]

Result using Gaia: Belokurov et al. 2018
Elongated toward Vr: Sausage-like feature in halo stars
diskVφ(km/s)

haloVr (km/s)

Nearby stars in (E,Lz,I3) phase space
SDSS-DR7 Calibration Stars + Gaia DR2Carollo & Chiba 2021, ApJ, 981, 191(R,z) distributionof sample starsz (kpc)ESun

R (kpc)Lz

(E,Lz,I3) space fromSDSS-DR7 Calibration Stars + Gaia DR2 Carollo & Chiba 2021
thin disk

thick disk

(2I3)1/2

E

Lz

Gaia-Enceladus

LzE(2I3)1/2

(E,Lz,I3) space fromSDSS-DR7 Calibration Stars + Gaia DR2 Carollo & Chiba 2021GEGE(2I3)1/2

ELz

metal-poor halo
No GELz

E(2I3)1/2

Comparison with abundance ratios
SDSS-DR7 Calibration Stars + Gaia DR2E

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

3.4 Globular clusters
Zinn 1985, ApJ, 293, 424
Halo clusters, disk (bulge) clusters
Zinn 1993, ASP Conf. vol.48
HB morphology

35

HB type (color) vs. metallicity in Galactic globular clusters
Mackey & Gilmore 2004, MN, 355, 504

More red BHBs
for higher [Fe/H]-1.1Gyr-2.2 Gyr

Relative to the age of M92(~ 12.6 Gyr)

36

6kpc

37

Bulge/disk
Young haloOld halo

38

Clusters in the LMC, SMC,+ Fornax and Sgr dwarf galaxies

39

SMC

LMC

Sagittarius

FornaxYoung halo clusters → external origin!?

40

ω Centauri
Lee et al. 1999
CTIO 0.9m
EHB:
Extended HB

Z=0.0004
0.005

ω Centauri
Bedin et al. 2004

High helium abundance !?
Norris 2004

Na-O anti-correlation in GCs(Carretta+ 2010)
general properties of GCs ⇒ multiple stellar population!?

H-burning at high T(CNO, NeNa,MgAl cycle)
1. First-generation stars
changed Na & O
abundance insidethese stars (Na O)2. Gas was expelled via.
AGB & SN.
3. New-generation stars
formed from thisprocessed gas

3.5 Galactic satellites

SclCarFor

List of bright satellites

Leo II

Leo IPhe45

Milky Way & Galactic satellites

46

Fornax @ D=138 kpc
Buonanno+ 1999
(from HST image)de Boer+2012(from CTIO image)[Fe/H]=-0.747

HST/WFPC2 results(Weisz et al. 2014)
Mv > -8 mag

Mv < -8 mag

48

Keck/DEIMOS spectroscopic survey by Kirby+09,11
Sculptor

Metallicity Distribution Functions
in several dSphs

slit mask

49

Mean [Fe/H] vs. luminosity for dSphs
Kirby et al. 2008

Feltzing & Chiba 201350

Dark matter in the MW dwarf satellites
Mass ratio between DM and stars(Mass enclosed within stellar extent ~ 4 x 107M)The MW satellites are largely
dark matter dominated!

(old data)
51