Chap.3 銀河系の構造

• 銀河系の各恒星系成分の概観

– bulge, thick disk, metal-weak halo• 球状星団

– metallicity and age distributions• 銀河系衛星銀河

– spatial and metallicity distributions

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銀河系の古成分恒星系

Haloage > 10 Gyr(formationredshift > 2)

globular cluster

halo star

thick diskage > 10 Gyr(formationredshift > 2)

Bulge

Fossil records in Galaxy formationNear-field Cosmology 2

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

3.1 Galactic bulge

Bulge stars in the NIRCM-diagram(using photometric data)

bulge+disk disk

bulge

1Gyrisochrone

星間吸収が大⇒赤外線観測

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Zoccali et al. 2003

10Gyr isochroneat [M/H]=-1.3, 0.0

3 & 5 Gyr isochroneat [M/H]=-1.3, 0.0

HB

4

5

Distributions of [Fe/H] and [Mg/Fe] in Galactic BulgeBensby et al. 2013

Alves-Brito et al. 2010(for RGBs)

Abundance of α-elements for bulge/disk dwarfs(Bensby et al. 2013)

Filled grey: bulgeRed: thick diskBlue: thin disk

Bulge stars showhigher [α/Fe]

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

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COBE imageCOBE 2µmimage

BRAVA surveyfields

BRAVA (Bulge Radial Velocity Assay)Howard+ 2009

Spheroid

Bar

* Cylindricalrotation

* No classicalbulge with hotdynamics

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Orbits in a bar

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

Edge-on

Face-on

Galactic Bulge: summary• 古い年齢 ~ 10 Gyr• 広い金属量分布

– -1 < [Fe/H] < 0 metal-rich starsが最も多い。

• Type II 超新星爆発による金属汚染

– time scale < 1 Gyr• 箱形・ピーナッツ型で円筒運動

– バー構造の3次元構造 (pseudobulge)、古い星で構成された円盤の重力不安定

– classical bulgeの割合小

円盤の重力不安定で形成か？まだまだ謎が多い。 10

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Geneva-Copenhagen SurveyThick-disk-to-thin-disk

probability ratio(from stellar kinematics)

Spectroscopic sampleused in Bensby+ (2014)

Photometric metallicity

For thick-disk stars

Spectroscopic metallicity

3.2 Galactic disk

Metallicity distribution of nearby stars

For thin-disk stars

Gilmore & Reid 1983, MN, 202, 1025Discovery of the thick-disk component

Number density ofstars vs z

300pc1350pc

Exponential scaleheight hz

hz(kpc) σR(km/s) σz(km/s) <Vφ>(km/s)Thin disk ~0.3 34 18 220Thick disk ~1.0 61 39 200 12

Quillen & Garnett, astro-ph/0004210

Thick disk starsat [Fe/H] < -0.4, age >10Gyr 13

Edvardsson et al. 1993

Feltzing et al. 2003High [α/Fe] thick-disk stars Van der Kruit & Searle 1981

Thick disk in NGC4565

Thick disk

Thin disk

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[alpha-elements/Fe]= Type II SNe / Type Ia SNe

Bensby+ 2014

Thick disk

Thin disk

Chemical clock diagram

Age-Metallicity Relation (AMR)

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Presence of old but metal-rich starsgives a large dispersion in AMR

F・G-dwarfs near the Sun(Bensby et al. 2014)

Thick disk

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Metallicity distribution of the thick disk

fiducial metallicity range of the thick disk -1 < [Fe/H] < -0.4

(reproduced from Wyse & Gilmore 1995) (Chiba & Beers 2000)

But the rapidly-rotating disk component is extendedtoward lower metallicity: Metal-Weak Thick Disk (MWTD)

Thin disk vs. Thick disk

Thick disk

Thin disk

Thin disk

Thick disk

Lee et al. 2011

Toyouchi &Chiba 2014

SDSSsample

[Fe/H]

[Fe/H]

<Vϕ>(km/s)

[Fe/H]

Negative gradient

Positive gradient

Mean orbital radius(kpc)

Negativegradient

Positivegradient

3.3 Galactic halo

Ryan & Norris 1991, AJ, 101, 1865High proper-motion stars

[Fe/H]19

Unavane et al. (1996)

Halo stars (near the Sun)are old (>10 Gyr).Later accreted fraction< 10%

5 Gyr

1015

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[α/Fe] for Galactic stars (Venn et al. 2004)

Dwarf satellites

HaloThick diskThin diskRetrograde halo

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Duality in [α/Fe] ratios in halo stars(Nissen & Schuster 2010)

thick diskhalo

+

Vφ=220 km/s

Vφ=0 km/s

U,V,W velocitiesof nearby stars

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diskhalo -1.7

discontinuity

disk

halo

Chiba & Beers 2000Rotational properties of metal-poor stars

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K12: Kafle et al. 2012(using 4664 BHBs from SDSS)

Red cross: Deason et al. 2013(halo stars with HST proper motions)

β

Velocity anisotropy parameter β(r)

Radial anisotropy:β(r)=1- (σθ

2+σϕ2)/2σr

2rGCβ>0 β<0

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Bird et al. 2019using5600 K giants fromLAMOST and Gaia DR2

Loebman et al. 2019using simulation resultsby Bullock & Johnston 2005

β ~ 0.7Radially anisotropicover entire radii

Presence of temporal dips

Zinn 1985, ApJ, 293, 424Halo clusters, disk (bulge) clusters

Zinn 1993, ASP Conf. vol.48HB morphology

3.4 Globular clusters

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Mackey & Gilmore 2004, MN, 355, 504

-1.1Gyr-2.2 Gyr

Relative to the age of M92(~ 12.6 Gyr) 28

6kpc

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Bulge/disk

Old halo

Young halo

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

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LMC SMC

Fornax Sagittarius

Young halo clusters → external origin!? 32

ω CentauriLee et al. 1999

Z=0.00040.005

CTIO 0.9mEHB:Extended HB

ω CentauriBedin et al. 2004

High helium abundance !?Norris 2004

Na-O anti-correlation in GCs (Carretta+ 2010)

general properties of GCs ⇒ multiple stellar population!?

1. First-generation starschanged Na & O abundance inside these stars (Na⇑ O⇓)

2. Gas was expelled via.AGB & SN.

3. New-generation starsformed from thisprocessed gas

H-burning at high T(CNO, NeNa,MgAl cycle)

3.5 Galactic satellites

Scl Car For Leo II Leo I Phe37

List of bright satellites

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FornaxLeo I

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Mean [Fe/H] vs. luminosity for dSphs

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Kirby et al. 2008 More recent results

(old data)

Mass enclosed within stellar extent (~ 4 x 107M)in dSph galaxies

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Dark matter dominated!