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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
1
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
20
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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
24
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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
26
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
27
Mackey & Gilmore 2004, MN, 355, 504
-1.1Gyr-2.2 Gyr
Relative to the age of M92(~ 12.6 Gyr) 28
6kpc
29
Bulge/disk
Old halo
Young halo
30
Clusters in the LMC, SMC,+ Fornax and Sgr dwarf galaxies
31
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
38
FornaxLeo I
39
Mean [Fe/H] vs. luminosity for dSphs
40
Kirby et al. 2008 More recent results
(old data)
Mass enclosed within stellar extent (~ 4 x 107M)in dSph galaxies
41
Dark matter dominated!