distribusi kecepatan dan metalisitas bintang, formasi galaksi.pptx
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7/28/2019 Distribusi kecepatan dan metalisitas bintang, formasi Galaksi.pptx
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Fisika Galaksi
DISTRIBUSI KECEPATAN DAN METALISITAS
BINTANG, FORMASI GALAKSI DAN GALACTIC-
ARCHAEOLOGY
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thick disk
stellar halo
bulgethin disk
The Galaxy
How did the Galaxy come to be like this ?
What is the origin/formation epoch/mechanism and relation between the various components?
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Chemodynamics of the Galaxy: Ingredients
Astrometry
Distances, proper motions, radial velocities
kinematic motions, orbital parameters
(U, V, W) , (Rm, e, |Z|)
Chemical tagging
High resolution spectra of samples of stars
Derivation of abundances of elements of
different nucleosynthesis history
Photometry
scale lengths, scale heights, number densities etc
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7/28/2019 Distribusi kecepatan dan metalisitas bintang, formasi Galaksi.pptx
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• Komponen kecepatan bintang relatif terhadap Matahari :
∗ = cos cos + , cos sin + , sin
∗ = sin cos + , sin sin + , cos ∗ = sin + , cos
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• Dalam bentuk perkalian matriks
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The Milky Way components: kinematic definitions
Comp. f Vlag σu σv σw
(km /sec)
Thin 93% -12 39 20 20
Thick 7% -51 63 39 39Halo 0.6% -220 131 106 85
(Based on Hipparcos: Dehnen, Binney, 1998, Soubiran et al.2000, Robin et al. 2003)
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The Milky Way: Kinematic Samples
Reddy et al. 2003,2006; Bensby+; Fuhrman+
Samples grouped into the thin disk,
the thick disk and the halo based
on kinematic definitions.
No metallicity or age criteria employed
Thin Disk
Thick Disk
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THE STELLAR SAMPLE
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WHAT ARE THE THICK DISKS?
• Detected as exponential
excess of flux at a few thin
disc scale-heights above the
mid-plane.
• Made of older and metal-
poorer stars than the thin
disk.
• They are ubiquitous.
• Scale-length similar to that
in thin disks.
Burstein 1979
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HOW DO THEY FORM?
Possibility 1: Dynamical heating of a cold disk by disk
overdensities like giant molecular clouds or spiral arms.
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HOW DO THEY FORM?
Possibility 2: Dynamical heating of the thin disk due to satellites
crossing it.
Thickening + flare!
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HOW DO THEY FORM?
Possibility 3: in situ formation. Disks form originally thick.
Disk forms thick and thin disk forms from gas accreted through
cold flows or which has not been spent in the first star formation
burst.
Cold gas
flow
Cold gasflow
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HOW DO THEY FORM?
Possibility 4: Accretion of a thick disk by the thin disk.
Kinematical signatures!
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WHAT IS THIS LETTER ABOUT?
Possibility 1: Dynamicalheating of a cold disk bydisk overdensities like giantmolecular clouds or spiral
arms.
Possibility 2: Dynamical
heating of the thin disk due
to satellites crossing it.
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SIMULATION PROPERTIES FOR POSSIBILITY 1
• Simulations designed for creating plausible z = 2 progenitors of Milky
Way-like spirals.
• N-body simulation with sticky-particle algorithm for representing cold
gas.• Initial disk with a constant disk and gas density truncated at R = 6
kpc and with scale-height hz = 500 pc.
• Code run in isolation for 1 Gyr and then, for 6 Gyr, includes cold disk
flows which increase galaxy mass.
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SIMULATION PROPERTIES FOR POSSIBILITY 2
• Initially contains a fully-formed spiral galaxy.
• The galaxy suffers one or a few minor mergers with varying orbital
parameters.
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POSSIBILITY 1: RESULTS
Gravitational instabilities appear soon, scatter stars and formclumps with masses up to a few 108 M
ʘ.
The stellar scattering caused by the clumps increase even more
the disk thickening.
hz = 500 pchz = 2 kpc
Clumps merge and form a bulge.
hz = 1.5 kpc
Cold flows increase the thin disk mass and reduce the thick disk
scale-height.
Cold gas
flow
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POSSIBILITY 1: RESULTS
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POSSIBILITY 1: RESULTS
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POSSIBILITY 1: RESULTS
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POSSIBILITY 2: RESULTS
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POSSIBILITY 2: RESULTS
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POINTS IN FAVOUR OF POSSIBILITY 1
• Observed thick disks do not flare substantially (in general).
• Although minor mergers occur, they are far more frequent at high
redshift, where a large disk gas fraction prevents the thin disk to be
thickened by this mechanism (Moster et al. 2010).• Kinematical studies suggest that thick disks have formed in one
single event and not due to a succession of mergers (Moni Bidin
2009).
• Thick disk formation scale-time in Possibility 1 (around 1 Gyr) iscompatible with thick disk high α/Fe abundances.
• Observations of very clumpy edge-on galaxies in the HUDF.
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AND THE WINNER IS...
Possibility 1: Dynamicalheating of a cold disk bydisk overdensities like giantmolecular clouds or spiral
arms.
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BUT...
• Thick disks created in this Letter are not as massive as what recent
analysis suggest the thick disks are (Comerón et al. 2011, sent to
ApJ).
•Elemgreen & Elmegreen (2006) found clumpy edge-on disks in theHST Ultra-Deep Field, but those clumps are as large as the thick disk
scale-heights, so thick disks could be (partly) created thick and not
be the result of a heated thin disk.
• At least a part of the material in thick disks comes from the accretionfrom external material as a significant fraction of counterrotating stars
has been found in one thick disk (Yoachim & Dalcanton 2008).
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