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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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• 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).