Formation of high-redshift disks and pseudo-bulges in nearby - - PowerPoint PPT Presentation

Formation of high-redshift disks and pseudo-bulges in nearby galaxies

Takashi Okamoto

(Kobe-branch, CCS,Tsukuba Univ.）

• T. Okamoto 2012 MNRAS 428, 718

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High-redshift disks

• Galaxies at z ~ 3 often

show disky morphology.

• What are they and what

will they be at z = 0?

• The size is comparable to

the galactic bulge.

Stark+’08

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Bulges

• Two classes of bulges
• Classical bulge
• ellipsoid (Sérsic index n > 2)
• mainly supported by velocity dispersion
• formed by a major merger?
• Pseudo-bulge
• boxy/peanut, disky (n < 2)
• non-negligible rotation
• formed via secular evolution of

a disc?

Athanassoula 05

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Bulge formation (standard pictures)

merger bulge formation newly accreted gas forms a new disk

classical bulge pseudo-bulge

disk instabilities (bar, clump, etc.) bulge formation

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Bulges of nearby large disk galaxies

• More than half of nearby large disks have

pseudo-bulges (Weinzirl+09, Kormendy+10)

• Is this fact consistent with the CDM model?
• How do the pseudo-bulges form under

CDM？

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This study

• Cosmological hydrodynamic simulations of galaxy

formation

• Two Milky Way-sized halos (Aquarius halos) in

100 h-1 Mpc comoving box (Aq-C, Aq-D)

• What type of bulge forms?
• How they form?

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Aq-C

• no significant mergers

below redshift 4

• disc formation begins

around redshift 2

• there is a bar below

redshift 1

• The orientation of the

disc changes with redshift

face-on edge-on stars gas

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Aq-C

• no significant mergers

below redshift 4

• disc formation begins

around redshift 2

• there is a bar below

redshift 1

• The orientation of the

disc changes with redshift

face-on edge-on stars gas

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Aq-D

• no significant mergers

below redshift 4

• disc formation begins

around redshift 2

• can’t see bar-like

structure

• clumpy star formation

below redshift 1

stars gas face-on edge-on

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Aq-D

• no significant mergers

below redshift 4

• disc formation begins

around redshift 2

• can’t see bar-like

structure

• clumpy star formation

below redshift 1

stars gas face-on edge-on

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Surface density profiles

• Fit the bulge by the Sérsic profile:
• Aq-C: n = 1.2
• Aq-D: n = 1.4

pseudobulge-like

r < 3 kpc でバルジ成分が卓越

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Bulge shapes

Both bulges are pseudo-bulges

face-on edge-on

• A bar in Aq-C
• Both bulges have disky

contour shapes in edge-

• n
• Weak signature of

boxy-bulge in Aq-C

• Diamond shape of Aq-

D’s bulge is a strong evidence of disky bulges

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Kinematic properties

• A signature of cylindrical

rotation in Aq-C’s bulge

• Aq-D’s bulge is mainly

supported by rotation

• Aq-C’s bulge has large

velocity dispersion but the non-circular orbit is confined in a thin oblate (small σz) .

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Evolution of surface density profiles

• By z = 2ー3, the bulges have formed as disks with small scale length.
• From z ~ 2, the main disks with large scale length form around the disky bulges.
• The bulge masses at z = 2 account for 70% (Aq-C) and 87%(Aq-D) of those at

z = 0.

The main process of the pseudo-bulge formation is NOT the secular evolution in these simulations

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Evolution of the galaxies

• At each redshift, distributions of stars and gas within 0.1 Rvir

are shown in face-on and edge-on.

• The progenitors are always disk-like.

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Star formation histories of the bulges

• Formation histories of

stars with in 3 kpc at z = 0

• Bulge stars are mainly

formed by high- redshift starbursts

• Mostly in ¡situ.

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Merging histories of the galaxies

• No clear correlation

between mergers and star formation activities

• Quiet merging histories

at z < 4.

• High-z major mergers

are quite ‘wet’

SFR merging mass ratio Mgas galaxy mass (M* + Mgas)

Mergers are irrelevant to pseudo-bulge formation

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Gas distribution during starbursts

• Gas distribution when

star formation rate is highest.

• Starbursts occur in

a gas disks.

• Pseudo-bulges form

as high-redshift disks.

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Velocity maps of the star-forming gas at z = 3

Aq-C Aq-D LoS mean velocity LoS velocity disp.

• Similar to the
• bserved high-

redshift disks.

• Velocity dispersion

is significantly lower than the LoS velocity probably because feedback implementation.

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Formation time-scale

• f the bulges
• blue filled circles: pseudo-

bulges

• cyan filled circles: pseudo-

bulges in late-type disks

• blue open circles: inactive

pseudo-bulges

• red open squares: classical

bulges

• Simulated bulges are

inactive pseudo-bulges

bulge mass specific SFR of the bulges

Fisher+’09

Aq-C Aq-D

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Summary

• (inactive) pseudo-bulges naturally form in Milky Way-

sized galaxies

• The formation of pseudo-bulges has completed before

disk formation ⇒ secular evolution of disk is unimportant

• These pseudo-bulges form by high-redshift starbursts

triggered by rapid supply of gas.

• The progenitors of pseudo-bulges would be observed as

high-redshift disks

• Although secular evolution only accounts for 30 % (Aq-

C) and 13 % (Aq-D) of bulge mass pseudobulge, it does affect the spacial and kinematic properties of the pseudo-bulges.

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v/σ

• 半径の関数として vrot/σ を plot
• Aq-C は low-z ほど中心部で vrot/σが減少
• たぶん bar の影響
• Aq-D は逆に z = 1 からほとんど進化なし

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