Juntai Shen (Shanghai Astro. Obs.) 沈俊太 (上海天文台)

Dynamics of Disk Galaxies (Oct. 20, 2013 @ SNU)

Structure of Bulges and Bars

1

Bulges A disk galaxy = “bulge” + disk

1st order approx. But how do we define a bulge?

Bulge ≈ elliptical thing in the middle of a disk

Morphology / Photometry Face-on: bulge=extra light in the center Edge-on: bulge=brighter and thicker

center Kinematics

Rotation vs. random motions Surrogate definition is easy, but what

they are physically?

2 Kormendy & Kennicutt (2004) Athanassoula (2008)

Bulges: physical classification

Classical bulges

Pseudobulges Central disky comp. Boxy bulges

Peanut shape Side-on bars?

Different formation & evolution paths

Classification is difficult

3 Kormendy & Kennicutt (ARAA, 2004)

~ ellipticals: formed in mergers

Extra light or central thick component at small radii constructed from the disk by mostly internal secular processes

Classical bulges

4

≈ Mini-elliptical; similar morphological, photometrical, and kinematical properties

Major merger-made or made of clumps (Elmegreen et al. 2008)?

Sersic n > 2 Less rotation-dominated

Pseudobulges (I)

5

Extra light at small R Formed from disk by internal secular processes Retain some memory of their disk origin More rotation dominated

Cylindrical rotation Gas, dust, young stars Nuclear disky structures “Boxy bulges”: separate class

Pseudobulges (II)

6

Pseudobulges: lower n (

Boxy/peanut-shaped bulges

7

Sometimes show X-shaped extensions e.g. Bureau & Athanassoula (1999, 2005)

~45% edge-on disks have peanut-shaped bulges (Lutticke et al. 2000) Comparable to the fraction of bars

HCG 87

Boxy peanut-shaped bulges: side-on bars? Simulation of bended/thickened bars

Buckling/firehose instability (Toomre 1966) Bar formation buckling instability saturation

B/PS bulges Combes & Sanders 1981; Raha et al. 1991

Can also be explained by the vertical instabilities of bar-supporting x1 orbits e.g. Pfenniger & Friedli 1991; Skokos et al. 2002; Patis et al. 2002

RZ σσ 3.0≤

Detailed study on Boxy/peanut-shaped bulges

9

3 types of bulges are likely to co-exist in the same galaxy

How to separate their contributions First step: to isolate out contribution of boxy

bulges See Zhao-Yu Li’s talk on Thursday:

“Tomographic study of buckled bars”

Boxy/peanut-shaped bulges

10

3 buckled bars with different buckling strengths Purely collisionless, no gas, simple

Li, Shen et al. in prep.

Note that the boxy bulge is shorter than the bar

Boxy/peanut-shaped bulges

11

3 buckled bars with different buckling strengths Surface density profiles

Well-fit by a single sersic profile

Li, Shen et al. in prep.

Comparison with Edge-on Galaxies Similar density profile along the minor axis for the

boxy bulge No central pseudobulge and extended “disk” in

our model More in Zhao-Yu Li’s talk on Thursday

Kormendy & Barentine (2010) Li, Shen et al. in prep.

The Milky Way bulge

COBE Near IR image of the Milky Way

Most of bulge stars are old (>5 Gyr, Clarkson et al. 2008) A wide range of metal abundances (McWilliam & Rich

1994; Fulbright et al. 2006; Zoccali et al. 2008)

Stellar Kinematics of the Bulge BRAVA (Bulge Radial Velocity Assay) survey

~10,000 M giants as targets Stellar kinematics covering the whole Bulge

Build a simple dynamical model to explain it

14

The Milky Way in NIR (COBE)

Modeling the Milky Way Bulge • A simple model of the

Galactic bulge matches the BRAVA data extremely well in almost all aspects: – b = -4o major axis – b = -8o degree major axis – l = 0o degree minor axis – Surface density

• A tale of two instabilities

15

Shen, J., et al 2010

Power of simplicity High resolution N-body simulations with millions

of particles

Cold massive disk, initial Q ~ 1.2

A pseudo-isothermal rigid halo with a core which gives a nearly flat rotation curve of ~220 km/s from 5 to 20 kpc

Inside solar circle, Mdisk/Mhalo ~ 1.5, max disk

A good starting point

16

Modeling the Milky Way Bulge --- Surface Brightness Map

Sun DIRBE Composite map

– The bar angle from kinematic constraint is about ~ 20o – The bar’s axial ratio is about 0.5 to 0.6, and its half-length is ~4kpc

17 Shen, J., et al 2010

Modeling the Milky Way Bulge --- Match stellar kinematics in all strips strikingly well

18 Shen, J., et al 2010 Cylindrical rotation: hard to reproduce with a classical bulge

A Significant Classical Bulge is Excluded

The data excludes a pre-existing classical bulge with mass >~ 10% Mdisk; the MW is nearly a pure-disk galaxy! 19

Red clump: a good standard candle Along different lines of sight toward the Galactic

bulge, red clumps split into two groups McWilliam & Zoccali 2010; Nataf et al. 2010; Saito et al. 2011

Split Red Clumps in the Galactic Bulge

Nataf et al. (2010)

(0.27, -5.77) (-0.28, 5.76)

McWilliam & Zoccali (2010)

The full length of the structure is ~2.3 kpc in the radial direction. It tilts away from the Sun-GC line by ~ 20° “The double peaked RC is inconsistent with the tilted bar

morphology.” (McWilliam & Zoccali 2010)

X-Structure in the Milky Way?

McWilliam & Zoccali (2010)

End-to-end separations in the radial and vertical directions are roughly 3 kpc and 1.8 kpc, respectively.

Contribute ~7% of the boxy bulge light Orbits trapped around the vertically-extended x1 family

X-Structure in our model

Li & Shen (2012) McWilliam & Zoccali (2010)

As the longitude decreases, the peak at large distance becomes stronger with more distant particles.

The separation between the two peaks is roughly constant at different longitudes as in MZ10.

Comparison with Observations

Solar perspective

Two vertical lines mark the peak positions in (+1, -8)

As the latitude decreases, the separation between the two peaks also decreases.

The separation increases from ~2 kpc at b = ±5.5°to ~3 kpc at b = ±10.25°.

Comparison with Observations

Solar perspective

Two vertical lines mark the peak positions in (+1, -8)

Comparison with Observations

Saito et al. (2010)

b = -6°

b = -5°

Li & Shen (2012)

Solar perspective

Similar result in Ness et al. (2012)

Vertical Metallicity Gradient can be reproduced

with a secularly evolved bar/boxy bulge model

not a strong argument for the existence of a classical bulge in the Milky Way.

Mixing during the bar and buckling instabilities is incomplete, and therefore radial metallicity gradients in the initial disk can transform into gradients in the boxy bulge.

26

Martinez-Valpuesta & Gerhard (2013)

A similar simple bar model as in Shen et al. (2010)

Model Observations

Cautionary notes

27

Even unbarred galaxies can form boxy bulges Sellwood & Merritt (1994) Need to better understand their

relevance to real obs. Not all pure disk galaxies have

built a pseudobulge M33 has only a nucleus, not a

classical or pseudobulge Or a pseudobulge has faded out

into the disk? (KK04) Requires further study

Summary We seem to have somewhat coherent pictures on

different types of bulges Formation of boxy bulges due to bars is easier to

understand – helps to understand the formation and structure of composite bulges

Disk buckles to make boxy / peanut-shaped bulges – main driver shaping the MW bulge

The MW bulge is consistent with being a bar viewed edge-on

There is an X in the MW bulge! Its properties qualitatively match obs.

Further evidence that MW bulge formation is shaped mainly by internal dynamical instabilities 28

29 Credit: Zhao-Yu Li

That’s where I am gonna put the big black hole!

Thank you!

30

슬라이드 번호 1BulgesBulges: �physical classificationClassical bulgesPseudobulges (I)Pseudobulges (II)Boxy/peanut-shaped bulgesBoxy peanut-shaped bulges: side-on bars?Detailed study on Boxy/peanut-shaped bulgesBoxy/peanut-shaped bulgesBoxy/peanut-shaped bulgesComparison with Edge-on GalaxiesThe Milky Way bulge�Stellar Kinematics of the BulgeModeling the Milky Way Bulge Power of simplicityModeling the Milky Way Bulge --- Surface Brightness MapModeling the Milky Way Bulge ---� Match stellar kinematics in all strips strikingly wellA Significant Classical Bulge is ExcludedSplit Red Clumps in the Galactic BulgeX-Structure in the Milky Way?X-Structure in our modelComparison with ObservationsComparison with ObservationsComparison with ObservationsVertical Metallicity GradientCautionary notesSummary슬라이드 번호 29Thank you!SummaryFuture workBuckling instability and ellipticals슬라이드 번호 34Two bulge populationsDynamics of barsBars & ellipticals: fundamentally different!Naïve explanation of bar formationChallenges in disk galaxy formation of evolutionRed Clump StarsRecent work on the Bulge (I)The Milky Way as a pure-disk galaxyHighlighted by a Christian websiteHighlighted by a Christian websiteThe Milky Way is not specialNGC 4565: another giant pure-disk galaxyMany nearby spirals are nearly bulge-lessTake-home messagesPossible origins of bulgesClump-origin clumpsBekki & Tsujimoto 2011Inoue & Saitoh 2012MV & Gerhard 2013