Merger History and Impact on Star Formation at z<=1
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Merger History and Impact on Star Formation at z<=1 & [Constraints on bulge assembly via mergers since z<=4 ]. Shardha Jogee University of Texas at Austin. Collaborators - S. Miller, K. Penner , GEMS collaboration (H. W Rix, R. Skelton, R. Somerville,

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Collaborators

Merger History and Impact on Star Formation at z<=1

&

[Constraints on bulge assembly via mergers since z<=4 ]

Shardha Jogee

University of Texas at Austin

Collaborators

- S. Miller, K. Penner , GEMS collaboration (H. W Rix, R. Skelton, R. Somerville,

E. Bell, C. Wolf, Z. Zheng) & C. Conselice

- S. Khochfar, R. Somerville, P. Hopkins, A. Benson, A. Maller

- I. Marinova, T. Weinzirl, F. Barazza


Collaborators

Merger History & Impact on Star Formation at z<=1

Jogee et al & GEMS team 2008, 2009 (arXiv:0810.5617)

Goals

What is relative importance of different galaxy assembly modes as f(z) :

major mergers, minor mergers, cold gas accretion, secular mode

1) Provide empirical constraints on major + minor merger history out to z~1

2) Compare with predictions from LCDM-based models

3) By how much is <SFR> enhanced in normal vs visibly merging system?

4) What % of the SFR density comes from visibly merging galaxies ?


Collaborators

Ingredients

- ACS F606W 0.1” images from GEMS (Rix et al 2004)

- Spectro-photo z + stellar masses from COMBO-17 (Borch+06; Wolf+04)

- UV and IR-based SFR from COMBO-17 & Spitzer (Bell et al 2007)

- z ~ 0.2 to 0.8 (Tback~3 to 7 Gyr)

- High mass (M/M0 >= 2.5x1010):

Complete for red seq and blue

cloud : N~800 galaxies

- Interm. mass (M/M0 >= 1x109):

Complete for blue cloud only

N~3700 galaxies


Collaborators

Methodology : identifying mergers

  • Method 1

  • Visual classification of ~3700 galaxies by 3 classifiers

  • Identify systems (of M>Mcut) which show evidence of having experienced

  • a merger of mass ratio >1/10 in the last visibility timescale

  • Method 2

  • Automated CAS criterion based on asymmetry A and clumpiness S

  • A> 0.35 and A >S


Collaborators

Visual classification of Interacting vs Non-Interacting systems

Non-interacting E-Sd

Non-Interacting Irr1

Galaxies with small –scale asymmetries that can be internally triggered (e.g.,

via stochastic SF or low V/s) without any galaxy-galaxy interactions.

Mergers

Systems with evidence (e.g.,

morphological distortions) indicative of a merger of M1/M2>1/10 in last t_vis.

e.g., single remnants with tidal tails, warps, strongly asymmetric arms, double nuclei, galaxies bounded by a bridge,

e.g. very close (d<< t_vis * v) pair of overlapping galaxies


Collaborators

Example of interacting systems

2 at similar z

2 at similar z


Collaborators

Separate mergers into major minor, major/minor

Mergers

Systems with evidence (e.g.,morphological distortions) indicative of a merger of M1/M2>1/10 in last t_vis.

Clear Major (M1/M2>1/4)

- Double nuclei same L

- Contact pair w/ M1/M2>1/4

and z1~z2

- Train wreck

Clear Minor (1/10 < M1/M2 <1/4)

- Contact pair with M1/M2

~1/4 to 1/10 and z1~z2

- Single system where disk

has survived, but shows

a warp or strong tidal

signatures

Ambiguous: Major or Minor

% of minor or major

% of clear majors

% of clear minor


Collaborators

Test effect of bandpass shift and SB dimming on visual f

  • In last bin z =0.6--0.8

  • - rest-frame l of GEMS V

  • image shifts to near-UV

  • (3700-3290 A)

  • - SB dimming by factor of 8

  • Compare f from GEMS v vs deep, redder GOODS z

  • Results changes by less

  • than 1.07


Collaborators

Methodology : identifying mergers

  • Method 1

  • Visual classification of ~3700 galaxies by 3 classifiers

  • Identify systems (of M>Mcut) which show evidence of having experienced

  • a merger of mass ratio >1/10 in the last visibility timescale

  • Method 2

  • Automated CAS criterion based on asymmetry A and clumpiness S

  • A> 0.35 and A >S


Collaborators

Results


Collaborators

Merger fraction from CAS vs visual classifications

What are the visual types of the M*>1e9 systems picked by the CAS criterion (A>0.35 and A>S) ?

1) 44% (z~0.3) to 80% (z~0.7) are visually-classified non-interacting (Irr1, E-Sd) galaxies

 high contamination from non-interacting systems especially at z>0.5

2) the remaining are visually-classified merger systems [50% to 70% of latter are picked]


Collaborators

Mergers/interactions missed by the CAS criterion (A>0.35,A>S)

Non-Interacting galaxies picked by CAS criterion (A>0.35,A>S)


Collaborators

Merger history of massive galaxies since z~0.8 (last 7 Gyr)

For high mass (M>=2.5e10) galaxies

Interaction fraction f (for mass ratio >1/10) ~ 8% to 9%

fraction of clear major (M1/M2>=1/4) interactions ~1% to 3%

fraction of clear minor (1:4 to 1/10) interactions ~ 4% to 8%

fraction of ambiguous minor or major interactions ~ 1% to 2%

For an assumed visibility time of 0.5 Gyr, this implies that over Tb=3-7 Gyr (z=0.2-0.8) , every massive galaxy has undergone 0.7 interactions of mass ratio >1/10, of which 1/4 are major mergers, 2/3 are minor mergers, and rest are major/minor.

Jogee et al. & GEMS collaboration 2009


Collaborators

Compare merger rate of galaxies with LCDM models

  • Data

  • Rate= n f /Tvis for (major+minor)

  • Models

  • solid line = f(major + minor)

  • dotted lime = f_major

  • Models

  • - 3 SAMs w/ AGN feedback

  • - HOD w/ AGN feedback

  • - SPH cosmological

For high mass galaxies,the (major + minor) merger rate of models

- show factor of 5 dispersion

- bracket the observed rate &

show qualitative agreement

Jogee et al. & GEMS collaboration 2009


Collaborators

Star formation over last 7 Gyr

SFRUV ~ 0.1--25 Mo yr-1

Median (SFRIR/SFRUV) ~ 4

for 900 galaxies with both

Spitzer and UV data

 significant obscured SF


Collaborators

<SFR> in Mergers vs Non-Interacting Galaxies over last 7 Gyr

3 measures of SFR

1) SFRUV from LUV of COMBO-17

for full sample [N= 3698]

2) SFRUV + SFRIR from Spitzer

24 mu, detected in only 24%

of sample [N=878]

3) SFRUV + SFRIR-stacked from

stacking 24 mu frame (Zheng

et al 2007) for 87% of sample

Mean SFR of visible mergers is enhanced only by a modest factor (~1.6 to 2) w.r.t that of non-interacting galaxies

Similar results by Robaina et al. in prep

(Jogee et al. & GEMS collaboration 2009)


Collaborators

(Di Matteo, P. et al. 2007)

Statistical study of several hundred TREE-SPH simulations of major

mergers of different B/D, gas, orbital

parameters, etc

They find max SFR of most mergers is only enhanced by ~2 to3, compared to isolated case


Collaborators

SFR density from mergers over last 7 Gyr

For M*>=1e9 Mo & M*>=2.5e10

visible mergers account for less than 30% of the SFR density over z~0.2--0.8 (Tb=3 to 7 Gyr)

  • Decline in SFR density driven by shutdown in SF of normal galaxies

  • (Gas consumption by SF ? Decline in smooth gas accretion rate ? Transition of SF to lower masses )

(Jogee et al. & GEMS collaboration 2009)


Collaborators

Summary: Merger History & Impact on SF over 7 Gyr

1. Merger historyfor high mass (M>=2.5e10) galaxies

- Fraction of mergers (of mass ratio >1/10) ~ 8% to 9%

- For an assumed visibility time of 0.5 Gyr, this implies that over Tb=3-7 Gyr, every

massive galaxy has undergone 0.7 interactions of mass ratio 1/10, of which 1/4

are major mergers, 2/3 are minor mergers, and rest are major/minor.

2. Visual vs automated CAS methods

CAS merger criterion

 capture 50% to 70% of visually-classified mergers

 at z>0.5, has high contamination rate (80% at z~0.7) by non-interacting systems

3. Comparison with LCDM-based models

For high mass galaxies, the (major + minor) merger rate of models show a factor

of 5 dispersion and bracket the observed rate. Qualitative agreement

4. Impact on SF

For both M*>=1e9 Mo and M*>=2.5e10 Mo, visible mergers

- have their mean SFR enhanced by only ~1.6 to 2 wrt to non-interacting galaxies

- account for less than 30% of the SFR density over z~0.2--0.8 (Tb=3 to 7 Gyr


Collaborators

Constraints on bulge assembly and merger history since z<4

Weinzirl, Jogee, Khochfar, Burkert & Kormendy 2009, ApJ, in press (arXiv:0807.0040)

  • Bulge + Disk+ Bar decomposition of H image of high mass (M*/M0 ~1010 -1011 ) spirals

 Most (66%) high mass spirals have low bulge B/T < 0.2 (77% have n<2)

 Such bulges exist in barred & unbarred galaxies across different H-types (S0-Sc).


Collaborators

Comparing with Hierarchical Models

  • Models from Khochfar & Burkert (2005) Khochfar & Silk 2006)‏

  • - DM halo merger trees from the extended Press-Schechter formalism

  • - Semi-analytic prescriptions for star formation (Cox+08) , cooling, SN feedback

For Galaxies with a past major merger

 Major mergers at z<=2 lead

to bulges with present- day

B/T > 0.2

 Also true for most major

mergers at z<=4

(Weinzirl, Jogee, Khochfar, Burkert & Kormendy 2009)


Collaborators

Model vs data

Present-day Data Model galaxies Model galaxies Model Galaxies

B/T w/ major merger w/o major merger All

since z<=4 since z <=4

------------------------------------------------------------------------------------------------------------------

B/T <=0.266% ~1.6% ~66% ~68%

0.2 <B/T<= 0.4 26% ~5% ~13% ~18%

B/T > 0.4 8% ~13% ~1 % ~14%

  •  Major mergers since z<=4 fail seriously (by a factor of ~30) to account for the low B/T<0.2 bulges present in 2/3 of high mass spirals

  •  It is likely that such bulges are primarily built by minor mergers

  • and secular processes at z<=4

(Weinzirl, Jogee, Khochfar, Burkert & Kormendy 2009)


Comparing with hierarchical models

Comparing with Hierarchical Models

  • Make a quantitative comparison with the predicted B/T distribution from CDM-based models (Khochfar & Burkert 2005; Khochfar & Silk 2006)‏

  • DM halo merger trees from the extended Press-Schechter formalism (Somerville & Kolatt 1999)‏

  • Baryonic physics from semi-analytic prescriptions for SF, cooling, supernovae feedbackMajor merger (M1/M21/4) dynamics:

    Major mergers set B/T to 1; B/T declines after major mergers due to disk buildup by cold accretion

    A galaxy with a past major merger can have B/T0.2 at z=0 only if zlast2

Galaxies with a past major merger

Courtesy of Khochfar & Burkert


Distribution of b t and bulge index

Distribution of B/T and Bulge Index

  • Mean B/T, bulge index are consistent with other work (e.g., Laurikainen et al. 2007; Graham & Worley 2008).

  • 68% of bulges have B/T0.2; 77% have n2 . Such bulges exist in barred and unbarred galaxies across a wide range in Hubble type!

Weinzirl et al. 2008

Weinzirl et al. 2008


Collaborators

Galaxies with a past major merger


Distribution of b t data vs model

Distribution of B/T: Data vs Model

  • The fraction of model galaxies with a past major merger and B/T≤0.2 is 3%, more than 20 times smaller than the observed fraction (66%).

  • B/T≤0.2 bulges cannot have been built by major mergers!

Weinzirl et al. 2008


Collaborators

Constraining Galaxy Evolution…..

LCDM models: good paradigm for DM + structure on large scales

  • Predictions for galaxy evolution

  • = f (baryonic physics)

  • Possible areas of discord

  •  Substructure or missing satellite problem

  •  Angular momentum problem

  •  Problem of bulgeless and low B/T galaxies

(Springel et al. 2005)

Need empirical constraints on baryonic ‘physics’

 Merger history and its impact on SF  part 1 of this talk

 Bulge properties as f (M)  part 2 …

 Mechanisms redistributing angular momentum: mergers, bars

 Feedback (SF and AGN)


Collaborators

Merger fraction from visual classifications versus CAS

  • For high M/Mo>=2.5e10

  • CAS-based f agrees within a factor of less than two with visual f

  • For interm M/Mo>=1e9

  • CAS method overestimates f by a factor of 3 at z>0.5… as it picks up a large number of non-interacting galaxies (E-Sd and Irr1)

Jogee et al 2008, 2009


Properties and origin of bulges in high mass spirals

Properties and Origin of Bulges in High Mass Spirals


Motivation

Weinzirl et al. 2008 (ApJ submitted; arXiv:0807.0040) has two goals:

Quantify B/T and bulge index for nearby high mass galaxies

Make a detailed quantitative comparison with CDM-based models

Motivation

  • Bulges provide important clues about galaxy formation, but the absence of bulges is likewise interesting!

  • Classical bulges are often absent locally:

    • 15% of edge-on galaxies are bulgeless (Kautsch et al. 2006)‏

    • 20% of i<60○ low-mass disks are quasi-bulgeless (Barazza, Jogee, & Marinova, 2008)‏

    • 11/19 galaxies with D<8 Mpc and Vc>150 km/s have pseudobulges (Kormendy & Fisher 2008)

  • Must compare distribution of bulge-to-total mass (B/T) and bulge index to CDM-based models for high and low masses

  • Kautsch et al. 2006

    Barazza, Jogee, Marinova (2008)‏


    Sample

    Main sample is 146 i < 70○ galaxies, complete for M* > 1010 M⊙ and MB < -19.3

    Sample

    • Drawn from OSU Bright Spiral Galaxy Survey:

      • Bright local field galaxies with mB≤12

      • Reference sample for bars in the local Universe (Eskridge 2000; Marinova & Jogee 2007)

  • Use H-band light to trace stellar mass


  • Luminosity decomposition

    We perform 2D bulge-disk and bulge-disk-bar decomposition with GALFIT (Peng et al. 2002)‏

    Luminosity Decomposition

    • Galaxy light is emitted from physically and dynamically distinct components:

    • Most previous 2D decompositions have used only bulge-disk models (e.g., Allen et al. 2006)‏

    • Inclusion of the bar in 2D bulge-disk-bar decomposition is important:

      B/T and bulge index are overstated in 2D bulge-disk decomposition of barred galaxies (Laurikainen et al. 2005)‏

      60% of galaxies are barred in H-band (Marinova & Jogee 2007)‏

      Optical bar fraction is higher in galaxies without prominent bulges (Odewahn 1996; Barazza, Jogee, Marinova 2008; Marinova et al. 2008; Aguerri et al. 2008)‏


    Decomposition with galfit

    Stage 1:One Component

    Input guesses for single Sersic component

    Fit single Sersic profile

    Stage 2:Two Components(bulge+disk or bar+disk)‏

    Stage 1 outputs are input guesses for bulge(disk b/a, PA fixed to pre-determined values)‏

    Fit Sersic profile + exponential disk

    Stage 3:Three Components(bulge+disk+bar)‏

    Stage 2 outputs areinput guesses for bulge and diskInclude guesses for bar parameters.(disk b/a, PA fixed to pre-determined values)‏

    Fit Sersic bulge + exponential disk + Sersic bar

    Choose the best fit from Stage 2 and Stage 3 based on:

    2ResidualsModel parametersData image

    Decomposition With GALFIT

    • For 78% of galaxies, nuclear point sources were added to the best model(to account for AGN, HII nuclei, nuclear star clusters)‏


    Sample decomposition for ngc 4643

    Stage 1:One Component

    Stage 2:Two Components(bulge+disk)‏

    Stage 3:Three Components(bulge+disk+bar)‏

    Sample Decomposition For NGC 4643

    • B/T usually changes between Stage 2 and Stage 3 by a factor of <4

    Residual bar light

    Weinzirl et al. 2008

    Light redistributed from bulge & diskto bar


    Sample and method

    Sample and Method

    Results


    Distribution of b t and bulge index1

    Distribution of B/T and Bulge Index

    • Mean B/T, bulge index are consistent with other work (e.g., Laurikainen et al. 2007; Graham & Worley 2008).

    • 68% of bulges have B/T0.2; 77% have n2 . Such bulges exist in barred and unbarred galaxies across a wide range in Hubble type!

    Weinzirl et al. 2008

    Weinzirl et al. 2008


    Bar fraction vs b t and bulge index

    H-band bar fraction is greater by a factor of two for low B/T and low bulge index galaxies!

    Bar Fraction vs B/T and Bulge Index

    • H-band bar fraction is 58% (84/146), in agreement with other studies on the same data (Marinova & Jogee 2007; Laurikainen et al. 2004; Eskridge et. al 2000)‏

    • Is H-band bar fraction sensitive to B/T and bulge index?

    • Is there a relationship between bulges and bars?

      Secular evolution may build low-B/T, disky bulges

      Or, low-B/T galaxies with no ILR are more susceptible bars induced by swing amplification with a feedback loop (Julian & Toomre 1966; Toomre 1981; Binney & Tremaine 1987)‏


    Comparing with hierarchical models1

    Comparing with Hierarchical Models

    • Make a quantitative comparison with the predicted B/T distribution from CDM-based models (Khochfar & Burkert 2005; Khochfar & Silk 2006)‏

    • DM halo merger trees from the extended Press-Schechter formalism (Somerville & Kolatt 1999)‏

    • Baryonic physics from semi-analytic prescriptions for SF, cooling, supernovae feedbackMajor merger (M1/M21/4) dynamics:

      Major mergers set B/T to 1; B/T declines after major mergers due to disk buildup by cold accretion

      A galaxy with a past major merger can have B/T0.2 at z=0 only if zlast2

    Galaxies with a past major merger

    Courtesy of Khochfar & Burkert


    Minor mergers and secular evolution

    Minor Mergers and Secular Evolution

    Contribution of minor mergers:

    Satellite deposits stars in central region of the primary

    Gas inflow from tidally induced bars and tidal torques

    Included in modelNeglected in model

    Contribution of secular evolution:

    Bar-driven inflow between mergers

    Boxy/peanut bulges from bar bending/buckling

    Minor mergers add all stellar mass in satellite to bulge of primarySecular processes are neglected

    • Bulge formation mechanisms include major mergers (M1/M21/4), minor mergers (1/10<M1/M2<1/4), and secular evolution


    Distribution of b t data vs model1

    Distribution of B/T: Data vs Model

    • The fraction of model galaxies with a past major merger and B/T≤0.2 is 3%, more than 20 times smaller than the observed fraction (66%).

    • B/T≤0.2 bulges cannot have been built by major mergers!

    Weinzirl et al. 2008


    Summary future work

    Summary & Future Work

    • Sample: 146 i < 70○ galaxies; complete for M* > 1010 M⊙ and MB < -19.3

    • Model: Hierarchical CDM-based models from Khochfar, Burkert, & Silk

    • Results:

      Low B/T0.2 bulges are found in 68% of spirals; n2 bulges are found in 77%

      Fraction of model galaxies with past major mergers at z<=4 and B/T<0.2 is more than 20 times smaller than the observed fraction

    • Future observational work:

      Measure ages of bulges relative to bars and disks with IFU spectroscopy

    • Ongoing decomposition of the dense Coma cluster (ACS Coma Cluster Treasury Survey; Carter et al. 2008)

    • Study properties of massive disks at 1.5<z<3 from the GOODS NICMOS survey (Conselice et al. 2008)‏


    M l ratio

    M/L Ratio

    • Backup slide of Schneider plot for M/L ratio and stellar populations

    • Bell equation for stellar mass+photometric vs dynamical mass + definitions


    Swing amplifier

    Swing Amplifier

    • Diagram of swing amplifier loop. Destruction of ILR


    Stellar masses

    Stellar Masses

    • Bell equation for stellar mass+photometric vs dynamical mass + definitions


    Collaborators

    Extra slides :


    Collaborators

    Specific SFR vs Mass : the fractional growth of galaxies

    Only modest enhancement in SSFR of Dist/Int vs normal galaxies

    Larger SSFR (fractional growth) in low mass than high mass galaxies at

    z<1 (“downsizing”)

    See Noeske et al al (2007) : SFR as f(M) and proposal of staged SF model


    Collaborators

    Challenges for predicting how galaxies evolve

     Model predictions not unique/robust

    31Mpc/h

    1) Limited dynamic range + spatial resolution

    [N=1010, D=500Mpc/h, Resolution~5kpc/h]

    DM

    2) Halo occupation statistics

    3) DM halo merger  galaxy merger history

    Light

    4) Assumed baryonic physics

    - Model of ISM

    - Recipes for star formation and feedback,

    - Mechanisms to redistribute angular momentum (mergers, bars, dynamical friction)


    Collaborators

    Challenges for LCDM models of galaxy evolution

    LCDM models = good paradigm for how structure and DM evolves on large scales

    (Springel et al. 2005)

    Millenium Run : 1010 particles

    Follows DM in region D=15 Mpc/h

    Resolution = 5 kpc/h


    Collaborators

    Challenges for predicting how galaxies evolve

     Model predictions not unique/robust

    1) Limited dynamic range + spatial resolution

    Cannot simultaneously model large-scale

    environment and resolve galaxy

    components (bulge, bar, disk)

    [N=1010, D=500Mpc/h, Resolution~5kpc/h]

    31Mpc/h

    DM

    2) Halo occupation statistics

    3) DM halo merger  galaxy merger history

    Light

    4) Assumed baryonic physics

    Model of ISM, recipes for star formation and

    feedback, mechanisms to redistribute angular

    momentum (mergers, bars, dynamical friction)


    Collaborators

    Broad Questions For This Workshop

    1) Status of challenges to LCDM models of galaxy evolution?

    - Angular momentum problem

    - Challenge of galaxies with no bulges or bulges of (low B/T, n)

    - Substructure or missing satellite problem

    - Cusp–core controversy

     Latest empirical constraints on the history of (mergers, SF, and structural assembly)

     Are problems alleviated by improvement in resolution + baryonic physics (feedback)

    2. New challenges ?

    - massive disks at z~1.5 to 3 with high SFR/bulges but no signs of major mergers

    - mass function of very massive galaxies

    3. Relative importance of different galaxy assembly modes as f(z)

    major mergers, minor mergers, cold gas accretion, secular modes

    4. SF and AGN activity: triggers and feedback


    Collaborators

    (A) Challenge of galaxies with no bulge or low (B/T, n) bulges


    Collaborators

    (A) Challenge of galaxies with no bulge or low (B/T, n) bulges

    • 1) Major mergers build classical bulges

    • Violent relaxation of stars  spheroid of low v/s, n=4 (or 2<n <6)

    • B/T at z~0 depends on epoch of last major merger & subsequent disk buildup

    Every galaxy that had a major merger at an epoch when its mass was a significant

    fraction of its present-day mass should harbor a classical bulge with a significant

    bulge-to-total (B/T) ratio.

    • 2) Secular processes build disky pseudobulges and boxy bulges

    • Gas inflow driven by a bar in non-interacting galaxy  SF builds disky, high v/s, low n<2.5

    • stellar component = disky/pseudobulge (Kormendy 93)

    • Buckling instability + vertical ILRs make edge-on bars look peanut/boxy (Combes, Shlosman)

    • 3) Minor mergers build …. bulges

    • Gas inflow driven by induced bar and tidal torques  SF builds disky component?

    • Satellite accretion in central region builds/enhances bulge. Structure?


    Collaborators

    (A) Challenge of galaxies with no bulge or low (B/T n) bulges

    1) In low mass/late type galaxies: bulgeless galaxies are frequent

    - late type galaxies are often bulgeless (Boker et al. 2002)

    - 15% of edge-on SDSS galaxies are thin bulgeless disks (Kautsch et al. 2006)

    - 20% of i<60 SDSS galaxies at z<0.03 appear bulgeless (Barazza, Jogee, Marinova 08)

    (Kautsch et al. 2006)

    (Barazza, Jogee, Marinova 08)


    Collaborators

    (A) Challenge of galaxies with no bulge or low (B/T n) bulges

    2) Even high mass spirals show a high frequency of low (B/T, n) bulges

    - Most S0 -S0/Sa have bulges with Sersic n < 2 (Balcells et al 03; Laurikainen et al 07)

    - 11/19 galaxies with D<8 Mpc& Vc>150 km/s have pseudobulges (Kormendy & Fisher 08)

    - For a sample of 140 M*>1e10 spirals:

    66% have B/T < 0.2 & 77% have n< 2.

    SAM models predict that galaxies with a

    past major merger can only account for

    3% of spirals with such low B/T.

     Are remaining bulges built via

    minor mergers and secular modes?

    (Weinzirl et al. 08; See talks by Khochfar,

    Weinzirl, Balcells)

    (Weinzirl et al. 08)

    - Most of 400 spirals along Hubble sequence have B/T<0.25 (Graham & Worley 2008)


    Collaborators

    QUESTIONS/OPEN ISSUES

    Theory

    1) Can cosmological simulations produce enough bulgeless/low B/T galaxies ?

    2) Do main processes for removing low J gas differ in high vs low mass systems?

    3) Models have focused primarily on major mergers. How do we better incorporate

    bulge building via secular evolution, minor mergers, and cold gas accretion?

    [Talks: Burkert, Navarro, Governato, Dekel, Khochfar, Combes,,Shlosman, Cox, Stewart, Hopkins]

    Observations

    4) Fold in Ages + Kinematics+ Metallicity w/ structure of (B/T, n) of bulges

    5) How do bulge, bar, disks vary in field vs cluster enviroments ?

    [e.g.,Talks by Barroso, Juric, Brown, Balcells, Fisher, Weinzirl, Marinova, Graves]

    6) Direct empirical constraints on minor and merger history out to z~2

    [see talks by Balcells, Sanjuan, Robaina, Sketlon, Stewart, Conselice, Jogee]


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