The Role of Dissipation in

Galaxy MergersSadegh KhochfarUniversity of Oxford

Why should dissipation be important?

Perez-Gonzalez et al. (2005)

Dissipation during Mergers

Springel & Hernquist (2005)

Star bursts occur during mergers and the strength depends on the available fuel.

New born stars are not subject to the existing phase space constraints and can increase the phase space density.

Carlberg (1986)

Semi-analytic Modelling

Extended Press-Schechter Gas Cooling Reionising Background Star Formation Supernova Feedback Stellar Population Models Galaxy Merging via Dynamical Friction

Stellar Components

Bulge Stars:

Major Merger: Stellar disks get disrupted spheroid All available cold gas centre of the remnant Gas in the centre central star burst

Minor Merger: Stars of satellite to bulge Gas of satellite to disk

Disk Stars:

Parameterisation of Schmidt-Kennicutt law

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˙ M * = αMcold

tdyn

Stars in Bulges & Ellipticals

3 main distinct origins:

Former disk stars

quiescent

Central Starburst

star burst

Satellite starsSpringel & Hernquist (2005)

star burst

quiescent

Surface Mass Density

Springel & Hernquist (2005)

Effective radius of theStar burst component is ~ 5.7 time smaller than that of the scattered disc stars.

star burst

quiescent

Dekel & Cox (2006)

Surface Mass Density

Kauffmann et al. (2003)

cM

€

μ* =0.5M*

πR502(z)

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Mc = 3×1010 M sun

Morphology Dependence

Kauffmann et al. (2003)

EllipticalsSpirals

Where are all the Stars?

Khochfar & Silk (2006a)

The most massive Galaxies at each redshift are ellipticals galaxies.

With time massive disc galaxies start appearing.

Progenitors

• Progenitors of massive galaxies have already bulges

• Above MC no mergers between bulge less galaxies happen anymore

• no environment dependence

MC

Khochfar & Silk (2005)

Dry Mergers

Khochfar & Burkert (2003)

• At the characteristic mass scale the mass in progenitor bulges is roughly 50%

• Massive spheroids form by mergers of spheroids

Dissipation in Mergers vs Mass

MC

log M

~85 %Above MC bulges and ellipticals have on average a constant fraction of 85 % of stars made previously in disks M

quie

scen

t/Mbu

lge

Build-Up of the Relation

Early major mergers are gas-rich and tend to decrease the fraction of quiescent stars in bulges.

Satellite mergers in contrast increase the fraction of the quiescent population in bulges.

Khochfar & Silk (2006a)

Redshift Evolution

Bulges present at earlier times are more compact and smaller than their counterparts at low redshifts.

This effect is most significant for massive elliptical galaxies at high redshifts.

Environmental Dependence

Kauffmann et al. (2004)Khochfar & Silk (2006a)

So far….

Present day Es with masses > MC are determined by mergers of bulge dominated systems

Dissipation is more important for smaller Es except for the most massive Es at high z

Dissipation is more important at higher z Dissipation is more important for Es with

masses > MC

Size-Distribution

Kauffmann et al. (2003) Khochfar & Silk (2006b)

Size-Distribution

Khochfar & Silk (2006b)

Our results suggest:

€

p(x | M*) =1

2π (x − a)σ sb

exp −ln2 (x − a) /b[ ]

2σ sb2

⎛

⎝ ⎜

⎞

⎠ ⎟

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x ≡1− Mq / Mbul

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ln 1−Mq

Mbul

⎛

⎝ ⎜

⎞

⎠ ⎟∝ ln(Reff )

Dissipation Factor

Khochfar & Silk (2006a)

€

d ≡ ln 1−Mq,1

Mbul

⎛

⎝ ⎜

⎞

⎠ ⎟/ln 1−

Mq,2

Mbul

⎛

⎝ ⎜

⎞

⎠ ⎟

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Reff (z2) = Reff1/ d (z1)

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z1 ≤ z2

Size-Evolution

Massive ellipticals show a stronger size-evolution than less massive ones.

The size-evolution predicted based on the star-burst component agrees well with the observations. Khochfar & Silk (2006b)

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Re (z) = Rlocal1/ d

Size-Evolution

Massive galaxies could be up to five times smaller at high redshifts than now, because they are more likely to be formed during a gas-rich major merger.

Khochfar & Silk (2006b)

Trujillo et al. (2006)

RSF-Correlations

Schawinski, Khochfar et al. (2006)

Feedback effects correlate with sigma but not with the luminosity

Critical Black Hole Mass- Relation

MB

H

AGNs with black hole masses larger than the critical black hole mass shut off star formation and prohibit it in the future.

Schawinski & Khochfar et al. (2006)

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MBH ,C =1.26 ×108 σ

200

⎛

⎝ ⎜

⎞

⎠ ⎟3.45

The RSF-Correlation

Schawinski & Khochfar et al. (2006)

The Big Picture

Schawinski & Khochfar et al. (2006)

So far….

Generally the size of an E is a function of the star burst fraction

Gas-rich merger result in smaller Es The observed size evolution is in agreement

with the one predicted by LCDM Es of the same mass are smaller at high

redshifts Most massive Es show up to a factor of 3 in

size-evolution between z=0 and z=2

So far…

Assuming a critical BH mass-sigma relation accounts for the trend seen in the RSF galaxies

MBH- correlation is tighter when accretion dominates BH growth

MBH-L correlation is tighter when the BH growth is merger dominated

Red-Sequence & Blue-Cloud

Bell et al. (2004)Baldry et al. (2006)

Modeling Approach

Substructure (e.g. Kang et al. 2005)

Kang et al. (2005)

Modeling Approach

Substructure (e.g. Kang et al. 2005)

Cooling/heating (e.g. Dekel & Birnboim 2006; Cattaneo et al. 2005)

Cattaneo et al. (2005)

Modeling Approach

Substructure (e.g. Kang et al. 2005)

Cooling/heating (e.g. Dekel & Birnboim 2006; Cattaneo et al. 2005)

AGN Feedback (e.g. Croton et al 2006; Bower et al 2006)

Croton et al. (2005)Baldry et al. (2006)

Conclusions Dissipation is more important at high redshift Dissipation very important in the most massive Es at

high redshifts Dissipation is able to account for the size evolution

of Es Dissipation can account for the tightness of the MBh-

relation Shut off of star formation is the main key to produce

the color bimodality There are many different approaches to achieve a

shut off which show different successes and failures

Spheroids

Kormendy & Bender