the role of dissipation in galaxy mergers sadegh khochfar university of oxford
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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
€
˙ 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)
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.
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
Khochfar & Silk (2006b)
Our results suggest:
€
p(x | M*) =1
2π (x − a)σ sb
exp −ln2 (x − a) /b[ ]
2σ sb2
⎛
⎝ ⎜
⎞
⎠ ⎟
€
x ≡1− Mq / Mbul
€
ln 1−Mq
Mbul
⎛
⎝ ⎜
⎞
⎠ ⎟∝ ln(Reff )
Dissipation Factor
Khochfar & Silk (2006a)
€
d ≡ ln 1−Mq,1
Mbul
⎛
⎝ ⎜
⎞
⎠ ⎟/ln 1−
Mq,2
Mbul
⎛
⎝ ⎜
⎞
⎠ ⎟
€
Reff (z2) = Reff1/ d (z1)
€
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)
€
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)
€
MBH ,C =1.26 ×108 σ
200
⎛
⎝ ⎜
⎞
⎠ ⎟3.45
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
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
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