The Role of Dissipation in Galaxy Mergers

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The Role of Dissipation in Galaxy Mergers

• Sadegh Khochfar
• University of Oxford

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Why should dissipation be important?
Perez-Gonzalez et al. (2005)
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Dissipation during Mergers
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.
Springel Hernquist (2005)
Carlberg (1986)
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Semi-analytic Modelling

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

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Stellar Components

• Disk Stars
• Parameterisation of Schmidt-Kennicutt law

• 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

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Stars in Bulges Ellipticals

• 3 main distinct origins
• Former disk stars
• quiescent
• Central Starburst
• star burst
• Satellite stars

star burst
quiescent
Springel Hernquist (2005)
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Surface Mass Density
Effective radius of the Star burst component is
5.7 time smaller than that of the scattered disc
stars.
star burst
quiescent
Springel Hernquist (2005)
Dekel Cox (2006)
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Surface Mass Density
Kauffmann et al. (2003)
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Morphology Dependence
Ellipticals
Spirals
Kauffmann et al. (2003)
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Where are all the Stars?
The most massive Galaxies at each redshift are
ellipticals galaxies. With time massive disc
galaxies start appearing.
Khochfar Silk (2006a)
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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)
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Dry Mergers

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

• Massive spheroids form by mergers of spheroids

Khochfar Burkert (2003)
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Dissipation in Mergers vs Mass
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Above MC bulges and ellipticals have on average a
constant fraction of 85 of stars made
previously in disks
Mquiescent/Mbulge
MC
log M
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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)
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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.
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Environmental Dependence
Kauffmann et al. (2004)
Khochfar Silk (2006a)
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So far.

• Present day Es with masses gt 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
  gt MC

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Size-Distribution
Kauffmann et al. (2003)
Khochfar Silk (2006b)
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Size-Distribution
Our results suggest
Khochfar Silk (2006b)
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Dissipation Factor
Khochfar Silk (2006a)
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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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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)
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RSF-Correlations
Feedback effects correlate with sigma but not
with the luminosity
Schawinski, Khochfar et al. (2006)
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Critical Black Hole Mass-s Relation
AGNs with black hole masses larger than the
critical black hole mass shut off star formation
and prohibit it in the future.
MBH
Schawinski Khochfar et al. (2006)
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The RSF-Correlation
Schawinski Khochfar et al. (2006)
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The Big Picture
Schawinski Khochfar et al. (2006)
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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 z0 and z2

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So far

• Assuming a critical BH mass-sigma relation
  accounts for the trend seen in the RSF galaxies
• MBH-s correlation is tighter when accretion
  dominates BH growth
• MBH-L correlation is tighter when the BH growth
  is merger dominated

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Red-Sequence Blue-Cloud
Bell et al. (2004)
Baldry et al. (2006)
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Modeling Approach

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

Kang et al. (2005)
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Modeling Approach

• Substructure (e.g. Kang et al. 2005)
• Cooling/heating (e.g. Dekel Birnboim 2006
  Cattaneo et al. 2005)

Cattaneo et al. (2005)
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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)
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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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Star Formation in Elliptical Galaxies
RSF galaxy 1-3 Star formation in the last
Gyr Sample of 800 Es, all visually classified.
Schawinski Khochfar et. al (2005)
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Intrinsic Scatter
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Black Hole Mass-s Relation
AGNs in black holes with masses larger than the
critical black hole mass shut down star formation
and prohibit it in the future.
MBH
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AGN-Feedback
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Size-Evolution
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Surface Brightness
Kauffmann et al. (2003)
Bingelli et al. (1988)
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Dissipation in Mergers vs Redshift

• Spheroids formed at early times have higher
  fractions of stars being formed in a star burst
  event

Mquiescent/Mbulge
z
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Formation Epoch of Spheroids
The epoch of the last major merger is correlated
with mass of the spheroid and surface mass
density (fraction of quiescent stars in the bulge)
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Morphology Dependence
Mquiescent/Mbulge
Spirals
Ellipticals
log M
log M
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Bulges vs Ellipticals
Massive bulges have on average lower fractions of
star burst components then ellipticals of the
same mass and hence should have lower surface
mass densities
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Phase Space Density
Present day spiral galaxies are not likely to
merge into present day ellipticals in concordance
with CDM predictions.
Carlberg (1986)
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Summary

• Mc is connected to spheroid formation
• Progenitor galaxies with massive bulges result in
  remnants having constant surface mass density
• Mass fraction in spheroids from central star
  bursts becomes constant above Mc
• The star burst fraction increases with redshift
• In dense environments the star burst component in
  small galaxies is higher
• The fraction of star burst component is higher in
  ellipticals than in bulges
• Phase space constraints can be recovered
• Mc is not a universal mass scale

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A Universal Characteristic Mass Scale
Mquiescent/Mbulge
log M
log M
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Age distribution
Kauffmann et al. (2003)
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Surface Brightness
De Jong van der Kruit (1994)
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Concentrations
Higher concentration in more massive galaxies
Kauffmann et al. (2003)
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Dissipation in Mergers vs Mass
Mquiescent/Mstar burst
log M
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Formation of Bulges Ellipticals
Springel
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Density Structure of Remnantswithout Gas
20 bulge
10 bulge
no bulge
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The Sloan Digital Sky Survey

• DR1, Kauffmann et al. (2003)
• 14.5 lt r lt 17.77
• 0.03 lt z lt 0.1
• 122 808 galaxies, 20 of the survey
• Concentration indices, CR90/R50
• Galaxy size
• Stellar mass, M
• Surface mass density, m

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Spheroids
Kormendy Bender
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Density Structure of Remnantswithout Gas
The effective radius decreases depending on the
orbit geometry and structure of the merging
galaxies by a factor 0.8
initial
final
Naab Trujillo (2005)
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Density Structure of Remnantswithout Gas
The effective radius decreases depending on the
orbit geometry and structure of the merging
galaxies by a factor 0.8
initial
final
Phase space density
Naab Trujillo (2005)
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Elliptical/Bulge Formation
Simulation by T. Naab
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Parameter Space

• Mass ratio (e.g. Barnes Hernquist, 1992)
• Bulge-to-Disc ratio (Khochfar Burkert, 2003)
• Orbital parameters (Khochfar Burkert, 2006)
• Spin orientation (Khochfar Burkert, 2006)
• Mass to light ratios
• Scale length of stellar components
• Super-massive black hole (di Matteo et al., 2005)
• Gas fraction

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Progenitor Bulges and Reff
For progenitor bulges gt 50 of total mass
Shen et al. (2004)
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UV-CMR
The bluest UV-upturn Galaxy is at NUV-r 5.4 We
assume all galaxies with NUV-r lt5.4 have a young
stellar component Multiband photometric fitting
of the SED show that between 1-3 of the total
stellar mass is acquired within the last Gyr. gt
RSF Galaxies
Schawinski et al. (2006)
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Working Hypothesis

• Intrinsic scatter in black hole masses for a
  given velocity dispersion is the reason for the
  change in the fraction of RSF-galaxies with sigma
• Using the fraction of RSF-galaxies with sigma it
  is possible to predict a critical black hole mass
  at a given sigma at which feedback prohibits
  further star formation

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Intrinsic Scatter