Numerical Simulations of Mergers

Joshua E. Barnes

Institute for Astronomy, University of Hawaii

Topics

Spiral-Galaxy Mergers: Do They Make Ellipticals?

Ancient history: N-body experiments tested crucial aspects of merger hypothesis:

Remnant Luminosity Profiles

The radial profile at large r is a genuine signature of merging (violent relaxation & phase mixing) and tends to
ρ ∝ r-4

since f (E) is continuous at E = 0 (Jaffee 1987, White 1987).
  Remnant luminosity profiles
Barnes (1988)

In projection, merger remnants have r1/4-law profiles, but these are dominated by pre-existing bulges at small r.

Conservation of phase-space density precludes an r1/4-law profile in mergers of pure disks (Carlberg 1986); the profile flattens out as r -> 0.

Remnant Morphology

Brand-new merger remnant
Barnes (1992)
  Relaxed merger remnant
Barnes (1992)

Newly-merged galaxies (left) have lumpy and irregular structure, including `shells', `ripples', `loops', and 'plumes' commonly noted in peculiar systems (Schweizer 1982). Within a few crossing times (right), these features fade and a smooth triaxial object emerges.

Remnant Morphology

Brand-new merger remnant
Barnes (1992)
  Relaxed merger remnant
Barnes (1992)

Newly-merged galaxies (left) have lumpy and irregular structure, including `shells', `ripples', `loops', and 'plumes' commonly noted in peculiar systems (Schweizer 1982). Within a few crossing times (right), these features fade and a smooth triaxial object emerges.

Remnant Morphology

Brand-new merger remnant
Barnes (1992)
  Relaxed merger remnant
Barnes (1992)

Newly-merged galaxies (left) have lumpy and irregular structure, including `shells', `ripples', `loops', and 'plumes' commonly noted in peculiar systems (Schweizer 1982). Within a few crossing times (right), these features fade and a smooth triaxial object emerges.

Velocity Profiles: Merging Flunks a Test!

LOS velocity distribution for S+S remnant
Bendo & Barnes (2000)

Pure N-body remnants have velocity profiles with shallow leading edges and steep trailing edges -- but in ellipticals it's the other way around!
(Naab & Burkert 2001)

  Kinematic profiles for S+S remnant
Bendo & Barnes (2000)

Velocity Profiles: A Resolution?

The line profiles observed in ellipticals can be interpreted as signatures of embedded stellar disks -- a disk naturally yields a profile with the correct asymmetry (anti-correlation of h3 with v).

These observed profiles reinforce other (eg, photometric) evidence for disks in E galaxies.

  Disk LOS velocity profile

Velocity Profiles: A Resolution?

The line profiles observed in ellipticals can be interpreted as signatures of embedded stellar disks -- a disk naturally yields a profile with the correct asymmetry (anti-correlation of h3 with v).

These observed profiles reinforce other (eg, photometric) evidence for disks in E galaxies.

  Disk LOS velocity profile

Mergers of gas-rich galaxies can form disks.   Gas-rich merger forms disk

Velocity Profiles: A Resolution?

The line profiles observed in ellipticals can be interpreted as signatures of embedded stellar disks -- a disk naturally yields a profile with the correct asymmetry (anti-correlation of h3 with v).

These observed profiles reinforce other (eg, photometric) evidence for disks in E galaxies.

  Disk LOS velocity profile

Mergers of gas-rich galaxies can form disks.

Moreover, these disks often have interesting kinematics, including counter-rotation!
(Hernquist & Barnes 1992; Barnes 2002)

  Gas-rich merger forms disk

A Resolution, but...

But these disks are gas, not stars!





We need to find the right star-formation prescription to actually make this work.




A challenge for N-body simulators: exhibit a merger of two gas-rich disk galaxies producing a remnant with an embedded stellar disk (formed from the gas) and with velocity profiles similar to those seen in real ellipticals.

Hierarchical Mergers

In groups and clusters, merger remnants are likely to be invloved in further mergers. What do we know about such mergers?

One recent study finds that E+E mergers (contours) produce remnants with less rotation and more anisotropy than D+D mergers (shaded); most appear consistent with massive, boxy, slowly-rotating elliptical galaxies (filled squares).   Rotation and ellipticity of merger remnants
Naab, Khochfar, & Burkert (2006)

Formation of Galactic Nuclei

Massive black holes (MBH) appear to be universal in early-type galaxies. Models of E+E mergers must confront the dynamics of binary black holes.

Hypothesis: binary black holes are necessary to transform the steep cusps of small ellipticals into the shallow `cores' of bright ellipticals (Ebisuzaki, Makino, & Okumura 1991; Milosavljevic & Merritt 2001).

  Evolution of stellar density in merger with SMBHs
Milosavljevic & Merritt (2001)

Are N-body Simulations With MBH Reliable?

Worth doing... but validity of results remains uncertain.

Hierarchical Mergers: Questions

Do E+E merger simulations produce remnants with the right structure? Are line profiles consistent with observations?

Can decoupled cores and other kinematic features survive hierarchical mergers? If not (and it seems unlikely that they can) do detections of decoupled cores in cluster galaxies constrain hierarchical merging?

Can binary MBHs coalesce fast enough to avoid the formation of triple MBH systems and the inevitable slingshot ejections which would spoil the relationship between MBH and galaxy mass/velocity dispersion?

Feedback from Star Formation and AGN

Modeling the behavior of the ISM is hard even for in-active galaxies! A promising two-phase model leads to a simple effective equation of state; below some threshold density star formation is inoperative and the gas is isothermal, while at higher densities energy from supernovae boosts the pressure (Springel & Hernquist 2003).   Effective EOS of star-forming gas
Springel, DiMatteo, & Hernquist (2005a)
The accretion rate onto MBH particles is evaluated from the local gas density and pressure, and a small fraction (5%) of the energy liberated is coupled back to the surrounding gas. (This choice matches the observed MBH - σ relation.)

A Merger With AGN Feedback

Merger of galaxies with MBHs
Di Matteo, Springel, & Hernquist [source: chandra.harvard.edu]

A Merger With AGN Feedback: Wow!

Merger of galaxies with MBHs
Di Matteo, Springel, & Hernquist [source: chandra.harvard.edu]

Feedback: Applications

One possible consequence of feedback is to produce `red and dead' ellipticals. Without AGN feedback, too much gas is left to fuel star formation and remnants remain relatively blue. By terminating star formation, AGN feedback allows mergers to quickly transition to the red population (Springel, Di Matteo, & Hernquist 2005b).   Star formation rates in mergers w/ and w/o MBHs
Springel, Di Matteo, & Hernquist (2005b)

AGN feedback is most effective in massive galaxies (where MBH growth is most pronounced). Thus, this mechanism helps account for massive red galaxies at high redshift.

Feedback: Questions

Are galactic-scale outflows driven by energy input to the ISM alone, or are momemtum inputs (SN ejecta, AGN jets) significant?

In ultra-luminous IR galaxies, the energy reprocessed by the ISM from UV to IR is sufficient to unbind the baryons in a few Myr. Can models of this reprocessing justify the 5% coupling of AGN luminosity to gas used by Springel et al.?

In the Springel et al. simulations, feedback can stabilize pure gas disks. Can real galaxies do the same trick? Is there any evidence for such high gas fractions at early times?

Modeling Interacting Galaxies

Fitting a simulation to a real pair of interacting disk galaxies involves exploring a large parameter space.

Orbit  p , e , μ 3
Orientations i 1 , ω1 , i 2 , ω2 4
Time t P 1
View  θX , θY , θZ  3
Scale S L , S V 2
Center  α , δ , cz 3
Total   16

Parameter Space

The parameter space may be abstracted using cylindrical coordinates. The vertical coordinate represents parameters which can be chosen after a simulation has been run. The azimuthal coordinate represents disk orientations. The radial coordinate represents orbit parameters.   Interaction parameter space

Model Matching

The parameter space may be abstracted using cylindrical coordinates. The vertical coordinate represents parameters which can be chosen after a simulation has been run. The azimuthal coordinate represents disk orientations. The radial coordinate represents orbit parameters.

Simulating a real system involves guessing some set of initial conditions and integrating forward to see if they match the data. Trial and error may eventually yield a satisfactory match.

  Interaction parameter space

Matching the Mice

Finding a good match to both morphology and kinematics can require many experiments. Here is a subset of the runs John Hibbard and I did to match the Mice (NGC 4676).

NGC 4676 data and model
  Argument of pericenter survey
Modeling trade study [John Hibbard]

A Model of the Mice

Model of the Mice

A Model of the Mice: Action!

Model of the Mice

Galactic Identakit

Earlier stages of tidal interactions can be modeled with test-particle disks in massive spheroids. A spherical cloud of particles on circular orbits represents all possible disk orientations. The disk particles to display are selected interactively after simulation is run.

Identikit simulation
  Interaction parameter space

Galactic Identakit

Earlier stages of tidal interactions can be modeled with test-particle disks in massive spheroids. A spherical cloud of particles on circular orbits represents all possible disk orientations. The disk particles to display are selected interactively after simulation is run.

Identikit visulization
  Interaction parameter space

Model Matching: Objectives

Joshua E. Barnes (barnes@ifa.hawaii.edu)
Last modified: October 3, 2006
http://www.ifa.hawaii.edu/~barnes/talks/stsci/talk.html
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