Post on 31-Mar-2015
Interactions between Galaxies
Galaxy Dynamics
Françoise COMBES
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NGC 2207 and IC 2163– Hubble image
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Arp 188
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Arp 295
White contours: HI gas 21cm
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Nature of the interaction
Several propositions, and some propose magnetic interactions(force tubes)
In 1972, Toomre & Toomre: simulations at restricted 3-body(after Pfleiderer and Siedentopf, a few years before)
Interactions purely gravitationnalBisymmetry m=2Similarity with bars
Generation of two spiral arms
Self-gravity & corresponding amplification allow inner partsto develop contrasted density waves
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Comparison between potentialsof bars and tidal interactions
Different forces at large distancefrom centre, where the bar is weak
The tidal interactions are, on thecontrary, dominant at borders
μ is the mass ratio between thetwo galaxies
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Messier 51And its companion NGC 5195
Toomre & Toomre1972
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Interactions between galaxies•Frequent tidal Phenomena
•Formation of matter bridges between galaxies
•Burst of star formation
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Messier 51colors
DSS
2MASSNIR
Radio, VLA
Keel website
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The AntennaeToomre & Toomre1972
Hibbard's website
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The Antennae HSTformation of SSC(Super Star Clusters)
The Antennae, HI Hibbard et al 2001
Contours obtained at VLA+BVR colors
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The Mice
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Numerical simulations (Dubinski et al 1996)
The length of tidal tailsconstrains the amount ofdark matter, and mainlyits concentration
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Ensemble of galaxy mergers(Hibbard's website)
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Ring GalaxiesWhen the collision is head on, The two spiral arms wind out in a ring: concentric density wavescf Lynds & Toomre 76
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Horellou & Combes 1999
The rings are off-centered, and cannot be confused withresonant rings from barred galaxies
Also, another phenomenon to form rings: polar rings(in general seen edge-on)
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Several rings form successively,before wrapping out, and damping in phase space
Formation ofRing waves
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Spitzer PAH (8) off-centered rings
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Numerical Simulation
N-body + sticky106 particules350pc resolution
evolution during 1Gyr bar+spiral
Then collision 210 Myr
Mass ratio 1/13
Central ring 30deg inclination
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Splash of interstellar gasMessier 81, Messier 82, NGC 3077HI
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Reconstitution of the interactionSmall mass ratio, of the order of a few %
Several passages since the formation of Local Group
Magellanic cloudsare leading
Constraints on theDark matter of the MW
V ~200 km/s
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The Magellanic Stream
Detected in atomic hydrogen HI-21cm Equal amount of gas in the Magellanic Stream than in the Small Cloud SMC
The gas must have been dragged out of the SMC, according to simulations
Putman et al 98
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High Velocity Clouds (HVC) falling on the Galaxy
Origin still not well known
Their actual mass depends on their distanceRemnant of the Local Group formation? --> very massiveOr just debris from Magellanic Clouds?
Multiple Origins
Also, fountain effectAfter the formation of supernovae..
Wakker et al 99
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Interaction with Andromeda
The most massive galaxy of the Local Group, comparable to the MWIs only at ~700 kpcIts relative velocity is -140km/s
According to this radial velocity, the Approaching time is ~ 2 Gyr
But tangential velocity unknown
Soon proper motions with the satellite GAIA
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Simulations of the encounter with M31
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Formation ofPolar rings
Either by galaxy mergerswith perpendicular J
Or by gas accretion on external parts
cf LMC/MW
3D-shape of dark matter?
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Formation of polar rings
By collision?Bekki 97, 98
By accretion?Schweizer et al 83Reshetnikov et al 97
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Formation of PRG by collision
Bournaud & Combes 2002
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Merging Scenario: inclination of the ring
The inclination depends of But even if <55 impossibleto produce PR more inclinedthan 24 degrees
Rings are stable, t=8 Gyrs
Edge-on 10degrees
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Several rings propagate beforeWrapping out in phase space
Formation ofRing waves
Dissipation atRing formation
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Formation of PRG by accretion
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Accretion Scenario
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Accretion Scenario
Able to form inclined PR
NGC 660 Gas+stars Gas only
NGC 660 has a lot of gasProbably instable through precessionEven if self-gravitating
Not possible in merger scenario
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NGC4650: a case of accretion
No stellar halo detected aroundthe galaxyWhile it is expectedin the merging scenario
PR= 8 109Mo HI and4 109 Mo stars
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Polar rings and dark matter
Simulations show that dark matter is not concentrated (no cusp)And not flattened (on the contrary) flattening lower than E4
The case of NGC 4650A:Spherical Halo (Whitmore et al 87)
DM flattened along the equator (Sackett & Sparke 90, Sackett et al 94)
DM flattened along the polar ring (Combes & Arnaboldi 96)
Tully-Fisher relation for PRG:(Iodice et al 2002)
The HI width measures the dynamics of the PRWhile the luminosity in R or NIR measures the host galaxy
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Tully-Fisher for PRGs
TF in I bandIodice et al 2002
AM2020-504
UGC4261
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TF in K for PRGs & simulations15%peak
Ex Simulations
Circles: no masstriangles: with mass
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The PR are not circularThe two components are seen edge-on (selection effect)
The V observed in PR is the smallest, when the DM is flattened along equator
The more DM, the moreexcentric the PR is
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Tully-Fisher for the SO
"Mass" TF or "baryonic"Including the HI gas
Simulations show that PR are excentric
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TF of host galaxy vs Polar ring
Spiral galaxies
hosts
PRs
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Polar rings from cosmic gas accretion
Brook et al 2008
After 1.5 Gyr, interaction between the two disks destroys the PRG
Velocity curve about the same in bothequatorial and polar planes
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Warps & oscillations in z
Z(r,θ,t)=zo/2 [cos((Ω-νz)t-θ) +cos((Ω+νz)t-θ)]
Z(r,θ,t)=zo cos(Ωt-θ) cosνzt
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Decomposition in two progressive waves, of frequencyΩ p = Ω + νz et Ω - νz, the latter being retrograde
Can exist only beyond resonance(density wave theory)
The self-gravity, here again, will help to equal the precession rates
However, wave paquets will propagate towards the border ofthe galaxy, and damp, since the amplitude increases more and more
No reflexion possible, nor cavity amplification(as in SWING, WASER..)
Other mechanisms, like interaction between galaxies, orContinuous external gas accretion, with unaligned angular momentum
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Mergers between galaxiesDynamical friction: a mass M in a sea of stars
Chandrasekhar formula (43)
dv/dt = -v 16π2/3(lnΛ)G2mM f(0)
ρ = m f(0)
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Approximations of theChandrasekhar formula
Locale force, not globalForce at a distance ?
Self-gravity?
Deformation of companion?
Only simulations giveThe right order of magnitude
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Criteria for merging
Two spherical galaxies: depend on their énergiy E = v2/2of their momentum L = beFor two unbound systems, there exists a velocity vmax (Emax)Beyond wich no merger will occur
For spiral galaxies phenomena of resonance
The merger is then easier
L
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Formation of Ellipticals by merger
Merger of spirals of comparable mass ("major mergers")But also many more smaller masses ("minor mergers")
Obstacles: the number of globular clusters,The high density in phase space of the center in E-gal
NGC 7252 (Schweizer, 82, Hibbard 99)
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Hibbard's website
HI 21cm
Formation of tidaldwarfs
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Shells around elliptical galaxies
Very frequent phenomenon, technique of "unsharp masking"Malin & Carter 1983
NGC 3923: 25 shells
Up to 200kpc from centre
Aligned perpendicularlyto the major axis, for elongated galaxies
Wind randomly for galaxies round in projection
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Mechanism of "phase wrapping"
Phase wrapping (Quinn 1984, Dupraz & Combes 1986)
3D shape of elliptical galaxies? Dark matter?
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Dupraz & Combes 1986
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Gas in the shells?
Yellow: star shells
White: HI
Blue: Radiojets
RedCO obs
Charmandaris, Combes, van der Hulst 2000
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Hierarchical scenario
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Star Formation in mergers
E0 Sa Sbc Sd
Transfer of gas towards the center By bars driven by interactions
ProjectGALMER Di Matteo et al 07
Tree-SPH2 105 partSF+ feedback
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Gas inflow produce starbursts
Retrograde orbits more starbursts
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Direct Orbit gSa gSa
100kpc
dir
ret
ret
IN
OUTSense of gas flows
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Formation of Counter-rotations
Encounter between a spiraland an elliptical
Retrograde orbit
Tidal Forces Important at the border
The center is non affectedKeeps its orientation
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Elliptical + spiral
With or without gas,Efficient Mechanism
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Angular momentum trasnfer Solid r < 2kpcDash 2< r <5kpc
Dot-dash 5<r<10Dots r>10kpc
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Conclusions
Interaction between galaxies: formation of spiral arms, rings, warps, polar rings..
Intense Star Formation, starbursts
Formation of galaxies through mergers: hierarchical scenario
Formation of "super star clusters" which will become globular clusters
History of star formation: peak towards z=2, when galaxy clusersvirialise, and galaxies merge in large numbers
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Star formation history
Bouwens et al 2009