Interactions between Galaxies

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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 gravitationnal Bisymmetry
m2 Similarity with bars
Generation of two spiral arms Self-gravity
corresponding amplification allow inner parts to
develop contrasted density waves
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Comparison between potentials of bars and tidal
interactions Different forces at large
distance from centre, where the bar is weak The
tidal interactions are, on the contrary, dominant
at borders µ is the mass ratio between the two
galaxies
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Messier 51 And its companion NGC 5195 Toomre
Toomre 1972
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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 51 colors
DSS
2MASS NIR
Radio, VLA
Keel website
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The Antennae Toomre Toomre 1972
Hibbard's website
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The Antennae HST formation 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 tails constrains the
amount of dark matter, and mainly its
concentration
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Ensemble of galaxy mergers (Hibbard's website)
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Ring Galaxies
When the collision is head on, The two spiral
arms wind out in a ring concentric density
waves cf Lynds Toomre 76
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Horellou Combes 1999
The rings are off-centered, and cannot be
confused with resonant rings from barred
galaxies Also, another phenomenon to form rings
polar rings (in general seen edge-on)
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Formation of Ring waves
Several rings form successively, before wrapping
out, and damping in phase space
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Spitzer PAH (8m) off-centered rings
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Numerical Simulation
N-body sticky 106 particules 350pc
resolution evolution during 1Gyr ? barspiral
Then collision 210 Myr Mass ratio
1/13 Central ring 30deg inclination
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Splash of interstellar gas
Messier 81, Messier 82, NGC 3077
HI
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Reconstitution of the interaction
Small mass ratio, of the order of a few
Several passages since the formation of Local
Group
Magellanic clouds are leading
Constraints on the Dark 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
distance Remnant of the Local Group formation?
--gt very massive Or just debris from Magellanic
Clouds?
Multiple Origins
Also, fountain effect After 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 MW Is only at 700 kpc Its
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 of Polar rings Either by galaxy
mergers with 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 accretion? Schweizer et al 83 Reshetnikov et
al 97
By collision? Bekki 97, 98
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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 Q But even if Q lt55
impossible to produce PR more inclined than 24
degrees
Rings are stable, t8 Gyrs
Edge-on 10degrees
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Formation of Ring waves
Several rings propagate before Wrapping out in
phase space
Dissipation at Ring 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
Gasstars Gas only NGC
660 has a lot of gas Probably instable through
precession Even if self-gravitating Not possible
in merger scenario
NGC 660
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NGC4650 a case of accretion
No stellar halo detected around the galaxy While
it is expected in the merging scenario PR 8
109Mo HI and 4 109 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
PR While the luminosity in R or NIR measures the
host galaxy
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Tully-Fisher for PRGs
UGC4261
AM2020-504
TF in I band
Iodice et al 2002
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TF in K for PRGs simulations
15peak
Ex Simulations
Circles no mass triangles with mass
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The PR are not circular
The 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 more excentric 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 both equatorial and polar planes
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Warps oscillations in z
Z(r,?,t)zo cos(Ot-?) cos?zt
Z(r,?,t)zo/2 cos((O-?z)t-?) cos((O?z)t-?)
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? Decomposition in two progressive waves, of
frequency O p O ?z et O - ?z, 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 of the 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,
or Continuous external gas accretion, with
unaligned angular momentum
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Mergers between galaxies
Dynamical friction a mass M in a sea of stars
Chandrasekhar formula (43) dv/dt -v
16p2/3(ln?)G2mM f(0) ? m f(0)
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Approximations of the Chandrasekhar
formula Locale force, not global Force at a
distance ? Self-gravity? Deformation of
companion? Only simulations give The right order
of magnitude
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Criteria for merging
Two spherical galaxies depend on their énergiy E
v2/2 of their momentum L be For 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
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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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Formation of tidal dwarfs
Hibbard's website HI 21cm
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Braine et al 2000, 01
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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
perpendicularly to 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
Radio jets Red CO obs
Charmandaris, Combes, van der Hulst 2000
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Hierarchical scenario
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Star Formation in mergers
Transfer of gas towards the center By bars
driven by interactions
E0 Sa Sbc Sd
Project GALMER Di Matteo et al 07 Tree-SPH 2
105 part SF feedback
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Gas inflow produce starbursts
Retrograde orbits ? more starbursts
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Direct Orbit gSa gSa
Sense of gas flows
OUT
dir
ret
IN
ret
100kpc
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Formation of Counter-rotations
Encounter between a spiral and an
elliptical ?Retrograde orbit Tidal Forces
Important at the border The center is non
affected Keeps its orientation
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Elliptical spiral
With or without gas, Efficient Mechanism
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Angular momentum trasnfer
Dot-dash 5ltrlt10 Dots rgt10kpc
Solid r lt 2kpc Dash 2lt r lt5kpc
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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 z2, when galaxy clusers virialise,
and galaxies merge in large numbers
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Star formation history
Bouwens et al 2009