Interactions between Galaxies

Galaxy Dynamics

Françoise COMBES

2

NGC 2207 and IC 2163– Hubble image

3

Arp 188

4

Arp 295

White contours: HI gas 21cm

5

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

6

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

7

Messier 51And its companion NGC 5195

Toomre & Toomre1972

8

Interactions between galaxies•Frequent tidal Phenomena

•Formation of matter bridges between galaxies

•Burst of star formation

9

Messier 51colors

DSS

2MASSNIR

Radio, VLA

Keel website

10

The AntennaeToomre & Toomre1972

Hibbard's website

11

12

The Antennae HSTformation of SSC(Super Star Clusters)

The Antennae, HI Hibbard et al 2001

Contours obtained at VLA+BVR colors

13

The Mice

14

15

16

Numerical simulations (Dubinski et al 1996)

The length of tidal tailsconstrains the amount ofdark matter, and mainlyits concentration

17

Ensemble of galaxy mergers(Hibbard's website)

18

Ring GalaxiesWhen the collision is head on, The two spiral arms wind out in a ring: concentric density wavescf Lynds & Toomre 76

19

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)

20

Several rings form successively,before wrapping out, and damping in phase space

Formation ofRing waves

21

22

23

Spitzer PAH (8) off-centered rings

24

Numerical Simulation

N-body + sticky106 particules350pc resolution

evolution during 1Gyr bar+spiral

Then collision 210 Myr

Mass ratio 1/13

Central ring 30deg inclination

25

Splash of interstellar gasMessier 81, Messier 82, NGC 3077HI

26

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

27

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

28

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

29

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

30

Simulations of the encounter with M31

31

Formation ofPolar rings

Either by galaxy mergerswith perpendicular J

Or by gas accretion on external parts

cf LMC/MW

3D-shape of dark matter?

32

Formation of polar rings

By collision?Bekki 97, 98

By accretion?Schweizer et al 83Reshetnikov et al 97

33

Formation of PRG by collision

Bournaud & Combes 2002

34

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

35

Several rings propagate beforeWrapping out in phase space

Formation ofRing waves

Dissipation atRing formation

36

Formation of PRG by accretion

37

Accretion Scenario

38

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

39

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

40

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

41

Tully-Fisher for PRGs

TF in I bandIodice et al 2002

AM2020-504

UGC4261

42

TF in K for PRGs & simulations15%peak

Ex Simulations

Circles: no masstriangles: with mass

43

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

44

Tully-Fisher for the SO

"Mass" TF or "baryonic"Including the HI gas

Simulations show that PR are excentric

45

TF of host galaxy vs Polar ring

Spiral galaxies

hosts

PRs

46

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

47

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

48

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

49

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)

50

Approximations of theChandrasekhar formula

Locale force, not globalForce at a distance ?

Self-gravity?

Deformation of companion?

Only simulations giveThe right order of magnitude

51

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

52

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)

53

Hibbard's website

HI 21cm

Formation of tidaldwarfs

54Braine et al 2000, 01

55

56

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

57

Mechanism of "phase wrapping"

Phase wrapping (Quinn 1984, Dupraz & Combes 1986)

3D shape of elliptical galaxies? Dark matter?

58

Dupraz & Combes 1986

59

Gas in the shells?

Yellow: star shells

White: HI

Blue: Radiojets

RedCO obs

Charmandaris, Combes, van der Hulst 2000

60

61

Hierarchical scenario

62

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

63

Gas inflow produce starbursts

Retrograde orbits more starbursts

64

Direct Orbit gSa gSa

100kpc

dir

ret

ret

IN

OUTSense of gas flows

65

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

66

Elliptical + spiral

With or without gas,Efficient Mechanism

67

Angular momentum trasnfer Solid r < 2kpcDash 2< r <5kpc

Dot-dash 5<r<10Dots r>10kpc

68

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

69

Star formation history

Bouwens et al 2009