1. Interactions between Galaxies Galaxy Dynamics Françoise COMBES

2. 2 NGC 2207 and IC 2163– Hubble image

3. 3 Arp 188

4. 4 Arp 295 White contours: HI gas 21cm

5. 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 gravitationnal Bisymmetry m=2 Similarity with bars Generation of two spiral arms Self-gravity & corresponding amplification allow inner parts to develop contrasted density waves

6. 6 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

7. 7 Messier 51 And its companion NGC 5195 Toomre & Toomre 1972

8. 8 Interactions between galaxies Frequent tidal Phenomena Formation of matter bridges between galaxies Burst of star formation

9. 9 Messier 51 colors DSS 2MASS NIR Radio, VLA Keel website

10. 10 The Antennae Toomre & Toomre 1972 Hibbard's website

11. 11

12. 12 The Antennae HST formation of SSC (Super Star Clusters) The Antennae, HI Hibbard et al 2001 Contours obtained at VLA +BVR colors

13. 13 The Mice

14. 14

15. 15

16. 16 Numerical simulations (Dubinski et al 1996) The length of tidal tails constrains the amount of dark matter, and mainly its concentration

17. 17 Ensemble of galaxy mergers (Hibbard's website)

18. 18 Ring Galaxies When the collision is head on, The two spiral arms wind out in a ring: concentric density waves cf Lynds & Toomre 76

19. 19 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)

20. 20 Several rings form successively, before wrapping out, and damping in phase space Formation of Ring waves

21. 21

22. 22

23. 23 Spitzer PAH (8  ) off-centered rings

24. 24 Numerical Simulation N-body + sticky 10 6 particules 350pc resolution evolution during 1Gyr  bar+spiral Then collision 210 Myr Mass ratio 1/13 Central ring 30deg inclination

25. 25 Splash of interstellar gas Messier 81, Messier 82, NGC 3077HI

26. 26 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

27. 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. 28 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? --> very massive Or just debris from Magellanic Clouds? Multiple Origins Also, fountain effect After the formation of supernovae.. Wakker et al 99

29. 29 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

30. 30 Simulations of the encounter with M31

31. 31 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?

32. 32 Formation of polar rings By collision? Bekki 97, 98 By accretion? Schweizer et al 83 Reshetnikov et al 97

33. 33 Formation of PRG by collision Bournaud & Combes 2002

34. 34 Merging Scenario: inclination of the ring The inclination depends of  But even if  <55 impossible to produce PR more inclined than 24 degrees Rings are stable, t=8 Gyrs Edge-on 10degrees

35. 35 Several rings propagate before Wrapping out in phase space Formation of Ring waves Dissipation at Ring formation

36. 36 Formation of PRG by accretion

37. 37 Accretion Scenario

38. 38 Accretion Scenario Able to form inclined PR NGC 660 Gas+stars Gas only NGC 660 has a lot of gas Probably instable through precession Even if self-gravitating Not possible in merger scenario

39. 39 NGC4650: a case of accretion No stellar halo detected around the galaxy While it is expected in the merging scenario PR= 8 10 9 M o HI and 4 10 9 M o stars

40. 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 PR While the luminosity in R or NIR measures the host galaxy

41. 41 Tully-Fisher for PRGs TF in I bandIodice et al 2002 AM2020-504 UGC4261

42. 42 TF in K for PRGs & simulations 15%peak Ex Simulations Circles: no mass triangles: with mass

43. 43 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

44. 44 Tully-Fisher for the SO "Mass" TF or "baryonic" Including the HI gas Simulations show that PR are excentric

45. 45 TF of host galaxy vs Polar ring Spiral galaxies hosts PRs

46. 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 both equatorial and polar planes

47. 47 Warps & oscillations in z Z(r,θ,t)=zo/2 [cos((Ω-ν z )t-θ) +cos((Ω+ν z )t-θ)] Z(r,θ,t)=zo cos(Ωt-θ) cosν z t

48. 48  Decomposition in two progressive waves, of frequency Ω p = Ω + ν z et Ω - ν 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

49. 49 Mergers between galaxies Dynamical friction: a mass M in a sea of stars Chandrasekhar formula (43) dv/dt = -v 16π 2 /3(lnΛ)G 2 mM f(0) ρ = m f(0)

50. 50 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

51. 51 Criteria for merging Two spherical galaxies: depend on their énergiy E = v 2 /2 of their momentum L = be For two unbound systems, there exists a velocity v max (E max ) Beyond wich no merger will occur For spiral galaxies phenomena of resonance The merger is then easier L

52. 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. 53 Hibbard's website HI 21cm Formation of tidal dwarfs

54. 54 Braine et al 2000, 01

55. 55

56. 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 perpendicularly to the major axis, for elongated galaxies Wind randomly for galaxies round in projection

57. 57 Mechanism of "phase wrapping" Phase wrapping (Quinn 1984, Dupraz & Combes 1986) 3D shape of elliptical galaxies? Dark matter?

58. 58 Dupraz & Combes 1986

59. 59 Gas in the shells? Yellow: star shells White: HI Blue: Radio jets Red CO obs Charmandaris, Combes, van der Hulst 2000

60. 60

61. 61 Hierarchical scenario

62. 62 Star Formation in mergers E0 Sa Sbc Sd Transfer of gas towards the center By bars driven by interactions Project GALMER Di Matteo et al 07 Tree-SPH 2 10 5 part SF+ feedback

63. 63 Gas inflow produce starbursts Retrograde orbits  more starbursts

64. 64 Direct Orbit gSa gSa 100kpc dir ret IN OUT Sense of gas flows

65. 65 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

66. 66 Elliptical + spiral With or without gas, Efficient Mechanism

67. 67 Angular momentum trasnfer Solid r < 2kpc Dash 2< r <5kpc Dot-dash 5<r<10 Dots r>10kpc

68. 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 clusers virialise, and galaxies merge in large numbers

69. 69 Star formation history Bouwens et al 2009