Interacting and merging galaxies

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1 Interacting and merging galaxies

2 The local universe

3 Interacting galaxies ~7 % of the luminous galaxies in the local universe are interacting galaxies or mergers Interacting galaxies are recognized from the effects of tidal distorsions tails and bridges. Close interaction leads to tidal stripping Direct collisions give rise to rings

4 Interacting pair Bridge Tail

5 a = GM/(R+r)2 r This part will be stripped a = GM/(R-r)2 R, distance to perturber

6 Condition for tidal stripping a = GM/R 2 a (nearest) = GM/(R-r) 2 a (fartherst) = GM/(R+r) 2 If r<<r : a = 2GMr/R 3 Tidal force > Gravitational force 2GMr/R 3 > Gm/r 2 Where a is the aceleration M is the mass of the perturber R the distance to the perturber m the mass of the perturbed galaxy r the distance to the centre of the perturbed galaxy

7 Arp 295

8 UGC4703

9 The Cartwheel

10 HI

12 Mergers - a few common concepts Major merger two galaxies of nearly equal mass involved in the merger Minor merger M/m > 3 Cannibal A very massive galaxy growing in mass on behalf of much smaller galaxies

13 Virgo cluster The cannibal M87

14 M87

15 Globular Clusters Many globular clusters may indicate a violent past With frequent starbursts

16 Jet

17 Mergers - a few common concepts Major merger two galaxies of nearly equal mass involved in the merger Minor merger M/m > 3 Cannibal A very massive galaxy growing in mass on behalf of much smaller galaxies Dry merger merger involving gas poor galaxies Wet merger merger involving gas rich galaxies

18 Merging galaxies NGC The antennae Blue = H I

19 Merging galaxies NGC The antennae Major wet merger Blue = H I

20 Other ongoing mergers

21 Arp 190

22 HGC 25

23 Early computer simulations of interacting galaxies Holmberg 1941 ApJ 94, 385 Early modelling of M51-type Galaxies (Toomre & Toomre 1972,ApJ 178, 623)

24 N-body simulations of mergers (no gas involved). The Mice. Hibbard and Barnes.

25 NGC 4674 The Mice

27 More detailed simulation

28 The Milky Way and the Andromeda galaxy will probably merge in a head-on collision in 3-4 Gyr

29 Dubinsky

31 Mergers

32 The famous Arp 220 Mergers

33 Minor merger

34 Minor merger in our Galaxy

35 Paul Harding. Model of remnants of minor mergers.

36 The Sagittarius tidal stream The galactic plane

37 The Sagittarius tidal stream The galactic plane

38 The Sagittarius stream. Model of debris. Law et al., 2005, ApJ 619, 807

39 Indication of a past merger in an elliptical galaxy. These shells are indications of past merger including a disk galaxy

40 Phase wrapping (Quinn, 1983, IAUS 100, 34)

41 Model of a major merger of two stellar disk galaxies Disk 1 Disk 2 Hernquist & Spergel, 1992, ApJ 399, L 117

42 Merger signatures survive for a long time on large scales

43 The hidden dragon.

44 NGC 5128 (Cen A)

45 This looks like a shell These structures are probably caused by gas ejected from the centre David Malin

Post merger signatures 1) If the galaxy went through a starburst, a spectrum with strong Balmer absorption lines (A-type) will develop after a few 100 Myrs.

46 Post merger signatures 1) If the galaxy went through a starburst, a spectrum with strong Balmer absorption lines (A-type) will develop after a few 100 Myrs. Sometimes these galaxies are called E+A galaxies (Elliptical with A type stellar spectrum). 2) Counter rotating cores. If the merging galaxies have their spin axes in opposite directions the post merger may contain a dynamically separate system in the central region, rotating in an opposite direction as compared to the host. 3) Shells and rippels. Mergers involving a disk galaxy may create shell like structures in the halo region. 4) Tidal streams caused by infalling dwarf galaxies being torn apart along their orbits.

47 H I maps superposed on optical images

52 NGC

54 Formation of tidal dwarfs? Duc & Mirabel 1997

55 SPH model of merger Duc et al. 2004, A&A 427, 803

56 SPH model of merger Tidal dwarf Formation strongly dependent on disk-orbit orientation Duc et al. 2004, A&A 427, 803

57 Tidal dwarfs have the same metallicity as the mother Normal dwarfs follow the L-Z relation Duc & Mirabel 1997

58 We have seen a few effects of galaxy interactions and mergers Interaction cause strong morphological distorsions, mainly bridges, tails and rings. The HI envelopes are ripped apart and can form isolated clouds and later tidal dwarfs. The tidal dwarfs do not contain much DM or old stars but the metallicity is higher than for normal dwarfs. These are probably rare events. Mergers can completely destroy previous major morphological signatures but the memory of disks may survive over long periods in the form of shells and ripples Mergers make galaxies more massive hierarchical evolution

59 Evolution in Cluster Galaxy Morphologies Mergers more active at higher z AC118 z=0.31 Fewer, more massive galaxies at lower z Coma z 0 The `Morphs : Dressler, Ellis, Smail, Oemler et al From Ellis 2006

60 How is star formation affected? Starbursts? Quenching?

61 Specific questions Can galaxy interactions (close encounters) trigger starbursts? Global, nuclear or both? Or do the interactions trigger bar formation which then triggers nuclear starbursts? Can mergers trigger global starbursts? Can interactions or mergers trigger AGNs? Can nuclear starbursts trigger AGNs? Can these effects significantly influence the evolution of the galaxy population?

62 What regulates the star formation? Negative and positive feedback processes + Gravitational collapse of gas clouds + Cooling atomic, molecular, dust + Gas compression from stellar winds - Sputtering on dust particles - Ionization - Heating and expansion of gas clouds The normal state is self regulated i.e starbursts are shortlived, effects of galaxy interactions are mostly controlled (my opinion.)

63 But there are exceptions. Optical H I M82 M81

64 M82 contains a circumnuclear starburst Gas outflow Hα emission from ionized hydrogen

65 What is a starburst? No well-established definition exists. Some efforts: The mean gas consumption timescale << Hubble age This relevant globally for e.g. BCGs (Blue Compact Galaxies) and locally for circumnuclear starbursts High star formation efficiencies (SFE) The normal efficiency is in star forming regions is ~5%. In starbursts it may be more than 10 times higher.

66 What is a starburst? High SFR (Star Formation Rate), typically > a few M o /yr This will exclude BCDs (Blue Compact Dwarfs, M B >-17) and circumnuclear starbursts High SFR/unit area Problems: Galaxies with normal SFE but high column densities will be included (like edge on late type disk galaxies) High SFR/<SFR>, the b parameter <SFR> is the mean SFR over the lifetime of the galaxy. Problems: If used globally, circumnuclear starbursts will not qualify. Hard to determine <SFR> for starburst regions.

67 A starburst dwarf - the blue compact galaxy (BCG) ESO 338-IG04. The starburst affects most of the main body - a global burst G. Östlin

68 How important are they? Brinchmann et al. -04 the local universe: The majority of the star formation in the low-redshift Universe takes place in moderately massive galaxies ( Msolar), typically in high surface brightness disc galaxies. Roughly 15 % of all star formation takes place in galaxies that show some sign of an active nucleus. About 20 % occurs in starburst galaxies if a starburst is defined as one in which SFR/<SFR> >2-3 About 3 % occurs in starburst galaxies if a starburst is defined as one in which SFR/<SFR> >10. Agrees with Kennicutt et al. (-87) who found that ~6% of SF occur in regions of enhanced SF

69 Searching for starbursts in interacting and merging Galaxies Observables Broadband colours Hα fluxes or other hydrogen recombination lines, but not Lyα (resonant scattering eventually followed by dust absorption) Higher z: [ΟΙΙ]3727 FIR Radio continuum CO, HCN L FIR /L CO is a measure of the star formation efficiency L HCN /L CO is a measure of the dense gas mass fraction

71 Larger scatter and bluer colours indicate higher star formation rate in Arp objects but. Normal Hubble galaxies Arp galaxies Larson & Tinsley 1977

72 ..in this complete sample we see no effect of interaction on the star formation : Isolated : Interacting pairs : Merging Bergvall, Laurikainen, Aalto, 2003, A&A 405,31

73 The most up-to-date data of Arp objects show the same scatter as normal galaxies. And their colours agree with model predictions of a normal star formation history. Arp galaxies are not involved in global bursts

74 Arp galaxies observed with Spitzer

75 Spitzer

76 Smith et al., 2006, astro.ph 3408, The Spitzer Spirals, Bridges, and Tails Interacting Galaxy Survey Abstract. We present Spitzer mid-infrared images from a survey of threedozen pre-merger strongly interacting galaxy pairs selected from the Arp Atlas. The global midinfrared colors of these galaxies and their tidal tails and bridges are similar to those of normal spiral galaxies, thus this optically selected sample of interacting galaxies does not have strongly enhanced normalized star formation rates in their disks or tidal features. Despite distortion and disturbance these systems continue to form stars at a normal rate on average. The morphology of these galaxies is generally smoother in the shorter wavelength IRAC bands than at 8 µm, where dozens of clumps of star formation are detected.

77 Nikolic, Cullen, Alexander, 2004, astro-ph/ galaxies with companions from SDSS Pairs of galaxies show weak (max 40%) SF enhancement in the very centre (< about 2 kpc)

78 Although no support of global starbursts we do find an increase of ionized gas in the centres Keel et al. 1985, AJ 90, 708 Kennicutt et al. 1987, AJ 93, 1011 Laurikainen and Moles, 1995, ApJ 345, 176 Donzelli & Pastoriza 1997, ApJS, 111, 181 Barton et al. 2000, ApJ 530, galaxies from CfA2 with separation <70 kpc Bergvall et al , ~100 isolated pairs and mergers and isolated single galaxies Lambas et al. 2003, MNRAS 346, pairs from 2dF Nikolic et al. 2004, astro-ph/ , pairs from SDSS Bergvall et al Isolated Interacting Kennicutt et al. 1987, AJ 93, 1011

79 The far-ir emission originates from the central source - AGN or starburst The optical spectrum comes from an optically thin layer

81 ESO Clear excess in NIR AGN or starburst

82 Luminosity Function of normal galaxies * Schechter function Ultraluminous infrared galaxies Infrared galaxies

83 Luminous Infrared Galaxies LIRGs or LIGs L Bol,LIRG >4L * or L Bol,solar Very LIRG: L solar < L VLIRG <10 12 L solar Ultra LIRG: L solar < L ULIRG In all these cases the bulk of the energy is emitted in the far IR, peaking in the region betrween 60 and 100 µm. The energy source is dust enshrouded.

84 Famous ULIRGS - The superantennae

85 Important facts about LIRGs They are extremely rare objects. From the LF we find that within the volume V r <10000 km/s there is only one LIRG. Indeed the only LIRG we find is Arp 220. The LRIGs contribute with <6% of the total FIR emission in the local universe. The frequency of mergers increases with luminosity. Almost 100% of VLIRGs and ULIRGs have merger properties.

86 ULIRGs, mergers probably necessary but not sufficient Sanders et al. 1988

87 M 82 Aromatic Features in Emission (AFE) Destroyed by hard emission, e.g. an AGN

88 What heats the dust? Hot stars? Cold stars? AGNn? Shocks?

89 Spitzer data selecting Starburst galaxies at z~2 (Webb et al. 2006)

90 AGN or starburst? Spitzer/X-ray observations show that LIRG ULIRG Starburst dominated Starburst/AGN mix AGN dominated

91 Arp ULIRG

94 Arp 220 Hα Expanding supershell

95 The molecular emission is highly concentrated within 1kpc or even smaller, cf Arp220 Two disks are merging, as seen in the dispersion, and mm continuum CO on the optical HST image Downes & Solomon 98

96 What is happening? High molecular mass? High star formation efficiency?

97 Composite SFR law for normal disk galaxies, nuclear regions of normal disks and starbursts Starbursts Normal disks Centres of normal disks Schmidt-Kennicutt law Σ SFR Σ gas 1.4 Kennicutt 1998, ApJ 498, 541

98 Searching for starbursts in interacting and merging Galaxies Observables Broadband colours Hα fluxes or other hydrogen recombination lines, but not Lyα (resonant scattering eventually followed by dust absorption) Higher z: [ΟΙΙ]3727 FIR Radio continuum CO (mol. gas mass), HCN (dense gas mass) L FIR /L CO is a measure of the star formation efficiency L HCN /L CO is a measure of the dense gas mass fraction

99 L IR /L HCN is independent of L IR (Star formation rate/dense gas mass) is the same for spirals and ULIRGs. The star formation efficiency proportional on the amount of dense gas present.

100 What is the timescale of a merger? The dynamical collapse time e.g. assume r=10kpc M=10 10 solar masses t=220 Myr A statistical investigation from the Sloan survey indicates that the duration of the starburst is not much longer. Why??? What stops it?

101 High redshift

102 Merger rates f = f 0 (1+z) m What is the value of m?

103 0<z<1

104 1<z<2

105 2<z<3

106 Conclusions regarding merger evolution (Conselice et al. 2003, AJ 126, 1183) The merger frequency is a strong function of luminosity, mass and redshift. For M B >-20 and M * < solar masses the merger rate peaks at z~1. m= out to z=1 and m=0.5-1 in the interval z=0-3. For M B <-21 and M * >10 10 the merger rate rises steeply with m=4-6 up to a merger fraction of f~0.5 at z~2.5. For M * >10 9 s.m. there is a maximum accretion rate of s.m./gyr at z~2.5.

107 Lin et al. 2004, ApJ 617, 9 From DEEP2 survey Weak increase in merger rate with redshift. m around 1 at z=1. Thus about 9% of L * galaxies have undergone a merger since z=1. The disagreement between this and the data from Conselice et al. is not well understood.

108 Star formation rates from merger rates Accretion rate increases a factor of a few over the redshift interval -> possible increase in merger induced SF with corresponding factor -> (very roughly) SF in massive starbursts 10-20% of total SF at high redshifts. Mergers cannot build up L * galaxies over the redshift interval z=0-3. Bell et al. (2005, ApJ 625, 23) find that mergers contribute with only 30% to the starbursts at z=0.7.

109 Hibbard & Vacca 1997 Interpretation problem

110 Infrared luminosity functions (From Le Floche 2006) < 10 Local IRAS LF 1000x! LF05 Faint-end slope constrained from the deep source counts in GOODS-N as well as stacking analysis with COMBO-17 (Zheng et al. 2005) Galaxies and Structures through Cosmic Times, March 29 th, 2006

111 Massive gas-rich mergers contribute with about 10% of the total energy output at z=1. At z=2-3 a new extreme L(far-IR)/L(optical) galaxy population seems to be present. Massive gas-rich mergers may have fundamental role in galaxy formation.

112 Basic galaxy formation scenario: Halo bulge/massive central black hole - disk Bulge mass correlates with BH mass born hand in hand Models forbid collisionless mergers thus gas-rich more likely Bulge mass

113 The importance of mergers at very high redshifts. Models of disk galaxy formation including AGN feedback (di Matteo, Springel, Hernqvist 2005, Nature 433, 604 Robertson et al. 2006, ApJ 645, 986)

114 SPH models of disk formation Old models of dissipative galaxy formation in ΛCDM universe fail to produce large disks. In general spherical objects (sometimes with too small disks) are formed due to the low angular momentum transfer from the dark matter halo. New merger-driven scenarios, including star formation and black hole feedback in a multiphase medium give more realistic results. Here angular momenum of the final disk can be acquired from the orbital angular momentum. This will allow the formation of an extended disk.

115 Important parameters in gas-rich merger induced disk formation Star formation feedback and the strength of the pressurization of the ISM Presence of massive black holes Progenitor mass fraction Orbit and disk orientation Progenitor mass ratio Almost all gas-rich (fraction > 50%) mergers result in rotational supported structure

116 Star Formation during a Major Merger (from T.J. Cox)

117 The following siumulation shows a model run by di Matteo et al., including the formation of acentral massive black hole and its feedback on the infalling gas clouds. Only the gas component is shown. Web-address:

118 Star Formation during a Major Merger (from T.J. Cox)

119 Orbits and angular momenta Prograde coplanar orbits favour disk formation Large pericentric distances lead to larger angular momentum and larger disks. However, longer merging times means more gas consumed before merger and substantial angular momentum loss to the DM halo. This weakens the strength of the remnant disk. Therefore the angular momenta of the merging disks may contribute most substantially to the angular momentum of the remnant disk.

120 Gas rich mergers are fundamental for disky ellipticals And even more so for massive disk galaxies (S0, Sa, Sb)

121 Gas fraction Major merger progenitors must have original gas fractions f(gas)>0.8 in order to satisfy f(gas)>0.5 in the final disk remnant.

122 Gas fraction Major merger progenitors must have original gas fractions f(gas)>0.8 in order to satisfy f(gas)>0.5 in the final disk remnant. How massive are disks in massive S galaxies?

123 Dark matter halo solar masses Stellar halo 10 9 Bulge Disk MilkyWay ruleof thumb data

124 Bulge-to-disk ratio

125 Minor mergers A substantial part of the thick disk may have formed from multiple early mergers of gas-rich dwarfs

126 Major mergers with f gas < 80% Major mergers with f gas < 80% tend to form spheroid-dominated systems. These obey the elliptical scaling relations such as the Fundamental Plane. If E:s form through mergers, these still have to be relatively gas rich (>20-30%) to explain the high phase-space density in the centres of E:s (stars form after the merger).

127 Major mergers with f gas > 80% Disk dominated mergers will not satisfy the E scaling relations because they will be rotationally supported E galaxies can form if most gas is consumed during merger The models cannot explain bulgeless galaxies, i.e. late type disks. These probably have experienced no major mergers.

128 Reduction of gas consumption during the merger Formation of merger induced gaseous disk Feedback from massive central black hole. Little impact on global structure (larger than r eff ) but may influence the central starburst.

129 Conclusions Interactions induce a moderately increased star formation in a small region in the centre, typically < 1 kpc. Major mergers can start major starbursts that survive over at maximum a few 10 8 yr Major mergers can cause morphological transformations, e.g. from gas rich disks to ellipticals The contribution from interaction/merger induced starbursts to the local star formation rate is low, <10% The importance of mergers increases with redshift but maybe not dramatically so except for the most massive cases. There are contradictory views on this matter (the derived m varies from 1 to 5). Gas rich mergers may turn out to be a necessary condition to create massive disk galaxies. Minor mergers can create thick disks and feed galaxies continuously with fresh gas.

Peculiar (Interacting) Galaxies

Peculiar (Interacting) Galaxies Not all galaxies fall on the Hubble sequence: many are peculiar! In 1966, Arp created an Atlas of Peculiar Galaxies based on pictures from the Palomar Sky Survey. In 1982,