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IV. Interacting Galaxies

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Examples of galaxies in interaction:

Pairs of galaxies (NGC 4038/39)

M 51 – type (satellites)

Arp 220 – type

Compact galaxy groups

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B.A. Vorontsov-Velyaminov: Atlas and

Catalog of Interacting Galaxies (1959)

Search of Palomar Sky Survey for candidate interacting galaxies

(limiting magnitude ca. 16 mag)

Morphological classification of galaxy interactions and environment

of galaxies

355 galaxies classified

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Halton Arp: Atlas of Peculiar Galaxies

(1966)

Based upon Vorontssov-Velyaminov‘ catalog

338 selected objects observed mit 5m Mt. Palomar

Morphological classification focalized on spiral galaxies

Hypothesis that magnetic fields are responsable for observed

pecularities

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Arp Atlas of Peculiar Galaxies

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Based upon IIIa-J SRC southern sky survey

6445 peculiar galaxies selected out of a sample of 77838

objects

Categories:

- Galaxies with interacting companions

- Interacting doubles, triples to quintets

- Chains

- Groups

H. Arp & B.F. Madore: A Catalogue of Southen

Peculiar Galaxies and Associations (1987)

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Gravitional passages lead to Tadpole Galaxy

tidal perturbations.

1. Tidal Interactions

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Perturbations passing a

galaxy in retrograde

direction are less perturbant

(galaxy structure survives)

than prograde encounters.

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tidal arms: one only of HI,

therefore not visible in the

optical; one containing dust

and visible as dust lane.

Tidal arms in M51

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15 Schweizer, 1982, ApJ 252, 455

M82

M81

NGC 3077

Gas bridges within the

M81 Group feed the high

star formation in M82 by

gas infall.

Gas Bridges

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NGC 3314

2. Merging Galaxies

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Die Superantenne NGC 7252

video

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e.g. NGC 4260 shows

a double-core structure

with extremely high star

formation.

3. Double Cores as Merger Relics

X-ray observations even allow to

discern 2 bright sources that can

only be interpreted as BHs.

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4. Ultra-luminous Infrared Galaxies

ULIRGs are characterized by

extraordinarily high infrared

luminosities and hot dust.

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All ULIRGs are

merging galaxies

without any

exception.

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5. The Antennae

The Antennae is a ULIRG

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In merging galaxies

Super-Star Clusters

are formed in the

densest regions.

6. Star and Galaxy Formation in Tidal Arms

Mirabel et al. (1992)

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NGC 5291

Duc &Mirabel (1998)

Arp 188, the Tadpole Galaxy:

bright knots as DG

precursors?

bright star-formation knots

are structured within the tail.

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In numerical simulations mass is

concentrating in the central

regions but also clumping within

the tidal arms. If the masses are

large enough one can denote the

clumps as Tidal Tail DGs.

7. Mergers in the deep Universe

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Gas/dust and/or stellar disks/rings decoupled

from the galaxy body

Kinematically decoupled nuclei

Photmetric signatures („fine structure“):

- Shells/ripples

- filaments

- Boxiness of the isophotes

- „X“ shape of isophotes

9. Signatures of past interactions:

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9.1. Harassment NGC 4438 as member of the Virgo

Cluster reveals a slightly perturbed

disk structure in the optical, but an

irregular octopus-like structure in

Hα. The disturbing process is called

Harassment, an extremely rapid

passage of a perturber.

9.2. Direct Collisions

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Although the companions around

the Cartwheel Galaxy seem to be

disconnected, a fountain-like HI

trail traces the link to the past

intruder.

9.3. Ring Galaxies

Some galaxies show bright rings

partly with spokes, untypical

structures for rotating disks.

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Warps are (point-)symmetric distortions

of the disk and probably the result of

close passages such as of satellite

galaxies.

NGC 5719

Shells and warped rings in E‘s are

indicative for gas accretion.

9.4. Gas Accretion

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9.5. Kinematicly decoupled components

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5. Satellite Infall:

Minor Mergers

In NGC 1531 a satellite

DG approaches the galaxy

and exerts perturbations

to the disk, leading to disk

thickening and bending of

arms.

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In NGC 6872 the

approach of the

satellite DG

IC 4970 triggers

bar formation.

The Magellanic Clouds

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Tidal streams around NGC 5907

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5.3. Minor-merger Models Observational facts:

Sgr I : DG at d = 24 kpc, DGC = 16 kpc

Tidal Streamers within the MWG and M31

Disk thickening in Galaxies with close Satellites

but: thin Galactic Disks (the solar vicinity) (Edvardsson et al. 1993)

Theoretical requirements:

Hierarchical Merging

but: total accreted mass of small companions from CDM

scenario: macc 6.109 M (Tremaine 1980)

Questions:

Disk Heating

Star-formation Trigger

Bulge Formation

Timescales and Structural Effects of Perturbations

Simulations:

Realistic Models and Initial Conditions?

Adequate Numerical Treatment?

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Model by Katheryn Johnston

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Computer model of a small satellite galaxy orbiting a

larger (edge-on) disk galaxy. As the satellite orbits,

stars are stripped from the satellite and orbit in the halo

of the larger galaxy. (Kathryn Johnston, Wesleyan): see

the bending and tumbling of the satellite‘s figure axis!

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Initial Conditions:

Msat = 109 M

DGC,sat = 18 kpc

v = 200 km/s

i = 20°

Results:

• Disruption of

satellite with mass

loss 80%;

• Satellite sinks

into equatorial

plane within 1.5

Gyrs;

• Satellite moves

radially inwards

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Results:

• Satellite spirals

towards the center

leaving behind a

spur of stripped-

off stars: tidal

streamers?

• 20% of the

original mass

forms a still bound

unit in the center;

the displacement

with respect to the

center (0,0) of the

giant galaxy is

indicative for an

orbiting (double)

core.

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shows a double core

Satellite trajectories

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face-on perturbations

disk heating

0.85 Gyrs

2.45 Gyrs

750 Myrs 900 Myrs

1.05 Gyrs 1.20 Grys

1.35 Gyrs 1.50 Grys

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Close encounters of galaxies lead to

energy transfer according to the

Impuls Approximation

measurable as disk thickening.

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h24

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rvb3

MMG4ΔE