The Origin and Dynamicsof the Local Group of Galaxies
T. Sawa, C. Sato, H. Shirokane(Aichi University of Education)
andM. Fujimoto (Nagoya University)
Ｍ３１，Ｍ３２，ＮＧＣ２０５

Introduction• Since the discovery of the Magellanic Stream, thedynamics of the Magellanic System is extensively
studied. Tidal models of the Galaxy-LMC-SMC
system success to reproducing the geometricaland dynamical structure of the MS (Murai andFujimoto 1980, Gardiner, Sawa and Fujimoto1994), and orbits of the LMC and SMC are welldetermined.
• However, some problems are remain.

LMC & SMC1

Remaining Problems• The origins of the LMC and SMC
Did they initially form as satellite galaxies of theGalaxy, or did they fall into the Galaxy from anotherregion.
• Large orbital angular momenta of the Magellanic
Clouds around the Galaxy
The tidal model in which the Magellanic Clouds
formed in the neighborhood of the Galaxy cannot
explain such large orbital angular momenta.

Some questions about must be answered
through a more-global model for the LGG.
M33

Sky Distribution of LGG• Many dwarf galaxies are distributed near thegreat circle perpendicular to the galactic plane.
• Magellanic Stream lies along this great circle.
• Members of M31 are distributed near thiscircle

suggestion
A big dynamical event occurred in theneighborhood of the Galaxy about 10 Gyr ago

NGC31092

3-dimensional distribution of LGG• Many LGG members are distributed in acoplanar way
New idea
An off-center hydrodynamical collision occurred
some 10 Gyr ago between the primordial gas-rich
M31 and the similar Galaxy, and compressed thehalo gas to form the LGG dwarf galaxies.
NGC55

3

Face-on view

MW group

M31 group

Other group

Edge-on view

MW group

M31 group
Other group4

Galaxy interaction anddwarf galaxies• A many young dwarf galaxies are observed
around the violently merging gas-richgalaxies （Deeg et al. 1998，A & Ap Suppl ．
129，445）
It suggests the possibility that dwarf
galaxies of Local Group are formed by theoff-collision between the Galaxy and M31

Our Model (SF Model)
Sawa and Fujimoto 2005,
PASJ, 57
Many dwarf galaxies of LGG is formed by theoff-center collision of proto-M31 and protoGalaxy in early universe, and scattered in thepresent position.
SMC5

Scenario of our model

Two massive
proto-galaxies
made a offcenter collision.

The newborn dwarfgalaxies are scattered on
the orbital plane of theGalaxy and M31.

M31 is again approaching us.

Assumption• M31 and the Galaxy have massive halo
• The radius of massive halo is 200 - 300 kpc• The radius of the proto galaxy is 120 kpc• The collision is occurred about 10 Gyr ago• LMC and SMC were formed by the collision
Leo I6

Basic equation
MA¨rA ＝ −∇AφG−FGAMG¨rG ＝ −∇GφA−FAG
ML¨rL ＝ −∇L （φA ＋ φG ＋ φS ） −FAL −FGLMS¨rS ＝ −∇S （φA ＋ φG ＋ φL ） −FAS −FGSMD¨rD ＝ −∇D （φA ＋ φG ＋ φL ＋ φS ）M：mass of galaxy
φ：gravitation potential
F ：Dynamical Friction
ＮＧC185 & 147

The orbit of M31
The orbits of LMC and SMC are well determinedby the presence of Magellanic Stream.
LMC and SMC were also formed by thecollision of M31 and the Galaxy.

These conditions give the restriction for theorbits of M31.
Magellanic Stream

7

Determination of the M31 orbit
The orbits of LMC and SMC of that byGardiner, Sawa & Fujimoto (1994) are used
Four bodies problem of M31-Galaxy-LMCSMC are solved.
We search the M31 orbits such as the LMC
and SMC are formed by the collision of M31
and the Galaxy.
LMC

Nearly perpendicularorbit to the galactic plane
Clock wiseμl＝＋３８±１６ μas/yr

Counter clock wiseμb＝−４９± ５ μas/yr

Adopted M31 orbit

8

Relation between the collisiontime and radius of massive halo
Number of possible orbits of M31
radius ＼ time

9-10 Gyr

10 -11 Gyr

11-12 Gyr

200 kpc

ー1230 kpc

ー250 kpc

ー4ー
12ー280 kpc

43ー

300 kpc

ー5ー

ΔVl=20 km/s , ΔVb=20 km/s

NGC6822

Orbits of the Galaxy group• Angle between the orbital planes of M31 anddwarf galaxy is less than 30°• Dwarf galaxy orbits the Galaxy counterclockwise seen from the present position ofthe sun• Dwarf galaxy located the formation site about
10-12 Gyr ago
Sextans A9

Orbit of M31 group• Angle between the orbital planes of M31
and dwarf galaxy is less than 30°• Orbital angular momentum of dwarf galaxy
has the same direction as that of M31• Dwarf galaxy located the formation site
about 10-12 Gyr ago

IC10A Sheet of score for model orbits
WLM

× Sculptor○ Sextans B

× Sagittarius○NGC55× LGS3

○ NGC3109

× SagDIG

×IC10○ IC1613○ Antlia

× NGC6822

×NGC147○ And II− Leo I○ DDO 210×And III− M33○ Sextans A

× IC5152

×NGC185○ Phoenix○ Sextans○ Tucana−NGC205○ Fornax○ Leo II

○ UKS2323-326 ×M32

○ EGB0427+63

× GR8× Pegasus

M31

○ LMC

○ Ursa Minor○And I− Carina○ Draco○SMC○ Leo A× Milky Way○○Possible orbits： 23/37 （62%） Our Galaxy

10

Simulation for dwarf galaxy
formation

3-dimensional distribution of orbits

Pegasus

10 Gyr Ago: Two massive proto-galaxies
made a off-center collision.

Proto MW

Proto M31

11

5 Gyr ago: The newborn dwarf galaxies are scatteredon the orbital plane of the Galaxy and M31.

MWM31

Preset time: M31 is again approaching us.

12Edge-on view

VLGRHigh Velocity Clouds

Observed HVC Distribution

13

A Model of the origin of HVCs
• Some HVCs are formed by collision of M31
and the Galaxy, too.
• We search the HVC orbits which satisfy thepresent position and radial velocity.

ＷＬＭ

VLGR

Comparison withobservation and model

Observation

Model14

Summary• LGG members of the Galaxy group and M31
group distribute in a coplanar way• It is possible to explain that these dwarf
galaxies and some HVC were formed by the offcenter collision between the Galaxy and M31(SF model)• The Galaxy and M31 need to have dark halo of
200 - 300 kpc radius, respectively• We predict the proper motion of M31 asfollows;µl＝＋３８±１６ µas/yrPhoenixµb＝−４９± ５ µas/yr

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