Local Group Dynamics

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Local Group Dynamics

Our Local Group of galaxies is a highly dynamical

system. Its present substructure, with subgroups of dwarf galaxies around

its two dominant spiral members, our Milky Way and

the Andromeda Galaxy M31, is probably only

a temporary scenario, which was not always as it is now.

According to an analysis of astronomers Gene Byrd, Muri Valtonen,

Marshall McCall, and Kimmo Innanen (see

Astronomy Vol. 21 No. 11 (Nov 1993):28

or Mercury Vol. 23, No. 1 (Jan/Feb 1994):20),

the Magellanic Clouds and Leo I have once been with the Andromeda Galaxy,

which had formed quite close to the Milky Way but drifted apart.

The galaxies of the Maffei 1 group,

notably IC 342 and Maffei 1 are thought to have been part of the group at

that time, until M31 suffered a cataclysmic merger with another dwarf galaxy

(which may have left the second nucleus in Andromeda’s core),

about 5 to 10 billion years ago.

As a result of this merger, IC 342 and Maffei 1 were ejected,

and the 3 dwarfs mentioned left their orbits around M31. The Magellanic

Clouds were captured by the Milky Way some 6 billion years ago, while

Leo I was ejected to a hyperbolic orbit, and will escape from the group

during the next few billion years.

As was already analysed in Binney and Tremaine’s fundamental work,

Galactic Dynamics, the Magellanic Clouds are probably on a collective

orbit (as they are gravitationally tied to each other), which is broken down

by a process called dynamical friction, as they pass through the halo

of our Galaxy. This makes their orbit decrease with time, so that the clouds

will spiral closer to the Milky Way, until in several billion years, they

will finally merge into our Galaxy.

Dynamical friction is a process which is due to the mutual gravitational

interaction of the stars of both galaxies. If a star, a cluster, or a small

galaxy crosses through a large galaxy, it is decelerated by the statistical

gravitational disturbations, passing kinetical energy over to the stars of

the larger galaxy. Therefore, the crosser is slowed down, but the “star gas”

of the larger galaxy is “heated”: This process will make the galaxy expand.

The slowdown will eventually lead to a merger, i.e. the crosser is “eaten”

by the larger galaxy, or even its core, which in turn absorbs the kinetic

energy of the collision and expands. This process explains well why the

Andromeda Galaxy, although significantly less massive, is about double as

large as the Milky Way galaxy, because of its early merger.

Also, in the future, interaction between the member galaxies and with the cosmic

neighborhood will continue to change the Local Group. Some astronomers speculate

that the two large spirals, our Milky Way and the Andromeda Galaxy, may perhaps

collide and merge in some distant future, to form a giant elliptical.

In addition, there is evidence that our nearest big cluster of galaxies, the

Virgo Cluster, will probably stop our cosmological

recession away from it, accelerate the Local Group toward itself so that it will

finally fall and merge into this huge cluster of galaxies, see our

Virgo Cluster & Local Group page.

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