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Tidal Streams in the Milky Way

Introduction

When two galaxies approach each other, gravity slows the motion of each. Strong enoughgravitational effects cause the galaxies to spiral towards each other. This will cause the two galaxiesto merge.

How they Form

When two galaxies that are gravitationally bound vary in mass by a large degree, the result is thecannibalisation of the smaller galaxy by the larger.

The gravitational attraction between the two galaxies causes the orbital energy to be divertedresulting in an increase in the galaxies internal energy. The orbit of the galaxy is reduced due to this.

Some orbital energy also dissipates as stars and gas are pulled out of either of the galaxies involved.This effect is known as tidal stripping and the groups of gas and stars affected are known as tidalstreams.

Examples

One tidal stream in the Milky Way is the Magellanic Stream. The Magellanic Stream was caused byinteractions between the Milky Way and the Magellanic Cloud. Tidal stripping is the likely cause of

its formation. The interaction took place around two hundred million years ago. The structure of theremaining tidal stream has been affected by interactions with the Milky Way s hot coronal gas.

Another example is the effect of the Sagittarius dwarf galaxy. This is the closest satellite galaxy to theMilky Way. Due to gravitational interactions between the satellite and the Milky Way it has beenpartially consumed and is starting to break apart. The merge of the galaxies has resulted in carbonstars and M giants being stripped from the satellite galaxy and spread out across the sky. This groupof stars is the tidal stream produced during the interaction.

The Palomar 5 galaxy is been nearly fully merged with the Milky Way. Ninety percent of its originalstars have been lost. The stars that have been gravitationally stripped are spread across the sky intwo tidal tails. These extend at angles greater than ten degrees.

It is likely that a tidal stream will form when the collision between Andromeda and the Milky Waytakes place in the future. Currently expansion with the Hubble flow has ceased due to the

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gravitational attraction between the two galaxies. They are approaching each other with a relativevelocity of 119 kilometres a second.

Detection

Tidal streams are often found on galaxies that contain dense molecular gas at their centre of massaround 10 9 solar masses. This and the detection of tidal streams are indications of a recent mergebetween galaxies.