Orbital Poles of Milky Way Satellites

(based on Palma, Majewski and Johnston 2002)

Orbital Poles of Milky Way Satellites

(based on Palma, Majewski and Johnston 2002)

Nick Cowan

UW Astronomy

February 2005

Nick Cowan

UW Astronomy

February 2005

Dramatis PersonaeDramatis Personae

Globular Clusters1. Red Horizontal

Branch (RHB)2. Blue Horizontal

Branch and Metal Poor (BHP+MP)

Dwarf Satellite Galaxies (DSG)

Globular Clusters1. Red Horizontal

Branch (RHB)2. Blue Horizontal

Branch and Metal Poor (BHP+MP)

Dwarf Satellite Galaxies (DSG)

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Galactic Formation SchemesGalactic Formation Schemes

Bottom-Up• Small-scale (GCs,

DSGs) form first.• Small pieces

coalesce into a galaxy.

Bottom-Up• Small-scale (GCs,

DSGs) form first.• Small pieces

coalesce into a galaxy.

Top-Down• Galaxy forms first• GCs and DSGs form

in the context of a large galaxy.

Top-Down• Galaxy forms first• GCs and DSGs form

in the context of a large galaxy.

Pole AnalysisPole Analysis

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Orbits will not be perfectly correlated:Orbits will not be

perfectly correlated:• Even in a spherical potential, tidal

forces will degrade orbits.• Potential is probably not spherical so we

expect precession of orbits.• Clumpy dark matter will scatter

satellites.• A time-dependant potential will scatter

satellites.

• Even in a spherical potential, tidal forces will degrade orbits.

• Potential is probably not spherical so we expect precession of orbits.

• Clumpy dark matter will scatter satellites.

• A time-dependant potential will scatter satellites.

Great Circle Pole Families Great Circle Pole Families

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Crossing Points of Great CirclesCrossing Points of Great Circles

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Correlation Function and [Fe/H]Correlation Function and [Fe/H]

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RHB and DSG

BHB/MP and DSG

Correlation Function and RgcCorrelation Function and Rgc

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Rgc > 25 kpc

8 < Rgc < 25 kpc

Arc Segment Family AnalysisArc Segment Family Analysis

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Determining Arc Segment FamiliesDetermining Arc Segment Families

1. Find the two nearest arc segments.

2. Lump them together.3. Find the next-nearest

arc segments.4. Lump them together.5. etc.6. Stop when you stop

believing.

1. Find the two nearest arc segments.

2. Lump them together.3. Find the next-nearest

arc segments.4. Lump them together.5. etc.6. Stop when you stop

believing.

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How far is too far?How far is too far?

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“I’m getting nervous, let’s stop here.”

Reality CheckReality Check

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ConclusionsConclusions

• BHB+MP GCs do not appear to track DSGs in their orbits around the MW.

• Some RHB GCs have correlated orbits.• Orbital Pole Analysis is a powerful tool

but better proper motion measurements will be needed to put more meaningful constraints on Galaxy formation schemes.

• BHB+MP GCs do not appear to track DSGs in their orbits around the MW.

• Some RHB GCs have correlated orbits.• Orbital Pole Analysis is a powerful tool

but better proper motion measurements will be needed to put more meaningful constraints on Galaxy formation schemes.