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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.
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