The Milky Way Galaxy

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The Milky Way Galaxy

• James Binney
• Oxford University

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Outline

• Why its important
• Components of the Galaxy
• Nearby stars
• Interstellar gas
• The bar
• The Galactic centre
• Globular clusters
• Star streams
• The dark halo

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Why bother?

• Its home!
• It can be studied in unique detail
• Its a highly typical galaxy

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Galaxy luminosity function

• Few galaxies LgtL
• Most light from galaxies with LL
• No accident that LL?

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Bulges and disks

• We live at edge of disk
• Disadvantage structure obscured by dust
• Advantage can study motions of nearby stars

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COBE Near IR View
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Dimensions

• Radius stellar disk 12 kpc37000 light years
• Distance Sun to centre 8kpc24000 l.y.
• Half-mass radius 40kpc?
• Thickness stellar disk 400pc1200 l.y.
• Stellar mass 51010 M
• Gas mass 5109 M

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The gas layer

• At Sun surface density 1020 atoms / cm2
• Gas layer 300 light years (3 1020cm) thick,
  so n0.3 atoms / cm3
• Density of air 1020 atoms / cm3
• So squashed to density air layer 1 cm thick
• Can see 1kpc when squashed could see only
  10cm through it
• Yuk!

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Star formation
Spitzer space telescope (IR)
Trifid nebula

• Stars form at rate few / yr

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Stars near the Sun

• Stars born on nearly circular orbits
• Stars have random velocities
• Spiral structure increases random velocities over
  time
• Derive age of solar neighbourhood 12.2Gyr

Hipparcos data
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Spiral structure

• Local Standard of Rest (LSR) on circular orbit
  around GC
• Shifts stars radially
• Sun may have shifted 2kpc

N-body simulation
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Pollution

• Pollution proceeds fastest near Galactic centre
• Older stars have fewer heavy elements
• Radial migration leads to big spread in Fe/H at
  given age

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Velocity space from Hipparcos

• Distribution of stars lumpy in velocity space
• Pointer to the Galactic bar and spiral structure

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Stars trapped by the bar
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Interstellar Gas

• Systematic effect circular streaming

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HI
CO
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The Galactic Bar

• Gas towards the GC moving away at 150km/s

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to Sun

• Expected if Galaxy barred

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If we could look down
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Near IR Photometry

• Galaxy brighter on left of GC

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• Individual objects (eg HB stars) also brighter on
  left

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The Galactic Centre
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Sgr A and Filaments
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Inclined Nuclear Disk
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The Black Hole

• Weak radio source Sgr A marks spot
• Orbiting stars reveal its force-field

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Globular Clusters
100 000 stars
M3
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Halo Disk Clusters
Halo
Disk
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• Disk clusters more metal-rich
• Also a population of field stars traced by blue
  horizontal branch stars RR Lyrae stars
• many from destroyed globular clusters

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Stellar halo(SDSS)
Bell et al (2007)
residuals
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Stellar streams
Belokurov et al (2007)
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Tidal streams (Pal 5)
Sloan digital sky survey (SDSS)
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Dark Halo
Milky Way
NGC 3198

• Hard to track around MW

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Dark halos cuspy?
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Gravitational microlensing

• Microlensing and magnitude of non-circular
  motions imply little DM at rlt5kpc

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Conclude

• Halo rather spherical
• Counts of gravitational microlensing events imply
  that its mass is not in stellar or planetary
  objects

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DM Searches

• If mGeV/c2, 106 /cm2/s at 300 km/s
• Seasonal variation in flux

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• If particles weakly-interacting, one
  occasionally impacts atomic nucleus
• look for events deep underground
  (shielded from cosmic rays)

Boulby, Yorks
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UKDMF Boulby
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Limits on cross section
SSM
CDMS Collaboration (06)
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Conclusion

• Understanding the Milky Way key for understanding
  the Universe
• Provides probe of constitution of Universe
• Much progress in the last decade
• Still many unresolved questions
• Will remain on the frontier of physics
  astronomy for the foreseeable future