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The Milky Way Galaxy
James Binney
Oxford University
Why it’s important
Components of the Galaxy
Nearby stars
Interstellar gas
The bar
The Galactic centre
Globular clusters
Star streams
The dark halo
Why bother?
• It’s home!
• It can be studied in unique detail
• It’s a highly typical galaxy
Galaxy luminosity function
• Few galaxies L>L*
• Most light from
galaxies with L~L*
• No accident that L~L*?
Bulges and disks
• We live at edge of disk
• Disadvantage: structure
obscured by “dust”
• Advantage: can study motions
of nearby stars
COBE Near IR View
Radius stellar disk 12 kpc=37000 light years
Distance Sun to centre 8kpc=24000 l.y.
Half-mass radius ~40kpc?
Thickness stellar disk ~400pc=1200 l.y.
Stellar mass ~5£1010 M¯
Gas mass ~5£109 M¯
The gas layer
• At Sun surface density ~1020 atoms / cm2
• Gas layer ~300 light years (3 1020cm) thick,
so n~0.3 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!
Spitzer space telescope (IR)
Star formation
• Stars form at rate
few / yr
Trifid nebula
Stars near the Sun
• Stars born on nearly
circular orbits
• Stars have random
• Spiral structure
increases random
velocities over time
• Derive age of solar
Hipparcos data
• Local Standard of Rest
(LSR) on circular orbit
around GC
• Shifts stars radially
• Sun may have shifted
N-body simulation
• Pollution proceeds fastest near Galactic centre
• Older stars have fewer heavy elements
• Radial migration leads to big spread in [Fe/H] at
given age
Velocity space from Hipparcos
• Distribution of stars lumpy in velocity space
• Pointer to the Galactic bar and spiral
Stars trapped by the bar
Interstellar Gas
• Systematic effect: circular
The Galactic Bar
• Gas towards the GC moving away at ~150km/s
to Sun
• Expected if
Galaxy barred
If we could look down
Near IR Photometry
• Galaxy brighter
on left of GC
• Individual objects
(eg HB stars) also
brighter on left
Sgr A and Filaments
Inclined Nuclear Disk
The Black Hole
• Weak radio source Sgr A* marks spot
• Orbiting stars reveal its force-field
Globular Clusters
100 000 stars
Halo & Disk Clusters
• 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
Stellar halo
Bell et al
Stellar streams
Belokurov et al (2007)
Sloan digital sky survey (SDSS)
Tidal streams
(Pal 5)
Dark Halo
Milky Way
NGC 3198
• Hard to track
around MW
Dark halos cuspy?
Microlensing and
magnitude of noncircular motions imply
little DM at r<5kpc
DM Searches
• If m~GeV/c2, ~106 /cm2/s at ~300 km/s
• Seasonal variation in flux
• If particles weaklyinteracting, one
occasionally impacts
atomic nucleus
• look for events deep
(shielded from cosmic rays)
Boulby, Yorks
Limits on cross section
CDMS Collaboration (06)
• 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