The Milky Way Galaxy
James Binney
Oxford University
Outline
• 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
Dimensions
• 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!
Star formation
• Stars form at rate few / yr
Trifid nebula
Spitzer space telescope (IR)
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
Spiral structure
• Local Standard of Rest (LSR) on circular orbit around GC
• Shifts stars radially
• Sun may have shifted ~2kpc
N-body simulation
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
Velocity space from Hipparcos
• Distribution of stars lumpy in velocity space • Pointer to the Galactic bar and spiral
structure
Stars trapped by the bar
Interstellar Gas• Systematic effect: circular
streaming
CO
HI
The Galactic Bar
• Gas towards the GC moving away at ~150km/s
• Expected if Galaxy barred
to Sun
If we could look down
Near IR Photometry
• Galaxy brighter on left of GC
• Individual objects (eg HB stars) also brighter on left
The Galactic Centre
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
M3
100 000 stars
Halo & Disk Clusters
Disk
Halo
• 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(SDSS)
residuals Bell et al (2007)
Stellar streams
Belokurov et al (2007)
Tidal streams (Pal 5)
Sloan digital sky survey (SDSS)
Dark Halo
• Hard to track around MW
NGC 3198
Milky Way
Dark halos cuspy?
Gravitational microlensing
Microlensing and magnitude of non-circular 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 weakly-interacting, one occasionally impacts atomic nucleus
• look for events deep underground (shielded from cosmic rays)
Boulby, Yorks
Limits on cross section
CDMS Collaboration (06)
SSM
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