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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! 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 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 HI CO 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 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 100 000 stars M3 Halo & Disk Clusters Halo Disk • 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) Sloan digital sky survey (SDSS) Tidal streams (Pal 5) Dark Halo Milky Way NGC 3198 • Hard to track around MW Dark halos cuspy? Gravitational microlensing 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 underground (shielded from cosmic rays) Boulby, Yorks Limits on cross section SSM CDMS Collaboration (06) 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