The Milky Way Galaxy

1. The Milky Way Galaxy James Binney Oxford University

2. 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

3. Why bother? • It’s home! • It can be studied in unique detail • It’s a highly typical galaxy

5. Galaxy luminosity function • Few galaxies L>L* • Most light from galaxies with L~L* • No accident that L~L*?

6. Bulges and disks • We live at edge of disk • Disadvantage: structure obscured by “dust” • Advantage: can study motions of nearby stars

7. COBE Near IR View

8. 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¯

9. 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!

10. Spitzer space telescope (IR) Star formation Trifid nebula • Stars form at rate few / yr

11. 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

12. Spiral structure • Local Standard of Rest (LSR) on circular orbit around GC • Shifts stars radially • Sun may have shifted ~2kpc N-body simulation

13. 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

14. Velocity space from Hipparcos • Distribution of stars lumpy in velocity space • Pointer to the Galactic bar and spiral structure

15. Stars trapped by the bar

16. Interstellar Gas • Systematic effect: circular streaming

17. HI CO

18. The Galactic Bar • Gas towards the GC moving away at ~150km/s

19. to Sun • Expected if Galaxy barred

20. If we could look down

21. Near IR Photometry • Galaxy brighter on left of GC

22. Individual objects (eg HB stars) also brighter on left

23. The Galactic Centre

24. Sgr A and Filaments

25. Inclined Nuclear Disk

26. The Black Hole • Weak radio source Sgr A* marks spot • Orbiting stars reveal its force-field

27. Globular Clusters 100 000 stars M3

28. Halo & Disk Clusters Halo Disk

29. 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

30. Stellar halo(SDSS) Bell et al (2007) residuals

31. Stellar streams Belokurov et al (2007)

32. Tidal streams (Pal 5) Sloan digital sky survey (SDSS)

33. Dark Halo • Hard to track around MW Milky Way NGC 3198

34. Dark halos cuspy?

35. Gravitational microlensing Microlensing and magnitude of non-circular motions imply little DM at r<5kpc

36. DM Searches • If m~GeV/c2, ~106 /cm2/s at ~300 km/s • Seasonal variation in flux

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

38. Limits on cross section SSM CDMS Collaboration (06)

39. 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