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The Milky Way Galaxy James Binney Oxford University.

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Presentation on theme: "The Milky Way Galaxy James Binney Oxford University."— Presentation transcript:

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


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


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

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

8 COBE Near IR View

9 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 £ 10 10 M ¯ Gas mass ~5 £ 10 9 M ¯

10 The gas layer At Sun surface density ~10 20 atoms / cm 2 Gas layer ~300 light years (3 10 20 cm) thick, so n~0.3 atoms / cm 3 Density of air ~10 20 atoms / cm 3 So squashed to density air layer ~1 cm thick Can see ~1kpc; when squashed could see only ~ 10cm through it Yuk!

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

12 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

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

14 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

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

16 Stars trapped by the bar

17 Interstellar Gas Systematic effect: circular streaming

18 CO HI

19 The Galactic Bar Gas towards the GC moving away at ~150km/s

20 Expected if Galaxy barred to Sun

21 If we could look down

22 Near IR Photometry Galaxy brighter on left of GC

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

24 The Galactic Centre

25 Sgr A and Filaments

26 Inclined Nuclear Disk

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

28 Globular Clusters M3 100 000 stars

29 Halo & Disk Clusters Disk Halo

30 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

31 Stellar halo (SDSS) residuals Bell et al (2007)

32 Stellar streams Belokurov et al (2007)

33 Tidal streams (Pal 5) Sloan digital sky survey (SDSS)

34 Dark Halo Hard to track around MW NGC 3198 Milky Way

35 Dark halos cuspy?

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

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

38 DM Searches If m~GeV/c 2, ~10 6 /cm 2 /s at ~300 km/s Seasonal variation in flux

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

40 UKDMF Boulby

41 Limits on cross section CDMS Collaboration (06) SSM

42 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

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