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 £ M ¯ Gas mass ~5 £ 10 9 M ¯

The gas layer At Sun surface density ~10 20 atoms / cm 2 Gas layer ~300 light years ( 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!

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 M 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

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

DM Searches If m~GeV/c 2, ~10 6 /cm 2 /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

UKDMF Boulby

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