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