Chemical & dynamical evolution of the Galaxy

James Binney

Oxford University

outline

• Secular heating

• Radial mixing

• Origin of the thin disc

Secular heating• Fluctuations in © from spiral structure and molecular

clouds scatter stars from near-circular orbits• Colour of MS stars related to mean age• So random velocity of MS stars increases with B-V

Aumer & B 09

Modelling heating

• Assume – SFR / e-t/t0

– ¾ / t¯

• Fit ¾(B-V) and N(B-V)

Aumer & B 09

The solar neighbourhood is old

• ¿max > 10.5 Gyr, best age 12.5 Gyr

• Peak cosmic SFR z = 1-2 $ ¿ = 7.8 – 10.5 Gyr

• Mean age 5.9 – 6.4 Gyr upper end of disc ages at 10 kpc in Scannapieco+09 simulations Aumer & B 09

Radial mixing

• Dominant effect of spiral structure is scattering @ corotation

• Does not heat disc but drives radial migration

Sellwood & B 02

Impact on chemical evolution

• Conventional evolution models (van den Bergh 62 ! Chiappini+97 ! Colavitti+08):– Disc a series of accreting annuli– Annuli evolve independently

• Schoenrich & B (09ab):– Annuli accrete from IGM & next outwards– Stars move radially on growing epicycles

(blurring) and epicenters “churned”– ¾ / t¯

Schoenrich & B (09)

Flow over R

From IGM per area

model

5

10

Schoenrich & B (09)

• Leads to – inhomogeneous

solar nhd– steep Z gradient

ISM

Local stars

d[Fe/H]/dR=-0.08

Schoenrich & B (09)

• Parameters fitted to Geneva-Copenhagen sample of Hipparcos stars

• Selection effects crucial

GCS

cylinder

Chiappini 97

No churning

Origin of thick disc

• Reid & Gilmore (83) identified thick disc by vertical profile

• Later thick-disc stars found to be old and high-®

• Overlap of thin & thick discs in Fe/H (Bensby+03) suggested hiatus in SF between two

thinB 09

Thick disc & SNIa

• In SB09 model infall rate declines monotonically & SFR determined by Kennicutt law so total continuity in SF

• But distribution of stars in (Fe/H,®) bimodal

• Reason: high SF in ~1Gyr before SNIa kick in

• Metal-poor stars in solar nhd not historical relic but reflect radial mixing

ISM tracks

Disc divided

Many predictions to test

Rotation vs chemistry

Chemistry vs z

Conclusions

• We are in the middle of an era of giant Galaxy surveys (Hipparcos, UCAC2, DENIS, 2MASS, SDSS, RAVE, HERMES, APOGEE, Gaia)

• From these data we should infer history of the Galaxy• Models have a crucial role to play• Dating stars is v hard; chemistry is a more accessible

proxy• Chemistry & dynamics entangled • Radial mixing has been too long neglected• The simplest model of chemodynamical evolution with

radial mixing suggests the thick disc formed quiescently and reflects dynamics of SNIa rather than lumpy accretion