Galaxy Formation and Evolution Galactic Archaeology

1. Galaxy Formation and Evolution Galactic Archaeology Chris Brook Modulo 15 Room 509 email: cbabrook@gmail.com

2. Lecture 5: Galactic Archeology

4. The Structure of our Galaxy

7. Old components of the Milky Way

8. Formation of the Milky Way

9. Formation of the Milky Way 2 collapse scenarios were postulated, based on kinematics and abundances

10. The stellar halo, bulge, thick and thin disks have different mean metallicities, as indicated

11. Age-Metallicity relation of the Components

12. The Milky Way’s history is reflected both in the abundances of key chemical elements in stellar atmospheres, and in stellar motions

13. The motions of local stars can be decomposed into circular (V), radial (U) and perpendicular to disk (W) components. Galaxy components the thin disk, thick disk and halo have different motions.

14. √ U2 +W2 (km/sec) Tangential orbital speed V (km/sec) Thin-disc stars follow nearly circular orbits, with most of their motion being tangential. Halo stars are equally likely to follow prograde or retrograde orbits and cross the midplane with high speeds.

15. Ratio of Iron to hydrogen, relative to that of the Sun Tangential orbital speed V (km/sec) These orbital distinctions are mirrored by differences in iron content, with halo stars being the most metal-poor, as if they were formed from relatively primordial material. Thin disk stars are the most metal rich.

16. Ratio of alpha elements to Iron, relative to that of the Sun Tangential orbital speed V (km/sec) The Galaxy’s different populations also differ in their alpha-to-Iron ratios, where alpha means elements such as oxygen and magnesium that are synthesised in core-collapse supernovae.

18. Ratio of alpha elements to Iron, relative to that of the Sun Tangential orbital speed V (km/sec) The Galaxy’s different populations also differ in their alpha-to-Iron ratios, where alpha means elements such as oxygen and magnesium that are synthesised in core-collapse supernovae.

19. Stellar Halo Formation Ryan & Norris 1991 Halo stars have high velocities compared to the local standard of rest (which rotates with the galaxy)- they also have low metallicity

20. Stellar Halo Formation Models of the accretion of multiple satellites. Do they look like the real MW halo? Johnston & Bullock 2005

21. Looking for accretion events Evidence of accretion from stellar kinematics. Stars may retain coherence in phase space longer than they will remain spatially associated Helmi et al. 1999

22. A problem?

23. Stellar Halo Formation Models of the accretion of multiple satellites. Do they look like the real MW halo? More sophisticated models seem to be able to account for this Johnston et al 2008, see also e.g. Robertson et al. 2005

24. Dual Stellar halo? See Carrollo, Beers et al. 2010

25. How have 2 halos formed?

26. In situ halo stars?i.e. not all halo stars come from satellites Is this the return of the original ELS rapid collapse scenario? That accretion plays a role in halo formation is not in doubt, and in particular the outer halo is almost certainly accreted. But the contribution of stars born in the disk and later knocked into the halo, is inner halo remains under debate Zolotov et al. 2009

27. Extremely Metal Poor Stars We can use old stars found in the halo of the Milky Way to learn about the earliest stages of galaxy formation. The particular abundances found in the lowest metallicity stars can tell us about the types of stars that first polluted the Universe.

28. Where are primordial stars found? Brook et al. 2007

29. Where are primordial stars found?

30. Where are primordial stars found? Primordial stars The oldest stars

31. Probing Dark Matter

32. Probing the shape of the Dark Halo

34. Probing the shape of the Dark Halo Yet CDM halos are triaxial/prolate (e.g. Jing & Suto 2002)

35. Probing the shape of the Dark Halo Can the effect of baryons explain the discrepencies with CDM? (again!) Adding baryons makes halos more spherical Kazantzidis et al. 2004

36. The Bulge The Galactic Centre

37. The bulge Metallicity Distribution Function

39. Bulge Formation: evidence from abundances Along with other galaxies, the bulge of the MW has been thought to have similarities to Elliptical galaxies: alpha-enhanced stellar populations, dominated by old stars, and seem to have formed on short timescales, possibly in less than 1 Gyr (e.g. Thomas et. al. 2005). Did it form in the same way as Ellipticals? Maybe through starbursts that are driven by mergers at high redshift?

40. The Bulge

41. Recent Bulge Observations Metallicity Gradient detected along minor axis. Recall that metallicity gradients may be signatures of formation mechanisms Moving away from the centre Ness et al. 2012

42. Recent Bulge Observations Metallicity distributions at different radii, all taken at lattitude -5° Indications of a complex overlap of components in the central regions? See Ness et al. 2012

43. The Thick Disk

44. Milky Way Thick Disk: properties • large scale height~ 0.6-1 kpc (e.g. Phelps et al `99) • unclear scale-length compared to thin disk (Juric 2008 cf Bensby et al. 2011) • ~5-10% of the mass of the thin disk • lags thin disk by~40 km/s • dynamically hot • old stars ~10 Gyrs (e.g. Gilmore & Wyse `95) • -1<[Fe/H]<-0.2 (peak~-0.6) • no vertical metallicity gradient • distinct chemical abundance patterns Thick disk info2 Kinematics, metal abundances and ages support the hypothesis that it is a distinct component

45. The Thick Disk: ages and metallicities Like halo stars, thick disk are old

47. Clues to Thick Disk Formation

48. Thick Disk Formation • A slow, pressure supported collapse (Larson 1976); • Enhanced kinematic diffusion of the thin disk stellar orbits • (Norris 1987); • A rapid dissipational violent dynamical heating of the early thin disk (Quinn et al. 1993, Jones & Wyse 1983) • stars accreted directly from satellites (Statler 1988; Abadi et al 2003) • collapse triggered by high metallicity (Wyse & Gilmore 1988). • Gas rich mergers at high redshift (disks born hot, Brook et al. 2004) • Star cluster “popping” (Kroupa et al. 2003) • Radial migration (Loebmann et al 2010, Schronich & Binney 2009) • -information of the metallicity, ages, and chemical abundances of thick disk stars can be compared to the predictions that the various scenarios make

50. Thick Disk Formation