Resolved Stellar Populations in the Milky Way

1. Resolved Stellar Populations in the Milky Way Ken Freeman Research School of Astronomy & Astrophysics Mount Stromlo Observatory The Australian National University Stellar Populations 2003

2. Overview of our Galaxy dark halo stellar halo thick disk thin disk bulge

3. Total mass ~ 2 x 1012 M_sun : ( 5 x 1011 M_sun out to 50 kpc) Wilkinson & Evans (1999), Sakamoto et al (2003) Stellar mass in bulge ~ 2 x 1010 M_sun disk 6 x 1010 M_sun halo 1 x 109 M_sun Ages of components: globular clusters ~ 13 Gyr; some outer clusters 1-2 Gyr younger thick disk : > 10 Gyr thin disk : ~ 10 Gyr from white dwarfs (Oswalt et al 1996, Legget et al 1998) 8 Gyr from old subgiants (Sandage et al 2003)

4. The thin disk is metal-rich and covers a wide age range The other stellar components are all relatively old (note similarity of [Fe/H] range for thick disk and globular clusters)

5. MOVIE Now show a numerical simulation of galaxy formation. The simulation summarizes our current view of how a disk galaxy like the Milky Way came together from dark matter and baryons • much dynamical and chemical evolution • halo formation starts at high z • dissipative formation of the disk

6. Simulation of galaxy formation • cool gas • warm gas • hot gas

7. Movie synopsis •z ~ 13 :star formation begins - drives gas out of the protogalactic mini-halos. Surviving stars will become part of the stellar halo - the oldest stars in the Galaxy • z ~ 3 :galaxy is partly assembled - surrounded by hot gas which is cooling out to form the disk • z ~ 2 :large lumps are falling in - now have a well defined rotating galaxy.

8. The metal-poor stellar halo abundance range [Fe/H] = -1 to -5 overlaps with the metal-poor tail of the thin disk Density distribution r ~ r -3.5, extends out to ~100 kpc Inner halo probably flattened, outer halo more nearly spherical

9. Kinematics of halo For [Fe/H] < -1.7, slow rotation (30 - 50 km/s); pressure-supported s = (140,105,95) km/s For [Fe/H] > -1.7 rotation increases, probably due to contribution of the metal weak tail of the thick disk Chiba & Beers 2000

10. Rotation of halo decreases with height above the galactic plane Chiba & Beers 2000

11. Little correlation of orbital eccentricity e with [Fe/H] (Chiba & Beers 2000) (Beers et al 2002) See the thick disk with its metal-poor tail, and a clump of high-e stars at [Fe/H] ~ -1.7 (ie at the [Fe/H] of the break in the Vrot - [Fe/H] relation. Could come from infalling gas with low angular momentum

12. Some thick disk stars in the solar neighborhood have [Fe/H] abundances as low as the most metal-poor globular clusters. Fraction F of (metal-weak thick disk) of the (total metal-weak population) near the sun increases with [Fe/H] What is this MWTD ? Did it form during collapse of disk ? Remnant of very early thin disk heated by early merger ? Accreted debris ? (Chiba & Beers 2000)

13. Would expect some accreted debris to settle to the disk eg decay of prograde satellite orbit around disk galaxy (Walker et al 1996) - dragged down into the disk plane by dynamical friction (against disk and halo) on timescale ~ 1 Gyr Rotation of debris would increase with z, as observed

14. Halo Streams Long orbital timescales  survival of identifiable debris eg Sgr tidal stream 2MASS M giants Ibata et al 1995 Majewski et al 2003

15. A large fraction of the halo stars in the meridional plane could be associated with Sgr debris colored points are different wraps of simulated orbit of Sgr (Helmi) black points are spaghetti halo giants Spaghetti collaboration : Morrison et al 2003

16. These tidal streams from the disrupting Sgr dwarf are interesting, but the ancient streams from small objects accreted long ago into the halo could be even more interesting. They are too faint to see in configuration space - may see them in phase space, eg (RG , VG ), or in integral space ie the space of integrals of the motion for stellar orbits, like energy and angular momentum (E , Lz )

17. Tidal Streams in the Galactic Halo(simulation of accretion of 100 satellite galaxies) y (kpc) RVGC (km s-1) x (kpc) RGC(kpc) (Spaghetti: Harding)

18. Accretion in integral space (E,Lz) Input - different colors represent different satellites • Output after 12 Gyr • stars within 6 kpc of • the sun - convolved with • GAIA errors Helmi & de Zeeuw

19. Accretion is important for building the stellar halo, but not clear yet how much of the halo comes from discrete accreted objects (debris of star formation at high z, as in movie) versus star formation during the baryonic collapse of the Galaxy Recent simulations of pure dissipative collapse (eg Samland et al 2003) suggest that the halo may have formed mainly through a lumpy collapse, with only ~ 10% of its stars coming from accreted satellites In any case, we may be able to trace the debris of these lumps and accreted satellites from their phase space structure

20. Chemical Properties of the metal-poor Halo a-enhancement associated with the short duration of star formation and enrichment large scatter in heavier elements at low [Fe/H], associated with a small number of discrete enrichment events insights into the nature of the earliest SN from detailed chemical abundances of very metal poor halo stars

21. Scatter in element ratios at lower [Fe/H] light s heavy s  elements have less scatter; Mg,Ti not rigidly coupled to Si,Ca r process Wallerstein et al 1997

22. The large observed scatter in [X/Fe] for metal-poor stars suggests that the neutron capture elements in metal-poor stars are products of only a few nucleosynthesis events - confirmed by simulations

23. White Dwarfs in the Halo Discovery of high proper motion WDs which could contribute some fraction of the dark halo density (Oppenheimer et al 2001) Much discussion - emerging view that these WDs are probably thick disk objects - no real consensus yet the number of true halo WDs appears consistent with the stellar halo(eg D. Carollo 2003, Salim et al 2003, Mendez 2002, Torres et al 2002)

24. • Oppenheimer WDs 2s contour for halo 2s contour for disk nearby M-dwarfs Many of these WDs are probably associated with the thick disk Reid et al 2001

25. The thin disk The thin disk is the defining stellar component of disk galaxies. It is the end product of the dissipation of most of the baryons, and contains almost all of the baryonic angular momentum Understanding its formation is an important goal of galaxy formation theory.

26. star formation history in galactic thin disk : roughly uniform, with episodic star bursts for ages < 10 Gyr, but lower for ages > 10 Gyr Rocha-Pinto et al (2000)

27. Solar neighborhood kinematics: Several mechanisms for heating disk stars: transient spiral arms, GMC scattering (eg Fuchs et al 2001), large-scale bending modes of anisotropic disk (*Sotnikova 2003), accretion events, star cluster dissolution (Kroupa 2001) Expect heating mechanisms to saturate after a few Gyr: stochastic heating : heated stars spend less time near galactic plane bending modes : heating decreases as vertical heating reduces the anisotropy What do observations show ?

28. old disk Velocity dispersions of nearby F stars appears at age ~ 10 Gyr thick disk Disk heating saturates at 2-3 Gyr Freeman 1991; Edvardsson et al 1993; Quillen & Garnett 2000

29. Structure of the thin disk exponential in R and z : scaleheight ~ 300 pc, scalelength 2-4 kpc (!) velocity dispersion decreases from ~ 100 km/s near the center (similar to bulge) to ~ 15 km/s at 18 kpc 2 log (velocity dispersion) 1.5 1 R (kpc) Lewis & KCF 1989

30. Moving Stellar Groups These are stars in the solar neighborhood with common motions and chemical properties : some are surviving fossils of star forming events in the disk. HR 1614 group (Feltzing 2000). Thin disk group, age ~ 2 Gyr, [Fe/H] ~ 0.2 Arcturus group (Eggen 1971). Old thick disk group, velocity V = -116 km/s relative to LSR, [Fe/H] ~ -0.6, These are nice examples of substructures surviving in the galactic disk. Gayandhi da Silva is working on the chemical homogeneity of these groups for her thesis at RSAA. These moving groups in the disk will become very interesting with RAVE and GAIA

31. Some moving groups are probably associated with local resonant kinematic disturbances by the inner bar : OLR is near solar radius (Hipparcos data) : Dehnen (1999), Fux (2001), Feast (2002) Sirius and Hyades streams - mainly earlier-type stars • Hercules disturb- • ance from OLR • mainly later-type • stars Dehnen 1999

32. Chemical properties of the nearby disk The age-abundance relation thick disk young disk old disk Edvardsson et al 1993

33. Chemical properties of the nearby disk : [X/Fe]  s thin disk thin + thick (see also Prochaska et al 2000; Bensby et al 2003; Yong et al 2003) Edvardsson et al 1993

34. Chemical properties of the nearby disk : [a´/Fe] = [(Ca, Si)/H] thin disk thin + thick (Rm is mean orbital radius) Edvardsson et al 1993

35. Abundance gradient in the old disk Abundance gradient for the old open clusters (age > Hyades) Friel 1995

36. More on the thick disk ... Most spirals (including our Galaxy) have a second thicker disk component, believed to be the early thin disk heated by an accretion event. In some galaxies, it is easily seen The thin disk The thick disk NGC 4762 - a disk galaxy with a bright thick disk (Tsikoudi 1980)

37. Near the sun, the galactic thick disk is defined mainly by stars with [Fe/H] in the range -0.5 to -1.0, though it does extend to very low [Fe/H] ~ -2.2. The element abundance data indicate that the thick disk has abundance patterns different from those for the thin disk, consistent with time delay between formation of thick disk stars and the onset of star formation in the current thin disk. Thick disks are very common - but not ubiquitous Formation pictures ... • a normal part of disk settling (Samland et al 2003) • accretion debris (Steinmetz et al 2003, Walker et al 1996) • early thin disk, heated by accretion events - eg the  Cen accretion event (Bekki & KF 2003)

38. If the heating by accretion picture is correct, the thick disk may be one of the most significant components for studying galaxy formation, because it presents a kinematically recognizable ‘snap-frozen’ relic of the (heated) early disk. Secular heating thereafter is unlikely to affect its dynamics significantly, because its stars spend most of their time away from the galactic plane.

39. Kinematics and structure of the thick disk rotational lag ~ 30 km/s (Chiba & Beers 2000) velocity dispersion in (U,V,W) = (46,50,35) km/s scale length = 3.5 to 4.5 kpc scale height from star counts = 800 to 1200 pc density = 4 to 10% of the local thin disk current opinion is that the thick disk shows no vertical abundance gradient (eg Gilmore et al 1995) not much is known about the radial extent of the thick disk - important, if the thick disk really is the heated early thin disk

40. My favored formation picture for the galactic disk Thin disk formation begins early, at z = 2 to 3 Partly disrupted during merger epoch which heats it into thick disk observed now The rest of the gas then gradually settles to form the present thin disk

41. The Galactic Bar- Bulge M31 small exponential bulge - typical of later-type galaxies. Unlike the large r1/4 bulge of M31 Pritchet & van den Bergh 1994 Launhardt 2002

42. Later type galaxies mostly have near-exponential bulges, rather than r1/4 bulges - hint that their bulges are not merger products - more likely generated by disk instability (eg Balcells et al 2002) Boxy bulges, as in our Galaxy, are associated with bars eg Bureau & KF 1999 - believed to come from bar buckling instability of disk. Our bar-bulge is ~ 3.5 kpc long, axial ratio ~ 1:0.3:0.3 pointing about 20o from sun-center line into first quadrant (eg Bissantz & Gerhard 2002)

43. The galactic bulge is rotating, like most other bulges: (Kuijken & Rich (2002) HST proper motions) Beaulieu et al 2000 K giants from several sources and planetary nebulae (+) • Velocity dispersion of inner • disk and bulge are fairly similar • not easy to separate inner disk • and bulge kinematically Bulge ends at |l| ~ 10o

44. Age and metallicity of the bulge Zoccali et al 2002 : stellar photometry at (l, b) = ( 0º.3, -6º.2) : old population > 10 Gyr. No trace of younger population. Extended metallicity distribution, from [Fe/H] = -1.8 to +0.2 (ie not very metal-rich at |b| = 6º ) Bulge MDF covers similar interval to (thin disk + thick disk) near sun

45. Abundance gradient in the bulge ( kpc ) Inhomogeneous collection of photometric ( ) and spectroscopic ( ) mean abundances - evidence for abundance gradient along minor axis of the bulge Zoccali et al (2002) Minniti et al 1995

46. Near the center of the bar/bulge is a younger population, on scale of about 100 pc : the nuclear stellar disk (M ~ 1.5 x 109 M_sun) and nuclear stellar cluster (~ 2 x 107 M_sun ) in central ~ 30 pc. (Launhardt et al 2002) ~ 70% of the luminosity comes from young main sequence stars.

48. The bulge globular clusters 3D kinematics of 7 globular clusters in the bar/bulge Their velocities show: • all of them are confined to the bulge region • the metal-poor clusters (o) are part of the inner halo • the metal-rich clusters include • a bar cluster • clusters belonging to a rotationally supported system Dinescu et al 2002

49. thick disk s = 42 km/s old thin disk s = 21 km/s continuos s ~ t 1/2 Cumulative ranked sum test: straight segments show age intervals over which the velocity dispersion remains constant.Abrupt changes of slope show appearance of discrete component Freeman 1991

50. Decay of a prograde satellite orbit