KIAA Lectures Beijing, July 2010 Ken Freeman, RSAA

1. KIAA Lectures Beijing, July 2010 Ken Freeman, RSAA Lecture 3: the Galactic bulge and the globular clusters

2. NGC 4594 : a classical r1/4 bulge

3. NGC 4565 : a boxy bulge

4. Our Galaxy has a small boxy bar-bulge

5. NGC 5907: no bulge at all

6. From lecture 1 …. Forming large galaxies with small or no bulges is currently difficult in CDM because of the relatively active ongoing merger history. Establishing the merger history of the Milky Way observationally is a major goal for Galactic Archaeology. We need to understanding how the Galactic bulge formed : is it even partly a merger product or did it form entirely through internal processes (eg disk and bar instability)

7. Different kinds of bulges Large classical bulges as in the Sombrero galaxy are believed to be merger products. Merger dynamics and violent relaxation leads to bulges with the characteristic r 1/4-law light distribution (Sersic index ~ 4). Classical bulges are common in early type galaxies but become progressively rarer towards later types. They share some structural, dynamical and population properties with the lower- luminosity ellipticals

8. Later type galaxies like the Milky Way mostly have small near-exponential boxy bulges, rather than r1/4 bulges. (eg Courteau et al 1996) Boxy bulges, as in our Galaxy, are associated with bars, believed to form via bar-buckling instability of disk. They are probably not merger products observations:Kuijken & Merrifield 1995, Bureau & KF 1999 Chung et al 2004 ... theory:Combes & Sanders 1981 ...

9. NGC 5746 [NII] 6584Å H [NII] 6548Å NGC 5746: gas kinematics in a boxy bulge show the signature of orbits in a bar potential (Bureau & Freeman 1999)

10. Bar-buckling to form a boxy/peanut bulge • The disk suffers a bar instability : very common for fairly cold disks • The bar buckles vertically, driven by horizontal and vertical resonances, and forms a boxy/peanut bulge: it takes a few bar revolutions to make this instability go (Combes et al 1990, Athanassoula et al 2007). • The whole process takes 2-3 Gyr after the formation of the disk • The rotation of boxy bulges is cylindrical: i.e. Vrot only weakly dependent on height above the plane velocity field structure

11. The maximum vertical extent of peanuts occurs near radius where vertical and horizontal Lindblad resonances occur ie where b =  - /2 =  - z/2 (remember: both  and z depend on the amplitude of the oscillation) Stars in this zone oscillate on orbits which support the peanut shape. Orbits supporting the peanut

12. So far, seen • classical bulges, probably merger products • boxy/peanut bulges, probably disk instability products There is a third kind of bulge-like structure which looks like an enhancement of the surface brightness profile above the exponential disk but appears to lie in the disk, from its shape and kinematics. These are the • pseudobulges (Kormendy 1993): they are believed to be generated by secular processes associated with the angular momentum transport by bars or weakly oval disks. They often show active star formation within the pseudobulge region. (Recall that bars are very common : about 2/3 of disk galaxies show some kind of bar structure in NIR images)

13. M83 in bluelight (L) and K light (near-IR) (R) The bar is much more obvious in the near-IR. The bar extends well beyond the central bulge.

14. NGC 6384 pseudobulge Kormendy & Kennicutt 2004

15. Another example of a starforming pseudobulge (HST) M. Carollo et al 1998

16. The kinematics of pseudobulges : V/ above oblate curve

17. The terms pseudobulge and secular evolution have become a bit mis-used. I think pseudobulge is best reserved for these flat enhancements that look like bulges only in their surface brightness profiles. There is nothing pseudo about the Galactic bulge. Secular evolution means slow relative to the dynamical time, like the slow transport of matter into the central regions via torques from a bar or oval disk. There is nothing secular about the bar-buckling scenario.

18. Kinematics of classical vs boxy bulges NGC 5866 NGC 7332 Falcon-Barosso et al 2004

19. Kinematics of classical bulge (NGC 5866): non-cylindrical rotation (SAURON) Falcon-Barosso et al 2004

20. Kinematics of boxy bulge (NGC7332): near-cylindrical rotation (SAURON) Falcon-Barosso et al 2004

21. The Galactic Bar- Bulge small exponential bulge - typical of later-type galaxies. Launhardt 2002

22. Age and metallicity of the bulge Zoccali et al 2003 : 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

23. 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 (2003) Minniti et al 1995

24. 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.

25. How did the Galactic Bulge form ? Later type galaxies like the Milky Way mostly have small near-exponential boxy bulges, rather than r1/4 bulges. (eg Courteau et al 1996) These small boxy bulges are probably not merger products: more likely generated by bar-buckling instability of disk. We might then expect some similarities of stellar population between the bulge and the surrounding disk and thick disk

26. Our bar-bulge is ~ 3.5 kpc long, axial ratio ~ 1: 0.3: 0.3 pointing about 15-30o from sun-center line into first quadrant (eg Bissantz & Gerhard 2002).

27. López et al (2006) find evidence of a longer flat bar lying in the disk of the Galaxy (7.8 x 1.2 x 0.2 kpc) from 2MASS counts and red-clump stars. The central boxy bar/bulge is the inner extended part of this longer flat bar GC 

28. The stars of the bulge are old and enhanced in -elements  rapid star formation history Are the bulge stars and thick disk stars different ? Not clear yet Here the data for the bulge stars and thick disk stars come from different sources [/Fe] higher for thick disk than for thin disk: higher still for bulge Fulbright et al 2007

29. bulge Differential analysis of O-abundance in bulge, thick disk and thin disk stars. The thick disk is O-enhanced relative to thin disk as usual, but the bulge and thick disk look very similar in this study. thick disk thin disk Meléndez et al 2008

30. The bar-forming and bar-buckling process takes 2-3 Gyr to act after the disk settles In the bar-buckling instability scenario, the bulge structure is probably younger than the bulge stars, which were originally part of the inner disk The alpha-enrichment of the bulge and thick disk comes from the rapid chemical evolution which took place in the inner disk before the instability acted

31. The galactic bulge is rotating, like most other bulges: Rotation (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| ~ 12o

32. 2 log (velocity dispersion) 1.5 1 R (kpc) Velocity dispersion of the thin disk As expected for exponential disk in R and z : scaleheight ~ 300 pc, scalelength 3-4 kpc. Velocity dispersion increases from ~ 15 km/s at 18 kpc to ~ 100 km/s near the center (similar to bulge). This makes it difficult to separate disk and bulge stars kinematically Lewis & KCF 1989

33. How to test whether the bulge formed through the bar-buckling instability of the inner disk ? Compare the structure and kinematics of the galactic bulge with an N-body simulation of a disk that has generated a boxy bar/bulge through bar-buckling instability of the disk (Athanassoula). Do the simulations match the properties of the Galactic bar/bulge (eg exponential stucture, cylindrical rotation ?)

34. N-body model seen from galactic pole

35. N-body model log intensity |b| COBE Minor axis surface brightness profiles The slope of log I(b) gives the length scale for the model

36. b = 0.5 b = 9.5 The kinematics of the model are as observed for boxy bulges: cylindrical rotation Detailed velocity data not yet available for the galactic bar/bulge: survey in progress. Model fits well to limited data available now

37. Vrot (km/s) l Rotation of bulge (5 < |b| < 10) model V rot (l ) gives the velocity scale for the model

38.  los (km/s) l Velocity dispersion of bulge (5 < |b| < 10) model

39. sun The ARGOS bulge survey We are doing a spectroscopic survey of the bulge with the AAT and AA to determine whether its kinematics are consistent with the bar-buckling scenario and to derive limits on any underlying classical bulge. (Melissa Ness, KCF et al) Observe at Ca triplet ~ 8600 Å, resolution = 13,000, SN ~ 70 Magnitudes chosen to cover entire sightline through the bulge 28 fields of 1000 stars each, in bulge and surrounding thin and thick disk (have spectra of 23,000 stars so far)

41. Sample Selection Criteria: 28,000 stars in 28 Fields Sample size sufficient to detect 5% merger generated bulge underlying an instability bulge. Selected stars in each field from 2MASS Mainly red clump giants along line of sight MK = -1.6, (J-K)0= 0.65 Colour cuts determined using Schegel reddening in each field Selection criteria do not exclude metal-poor stars: expect to find stars of the inner stellar halo. Also … In CDM cosmology formation scenarios, the first stars will be concentrated in the bulge region

42. Spectrum observed at Ca triplet with AAOmega See lines ofFe, Al, Ca, Ti, Si, Mg, O

43. Rotation Curves for 4 Fields of Latitude: From output velocities of ~ 23, 000 stars (error < 1.2km/s) V gc = Vhc + 220sin(l )cos(b) + 16.5[sin(b)sin(25) + cos(b)cos(25)cos(l − 53)] near side bulge far side [Athanassoula] cylindrical rotation

44. Find the expected metal-poor halo stars in the bulge region. They do not rotate as fast as the more metal-rich stars of the bulge (previously described by Paul Harding 1993) Are they just the stars of the inner halo, or are they the first stars, or is there no difference ?

45. The metal-poor stars in the bulge region rotate more slowly than the metal-rich stars: they probably belong to the inner Galactic halo

46. Where are the first stars now ? Diemand et al 2005, Moore et al 2006, Brook et al 2007 The metal-free (pop III) stars formed until z ~ 4 in chemically isolated sub-halos far away from largest progenitor. If its stars survive, they are spread through the Galactic halo. If they are not found, then their lifetimes are less than a Hubble time  truncated IMF The oldest stars form in the early rare density peaks that lay near the highest density peak of the final system. Now they lie in the central bulge region of the Galaxy. First stars are in orbits of fairly high eccentricity, rather similar to observed eccentricity distribution for metal-poor stars in the galactic halo

47. Distributions of the first stars and the metal-free stars Brook et al 2007

48. Distribution in present galaxy of debris from peaks selected at z > 12 (Moore et al 2006). Dashed cuve shows slope for metal-poor halo.

49. The Galactic Bulge - summary The bulge is not a dominant feature of our Galaxy - only about 20% of the light. The bulge is probably an evolutionary structure of the disk, rather than a feature of galaxy formation in the early universe. Structure and kinematics (so far) are well represented by product of disk instability. The -enhancement indicates that star formation in this inner disk/bulge region proceeded rapidly. Thebulge structuremay be a few Gyr younger than its stars.

50. The M31 bulge Rotation and velocity dispersion of its bulge are slightly larger than for the MW bulge peak Vrot ~ 100 km/s (0) ~ 140 km/s Simien et al 1979, McElroy 1983