Our Galaxy, the Milky Way

1. Our Galaxy, the Milky Way Ken Freeman, Research School of Astronomy & Astrophysics, ANU & UWA Public lecture August 12, 2010

2. The Milky Way on a winter’s night in the south

4. NGC 891 Keeler (1899)

5. The Kapteyn Universe 1922 Kapteyn’s Universe

6. Shapley and the distance to the Galactic center 12 kpc

7. Lindblad interpreted the pattern of stellar motions as due to Galactic rotation Oort worked out the theory of Galactic rotation

8. 30 kpc Need to tell you about dark matter now. There is more to the Galaxy than what we can see in this image. In fact, this is only a tiny fraction of it

9. NGC 2997 - a typical disk-like spiral galaxy - flat and rotating, but not rotating as a rigid body

10. halo The Westerbork telescope Rotation curve of a spiral galaxy : gravity provides the acceleration V2/R needed for the stars and gas to go around in circular motion. The gravitational field of the stars and gas alone is not consistent with the flat rotation curve. A dark halo is needed

11. 30 kpc Albert Bosma’s (1978) thesis from the Kapteyn Institute helped to make the case for the dark halos of galaxies

12. Rotation of the Galaxy Merrifield (1992)

13. M31 and the Milky Way are now approaching at 118 km s -1. Their separation is about 750 kpc M31 To acquire this velocity of approach in the life of the universe means that the total mass of the Milky Way is at least 120 x 10 10 M. The stellar mass is about 6 x 1010M, so the ratio of dark to stellar mass is ~ 20 118 km s -1 Milky Way The dark halo extends out to at least 120 kpc, far beyond the disk's radius of ~ 20 kpc (Kahn & Woltjer 1959)

14. The ratio of dark mass to stellar mass is typically about 20: 1 The dark halos extend 5 to 10 times further out than the disks of these galaxies Dark matter dominates the mass budget of the universe. It is very important for galaxy formation. The Big Bang was 13.7 Gyr ago. We see that galaxies are already forming 0.5 Gyr after the Big Bang. Without dark matter, this could not happen - it would take galaxies much longer to form

15. MOVIE Start by showing 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, through the merging of smaller objects in the cosmological hierarchy. • much dynamical and chemical evolution • halo formation starts at high z • dissipative formation of the disk

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

17. Movie synopsis •z ~ 13 :star formation begins - drives gas out of the protogalactic dark matter 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 disk galaxy. You saw the evolution of the baryons. There is about 10 x more dark matter in a dark halo, underlying what you saw: The dark halo was built up from mergers of smaller sub-halos Saw spiral structure developing in the gas Merging of galaxies is still going on now

18. Merging of galaxies is still happening now Merging stimulates star formation and disrupts the galaxies. This is NGC 4038/ 9 : two large merging spirals. The end product of the merger is often an elliptical galaxy. (In a few Gyr, the Milky Way will probably merge with M31)

19. NGC 5907: debris of small galaxy accreted by a larger galaxy Our Galaxy has a similar structure from the disrupting Sgr dwarf APOD

20. You could see spiral structure coming and going in the movie Our Galaxy has spiral structure, though it is difficult to map this structure from inside the Galaxy. Look first at the spiral structure of a nearby spiral M83

21. The nearby spiral galaxy M83 in blue light (L) and in the NIR (R) The blue image shows young star-forming regions and is affected by dust obscuration. The NIR image shows mainly the old stars and is unaffected by dust. Note how clearly the central bar can be seen in the NIR image

22. Sun Artist’s sketch of the spiral structure of our Galaxy, based on optical observations of young stars and radio observations of hydrogen and CO. APOD

23. For a long time, spiral structure was thought to be a steady wave, propagating around the Galaxy. Now we think it comes and goes, as in the movie - needs gas to maintain the spiral structure and star formation. Without replenishment of the gas, the star formation and spiral structure should go out in a few Gyr. Some gas comes from dying stars, but not enough. New gas may come from the infall of small galaxies and gas clouds - one of the big present puzzles. The Magellanic Stream in neutral hydrogen (APOD, 2010)

24. Most spirals (including our Galaxy) have a second thicker disk component . 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)

25. Our Galaxy has a significant thick disk It was discovered via star counts at the South Galactic Pole • it is about three times thicker than the thin disk

26. thin disk thick disk The discovery of the thick disk of our Galaxy: star counts towards the South Galactic Pole Gilmore & Reid 1983

27. The thick disk of our Galaxy: • its mass is about 10% of the mass of the thin disk • its stars are old (> 12 Gyr) and have less metals than the thin disk • its stars are enriched in alpha-elements (Mg, Si, Ca) so its star formation was rapid, ~ 1 Gyr

28. -1.0 -0.5 0.0 0.5 Thick disk chemical element ratios The ratio /Fe is about 2.5 times enhanced in the thick disk Mg enrichment in the thick disk: thick disk appears chemically distinct from the thin disk Fuhrmann 2008

29. -enrichment relative to Fe means rapid chemical evolution • Element-building occurs mainly via supernova explosions: exploding stars return chemically enriched gas to the interstellar gas - then new stars form. • The alpha elements (Mg, Ca, Si, Ti) come mainly from massive supernovae: • stars with masses > 10 M which explode within a few million years • Fe comes mainly from low mass supernovae which take 1-2 Gyr to explode • So a high alpha/Fe ratio means that the star formation and chemical evolution ended before the low mass supernovae had time to explode and enrich the interstellar gas. The thick disk stars formed mostly more than 12 Gyr ago. Then there was a pause in star formation, until the thin disk stars started to form about 10 Gyr ago. Thin disk star formation has continued at a more-or-less constant rate up to the present time.

30. How do thick disks form ? • large energetic star-bursts driven by early gas-rich mergers (Brook et al 2004). Clump cluster galaxies at high redshift may be such events. • accretion debris (Abadi et al 2003, Walker et al 1996) • early thin disk, heated by accretion events: 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 as merger activity dies down.

31. Clump cluster galaxy at z = 1.6: these are common at high redshift as galaxies are assembling (Bournand et al 2008)

32. Heating of the early thin disk by accretion of a small satellite galaxy

33. Chemical element abundance ratios in small galaxies Abundance ratios reflect different star formation histories LMC Pompeia, Hill et al. 2008 Sgr Sbordone et al. 2007 FornaxLetarte PhD 2007 Sculptor Hill et al. 2008 in prep + Geisler et al. 2005 CarinaKoch et al. 2008 + Shetrone et al. 2003 Milky-Way Venn et al. 2004 Venn 2008

34. We can use the chemical element abundance patterns to probe the formation of the Galactic thick disk. Small galaxies have distinctive and different abundance patterns: if the thick disk was built up partly by accretion of small galaxies, we will be able to recognise the imprint of these accreted small galaxies in the abundance patterns of the thick disk. This is called chemical tagging. It needs a huge number of stellar spectra. This kind of data does not exist yet: it is one of the goals for the HERMES survey. With the new HERMES instrument on the AAT, we will measure the chemical abundances of many elements for a million stars, mostly in the thin and thick disks

35. High resolution spectroscopy redder bluer High resolution spectrum 17A window on the solar spectrum revealing lines of Fe, Cr, Ti, V, Co, Mg, Mn, Nd, Cu, Ce, Sc, Gd, Zr, Dy

36. HERMES isa new high resolution multi-object spectrometer on the AAT spectral resolution 28,000 400 optical fibres over  square degrees 4 VPH gratings ~ 1000 Å First light ~2012 on AAT Will get spectra for a million stars The four wavelength bands are chosen to detect lines of elements needed for chemical tagging

37. The central regions of our Galaxy are dominated by the bar/bulge The boxy appearance of the bulge is typical of galactic bars seen edge-on. Where do these bars come from: they are very common ? About 2/3 of spiral galaxies show some kind of central bar structure in the infra-red.

38. The bars come naturally from instabilities of the disk. A rotating disk is often unstable to forming a flat bar structure at its center. This flat bar in turn is often unstable to vertical buckling which generates the boxy appearance. Shen, 2010

39. If this is right, then the stars in the bulge were once part of the inner disk We are doing a survey of about 30,000 stars in the bulge and the adjacent disk, to measure the chemical properties of stars (Fe, Mg, Ca, Ti, Al) in the bulge and adjacent disk: are they similar, as we would expect if the bar/bulge grew out of the disk ? We use the AAOmega fiber spectrometer on the AAT, to acquire medium-resolution spectra of about 350 stars at a time.

40. Melissa Ness

41. The Galactic bulge: mass = 2.1010 M The central black hole: mass = 4.106 M At the center of the bulge lies a supermassive black hole. Central BHs are common in galaxies: their mass is usually a small fraction of the bulge mass.

42. The black hole itself is invisible, but we can measure its gravitational effect on nearby stars. Turbulence in the earth’s atmosphere blurs out any details smaller than about 0.5 arcsec (0.25 mm at 100 m). To see stars close enough to the black hole (0.1 arcsec), adaptive optics are needed to correct for the effects of the atmospheric turbulence. AO image (0.060 arcsec) natural seeing image (0.26 arcsec) Gemini Observatory

43. The Galactic black hole is located at the The nearby young stars are moving very fast in the intense gravitational field of the black hole Star S0-2 has made a complete orbit in 15 years ! (0".1 ~ 1000 AU) A. Ghez, UCLA

44. Black holes in other galaxies Mass of the black hole • Mass of the BH • brightness of the bulge, but with fairly large scatter Brightness of the bulge Gültekin et al 2009

45. Central black holes in many galaxies, including the MW • Mass of the BH •  4 with smaller scatter Mass of the black hole The formation of the black hole and the properties of the surrounding bulge are closely related: maybe they formed together ? MW Random velocity of stars in the bulge, away from the BH

47. The End