Phil James

1. 22nd August 2007 STFC PhD Summer School, Durham Phil James The Structure of Galaxies Liverpool John Moores University Astrophysics Research Institute

2. Talk overview • Look at the diversity of galaxy Structures seen in the local Universe • Link structural properties with Content of galaxies (gas, stars, dust, dark matter, black holes) and Processes connecting these • Identify open questions; galaxies are far from being fully understood

3. I’m not going to mention this… Diagram courtesy Space Telescope Science Institute

4. …but we need to understand this Baldry et al. 2004 66846 SDSS galaxies 0.004<z<0.080

5. Red and blue sequence galaxies in the Virgo Cluster Image: CFHT

6. STRUCTURES: Field galaxies Image: A. Block

7. Structures: Disks NGC 2683 Image: D. Matthews & A. Block

8. Structures: Disks and Bulges NGC 4565 Image: Hugo, Gaul & Black (KPNO)

9. Structures: Disks and Bulges M 104 Image: HST

10. Structures: Bars

11. STRUCTURES: Elliptical galaxy

12. Galaxy Contents • Gas (atomic and molecular) • Stars • Dust • Black holes • Dark matter

13. Gas in galaxies • This is a ‘dissipative’ component – if 2 gas clouds collide, they can shock and radiate energy, so collisions are highly inelastic • If there is any initial angular momentum, this naturally leads to the formation of a disk:

14. Superthin galaxies These galaxies have littleor no bulge: pure disk systems.

15. Gas in galaxies • This is a ‘dissipative’ component – if 2 gas clouds collide, they can shock and radiate energy, so collisions are highly inelastic • If there is any initial angular momentum, this naturally leads to the formation of a disk • The gaseous disk then forms stars, once gas density is sufficiently large

16. Measuring star formation • Hα from gas ionized by hot young stars:

17. Red light spectrum of a galaxy

23. Measuring star formation • Hα from gas ionized by hot young stars • Mid/Far-infrared emission from hot dust around star formation regions (IRAS, Spitzer)

24. Dust in galaxies NGC 1410 Image Bill Keel

25. Spitzer IR Space Telescope

26. M81 observed by Spitzer

27. Measuring star formation • Hα from gas ionized by hot young stars • Mid/Far-infrared emission from hot dust around star formation regions (IRAS, Spitzer) • UV emission from young stars (GALEX) • Radio emission from star formation regions, or from supernova remnants

28. Star Formation (‘Schmidt-Kennicutt’) Law –SFR α (Gas density)1.4 Kennicutt 1998 ←Starburst nuclei ←Normal disks

29. Conversion of gas to stars • Star formation law works well, with wide applicability (normal galaxies and starbursts) • It is largely empirical, however – no physical basis for power law index • Does it apply to star formation in densest regions (globular clusters and nuclear clusters) or is there another mode of star formation for these?

30. Globular Cluster M3 Image K Teuwen

31. T. Böker et al. 2002 HST images of compact nuclear clusters

32. Some personal opinions (not all would agree…) • Gravitational collapse of gas clouds naturally leads to disks in undisturbed systems • Such disks will always start forming stars when a critical density is reached (note that there are ~no gas-rich, quiescent galaxies) • This star formation is continuous, at a broadly constant rate, in the absence of outside influences, and as long as the gas supply holds up

33. Star formation timescaleR-luminosity dependent extinction correction Bulge-dominated

34. Star formation timescaleR-luminosity dependent extinction correction Bulge + disk

35. Star formation timescaleR-luminosity dependent extinction correction Bulge-free

36. UGC 8508, Im UGC 9240, Im

37. Sm Im Mean R profile Mean Hα profile Difference, Hα-R

38. But many galaxies are not disks… • Q: Where do elliptical galaxies and spiral galaxy bulges come from? • A: This seems to require the presence of stars (a non-dissipative component, unlike the gas), and something to stir them up • Internal processes (bars, spiral arms) seem too weak – large bulges and elliptical galaxies probably need outside interference:

39. Simulation: J. Dubinski, U. Toronto

40. Some real interactions and mergers Atlas of peculiar galaxies, H. Arp

41. The Antennae, NGC 4038/4039. Colour Image: HST, B. Whitmore & F. Schweizer

42. Tadpole galaxy, Image:HST

43. Galaxy mergers – results from simulations • Colliding disc galaxies form long tidal tails and arms • After a close approach, they are likely to spiral together and merge • Gas becomes centrally concentrated, → nuclear starburst • Merger remnant density profiles resemble elliptical galaxies or bulges • Characteristic ‘relaxation’ timescales quite short – few x 108 years • Summary: undisturbed galaxies stay as thin discs, collisions make bulges or ellipticals

44. Under currently-favoured hierarchical cosmologies, mergers are common – most bright galaxies will have experienced at least one merger since their formation. ‘Minor mergers’ with dwarf galaxies may just build bulges or thicken disks; ‘major mergers’ of two large galaxies can make disks directly into ellipticals.

45. Bulge star-formation histories • Colours, population synthesis analyses show that most bulges are dominated by old stars, ~10 Gyr old • Bulges and ellipticals have little cold gas • Full understanding of this involvesfeedback processes • Feedback can come from stars (stellar winds and supernovae):

46. Galactic superwind in starburst M82

47. Bulges and feedback processes (contd.) The last decade has shown that bulges are closely linked to even more energetic phenomena than starbursts…

48. M31 Image: R. Gendler

49. Kormendy 1988a