1. Galaxies II AST 112

2. Galaxies Billions of them! Islands of millions or billions of stars All different shapes and sizes

3. Hubble Deep Field

4. Estimate: Galaxy Count Hubble Deep Field is some % of the total sky – Smaller than a 1mm x 1mm piece of paper held at arm’s length Count the galaxies and multiply by the ratio! ~130 billion galaxies in observable universe

5. Star Formation What exactly is required for star formation?

6. Star Formation Cold gas is required for star formation – Sometimes the gas “gets some help” and gets compressed by means other than gravity

7. Elliptical Galaxies Older (yellow and red) stars Not much structure Not much cold gas or dust

8. Spiral Galaxies Flat disks with arms, yellow bulges at center Disk and arms tend to be more blue

9. Lenticular Galaxies Like spirals: contain a disk and a bulge Do not contain spiral arms Not much star formation “Intermediate” between spirals and ellipticals

10. Irregular Galaxies No disk, not round Chaotic, “messy” structure

11. Spiral Galaxies Face-OnTilted Edge-On

12. Spiral Galaxies Sizes of disk and bulge vary from spiral to spiral M81: Larger bulgeM 100: Smaller bulge

13. Spiral Galaxies Some spirals’ arms are wound more tightly than others

14. Spiral Galaxies Many spiral galaxies have a bar Milky Way is a barred spiral

15. Spiral Galaxies One can observe a galaxy in different types of light It “picks out” elements of structure, some of which cannot be seen in visible light

16. Andromeda In Far-Infrared Dust glows in far- infrared Where is the dust confined to?

17. Andromeda In Visible Light We see starlight when we look in visible light What color does the disk show more strongly than the bulge?

18. Andromeda in Ultraviolet Light Bright newborn stars glow strongly in UV UV is a good map of star formation Where is star formation happening?

19. Triangulum Galaxy In Far-Infrared Dust glows in far- infrared Where is the dust confined to?

20. Triangulum Galaxy In Visible Light We see starlight when we look in visible light What color does the disk show more strongly than the bulge?

21. Triangulum Galaxy in Ultraviolet Light Bright newborn stars glow strongly in UV UV is a good map of star formation Where is star formation happening?

22. Spirals: Star Formation (Observations) Where’s the dust? – The disk or the bulge? Where are the younger (bluer) stars? – The disk or the bulge? Where’s the star formation? – The disk or the bulge? Where’s the cold gas? – The disk or the bulge?

23. Spirals: Star Formation (Observations) Red HII regions and blue open clusters reveal star formation Where exactly do we find these elements in this galaxy?

24. Spirals: Star Formation Thus far, we can conclude that: – The disk is full of gas and dust – The arms are full of star formation Why is there enhanced star formation?

25. Lin-Shu Density Waves You might think that a spiral galaxy’s shape is a fixed structure – If true, outer stars must orbit in same amount of time as inner stars – It’s not. Doppler measurements don’t show this at all.

26. Lin-Shu Density Waves Kepler’s Laws: – A star should orbit more slowly the farther out it is – A galaxy would “wind itself up” and destroy its spiral structure well within their current ages – No good!

27. Lin-Shu Density Waves The spiral structure is stable. The spiral arms are simply locations in the disk of high density – Stars move into and out of the arms! – It’s just like a traffic jam. One star enters it just as another is exiting. When gas and dust slam into the “traffic jam” and slow down, they compress – That’s why spiral arms show heavy star formation

28. Lin-Shu Density Waves Animations: http://en.wikipedia.org/wiki/Density_wave_theory

29. Lin-Shu Density Waves Why is star formation enhanced in the spiral arms? – Gas, dust and stars accelerated toward the high- density arms, “crash into it” – High density and compression trigger star formation

30. Elliptical Galaxies Elliptical galaxies have little or no disk Have little or no cold gas / dust Resemble the bulge of a spiral Most common type of galaxy in the universe

31. Elliptical Galaxies Most ellipticals are small – Dwarf ellipticals – Usually hang around larger spirals But the largest galaxies in the universe are giant ellipticals – M87: 1 trillion stars M87 Leo I

32. Elliptical Galaxies Due to lack of cold gas / dust, star formation rates are very low for ellipticals Blue stars have died So ellipticals are yellow / red

33. Irregular Galaxies Don’t have much structure Young stars More common toward beginning of the Universe LMC SMC

34. Irregular Galaxies Irregular galaxies often appear so because of one or more collisions Significant starburst activity is often seen in irregulars – Why?

35. Classification of Galaxies Here are some galaxies. Try to come up with a classification scheme.

36. Classification of Galaxies We can classify elliptical galaxies by how elliptical they are We can classify spirals by: – Size of the bulge – Tightness of the arms – Barred or not

37. Hubble Sequence

39. Quasars Quasar stands for quasi-stellar object Galaxies look like smudges in a telescope There are objects that look just like stars – A point of light – Their spectra have emission lines that didn’t correspond to anything we know of

40. Quasars They have the same redshift as distant galaxies Large telescopes can reveal more than just a quasi-star “Unknown lines” are known but highly redshifted lines

41. Quasars Distant galaxies often contain an active galactic nucleus (AGN) This is an SMBH that is actively devouring material Forms HUGE accretion disk