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Galaxies II

Galaxies II. AST 112. Galaxies. Billions of them! Islands of millions or billions of stars All different shapes and sizes. Hubble Deep Field. 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

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Galaxies II

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  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 Edge-On Face-On Tilted

  12. Spiral Galaxies • Sizes of disk and bulge vary from spiral to spiral M81: Larger bulge M 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 Leo I M87

  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

  38. 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

  39. 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

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

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