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Galaxies

Galaxies. PHYS390 Astrophysics Professor Lee Carkner Lecture 20. The Great Debate. In the early 20 th century astronomers could see many strange “nebula” Were they in our galaxy or independent distant galaxies? In 1923 Hubble found the distance to M31 using Cepheids

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Galaxies

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  1. Galaxies PHYS390 Astrophysics Professor Lee Carkner Lecture 20

  2. The Great Debate • In the early 20th century astronomers could see many strange “nebula” • Were they in our galaxy or independent distant galaxies? • In 1923 Hubble found the distance to M31 using Cepheids • Spirals too distant to be in our Milky Way

  3. Elliptical Galaxies • Classified by apparent ellipticity • Class = 10E where E = 1 – b/a • Dwarf ellipticals • MB ~ • Mass = • Size ~ 300 pc • Giant ellipticals • MB ~ • Mass ~ 1013 Msun • Size ~ • Largest things in universe

  4. Spiral Galaxies • Less variation in size, but more variation in structure compared to ellipticals • MB ~ • Mass ~ • Size ~ 5 to 100 kpc • Irregular galaxies often look like distorted spirals and have a similar range of properties

  5. Hubble Type • Spirals • Sa: • Sb: medium bulge, looser arms • Sc: • If bar present, SBa, SBb, SBc • Ellipticals • Others • Irregular: amorphous with no obvious structure

  6. Observing Galaxies • Need to apply K correction to account for red shift of light out of observed band msky ~ 22 B-mag/arcsec2 • Galaxies are diffuse • Have a distribution of magnitude that needs to be parameterized • Surface brightness as a function of radius

  7. de Vaucouleurs Profile • Can write in terms of the surface brightness at some radius (m(r)) m(r) = me + 8.3268[(r/re)1/4 -1] • radius within which ½ of the light is emitted • me is the surface brightness at re

  8. Rotation Curves • Rotation curve give mass • Sa have higher maximum rotation velocities • Brighter galaxies have higher maximum rotation velocities

  9. Tully-Fisher Relation • Sa: MB = -9.95logVmax+3.15 • Sb: MB = -10.2logVmax+2.71 • Sc: MB = -11.0logVmax+3.31 • Can measure Vmax from Doppler shifts and use to get MB and thus distance

  10. Star Formation • Sc galaxies are bluer than Sb or Sa • 21-cm and H II emission increase from Sa to Sc • Since Sc have more young stars, dust and gas they must have more star formation • The more disk (relative to bulge) the more star formation

  11. Elliptical Classes • cD • Normal (E) • Dwarf (dE) • Very small (few kpc) • Dwarf spheroidal (dSph) • Blue compact dwarf (BCD) • small ellipticals with star formation

  12. Dust and Gas • dE and dSph have almost no gas and dust • Other ellipticals have some gas • About ½ have dust • Ellipticals have very little star formation • Gas and dust may have been acquired after galaxy formation

  13. Faber-Jackson • L proportional to s04 • where s0 is the central radial velocity dispersion • Some of this deviation may be due to the galaxy capturing outside material

  14. Luminosity Function • Can examine the relative number of galaxies of a certain brightness and Hubble type • Spirals dominate in mass and luminosity • Strongly dependant on environment

  15. Next Time • Read 26.2 • Homework: 25.1, 25.3, 26.1a, 26.3

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