1. Galaxies PHYS390 Astrophysics Professor Lee Carkner Lecture 20

2. 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   Spirals too distant to be in our Milky Way

3. Elliptical Galaxies  Classified by apparent ellipticity  Class = 10E where E = 1 –    Dwarf ellipticals  M B ~  Mass =  Size ~ 300 pc   Giant ellipticals  M B ~  Mass ~ 10 13 M sun  Size ~  Largest things in universe

4. Spiral Galaxies  Less variation in size, but more variation in structure compared to ellipticals  M B ~  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   sky ~ 22 B-mag/arcsec 2  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 (  (r))  (r) =  e + 8.3268[(r/r e ) 1/4 -1]   radius within which ½ of the light is emitted   e is the surface brightness at r e

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: M B = -9.95logV max +3.15  Sb: M B = -10.2logV max +2.71  Sc: M B = -11.0logV max +3.31  Can measure V max from Doppler shifts and use to get M B 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  0 4  where  0 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