Classification of Galaxies

• Tuning fork: Zeilik Fig.21-1 -- Not an evolutionary diagram.
• Elliptical galaxies: Designation En where n = 10 (a - b)/a -- e.g., E0, E5
• Comparison of M32 (companion of M31Andromeda Galaxy) and M87.
• Using globular clusters to find distance to M87
• cD galaxies -- super-ellipticals with multiple nuclei (cannibalism?)
• Spiral galaxies -- dust is key
• Ordinary spiral (AAO)
• Barred spiral (AAO) -- why a bar?  Simulations show bar forms with less dark matter (halo)
• M74: An Sc galaxy in the visible and UV(UO)
• Spiral variations
• Edge-on spiral
• Radio view of M81 and size of galaxy
• Oddball galaxies
• Dwarf irregular (AAO) largest population by number
• M82(UO)
• M51(UO)
• M51 (NJIT/Rutgers)
• Cartwheel(HST)
• Centaurus A (AAO)

• Zeilik Table 21-1 -- Mass Estimates from Luminosity (visible matter)

• The morphological mix
• Observed (brightness limited):       77% spirals, 20% ellipticals,   3% irregulars
• Space-limited sample (to 9.1 Mpc): 33% spirals, 13% ellipticals, 54% irregulars
• Galaxy colors/spectral types
• Depends on morphology
• Dominated by relatively few giant stars
• Elliptical and early spiral -- K star spectra
• Late spirals -- G or F star spectra, with K spectra in central bulge
• Invisible radiations
• Radio -- H I (21 cm line), H II (recombination lines), CO (mm lines) -- for Sc spirals and Irr I galaxies, the H I is much more extended than the stars -- almost double in radius.
• IR -- measures amount of dust in galaxy
• X-rays from M31 (ROSAT)
• Galactic rotation curves -- mass estimates from dynamics (dark matter)
• Rotation Curves
• Velocity Dispersion  <v²> = 0.4 GM / r _1/2 where r _1/2 is the radius enclosing half the mass

A series of rotation curves for spiral galaxies. (Figure from Rubin, Ford, and Thonnard (1978), Ap. J. Lett., 225, L107.)

Theoretical Considerations

• Implications of Classification Scheme
• Brightness of bulge/nucleus different from disks (r ^{- 1/4} law for bulge, slower e ^{- ar} for disk)
• Halo part of spiral behaves like an elliptical

The surface brightness profile of the giant elliptical galaxy NGC 3379 is well represented by an r1/4 law.  (Figure from de Vaucouleurs adn Capaccioli 1979, Ap J Suppl, 40, 699.)




• Degree of tightness of spiral arms correlates with rate of star formation (more formation--less tight spiral)
• Either star formation affects dynamical evolution of galaxies or something else (dark matter?) affects both.
• Amount of dark matter plays an important role in distinguishing barred and unbarred spirals.
• Mergers and Interactions
• More merger animation
• What is dark matter?
• Dust?  No--dust is visible in IR
• Brown dwarfs?  HW problem will investigate that...
• Black holes?  Not enough X-ray binaries.
• Later, will see that dark matter problem only gets worse when we consider clusters of galaxies.
• Dark matter is central problem in comprehending the universe.
• Hubble Deep Field -- Lots of galaxies (HST)