Galaxy Characteristics Surface Brightness Alternative to Luminosity I(R) = Flux/area = erg/s/cm 2 /arcsec 2 I(0) – center flux I(R) = at radius R Define R where I=25, R 25 Often R 25 defines edge “I” depends on wavelength

Ellipticals Widest range of characteristics –Size, Brightness L E >25 billion L  – L E <few billion L  Surface Brightness goes as Re = effective radius (1/2 of light) For n>1, b=1.999n n=4, de Vaucouleurs Law, R 1/4 law – large E

Core brightness - radius/overall luminosity Large Luminosity Ellipticals faint core large core radius Shape Large Ellipticals – more spherical (E0) Motion of stars – very random Virial Theorem – relation of kinetic energy to potential (gravitational) energy of a system

Virial Theorem Mass distribution related to density, velocity Can also relate surface brightness, gives a 1/R dependence (I = 1/R) Systems that follow this are “relaxed” Relaxed systems seen in some spiral galaxy bulges, globular clusters Non-relaxed systems found in other spiral galaxy bulges, ellipticals – indicates they have yet to “settle down”

Faber-Jackson Relation Velocity dispersion =  (km/s) Related to Elliptical galaxy luminosity L v ≈2 x L  (  /200 km/s) 4 Why? Useful for getting distances to Ellipticals

Other Characteristics Stellar population –Cool star –Brightest = KM Supergiants Colors –Depends on luminosity/size/composition Brighter/larger = redder & metal rich Fainter/smaller = bluer & metal poor Gas - X-ray Mass – way big – Dark Matter!

X-ray Visible

Elliptical Characteristics cDEdEdSph MBMB -22 to to to to -15 Mass (M  ) D 25, kpc > ~

Spirals Observed features depend on wavelength –Bright stars, spiral arms – visible, UV –Faint stars, dust – IR –Gas – IR, radio Which wavelength makes the galaxy brighter? How does the surface brightness vary?

Surface Brightness Relation – h R = scale height (typically 1-10 kpc) Also have variation of brightness with h z typically h z = 0.1 h R Spirals with small I(R) have lots of H I – why?

Gas in Spirals H I dominant in disk Motion of disk – motion of H I Spider Diagrams – show velocity of disk motion –What should that look like?

H I visible beyond optical edge Molecular gas – confined to inner galaxy Sc, Sd, Sm galaxies – lots of H I S0 –Little gas –Some exceptions – ring-like structures Motion of gas – mass of galaxy –Tilt of galaxy –Velocity varies along disk

Goodie – another formula! V sys = velocity of galaxy through space V(R) = velocity at radius R (rotation curve) i = tilt from perpindicular (i=0 face on)  = angle from motion towards/away This can be solve for the rotation curve

What can velocities tell us? –Amount of velocity doesn’t correspond to what we see! –More mass is needed to produce velocities! –Dark Matter (in halo)! Tully Fisher Relation L  v max  v max = maximum rotation velocity  ≈ 4  depends on wavelength

Spiral Structure Use spiral arm tracers to map, measure –CO, H II, blue stars Why do spirals exist? –Density wave theory –Stochastic theory –?

Bars –Move at own rate, solid body rotation –Inward/outward motion? –S0, Sa bars – stars –Sb, Sc, Sm – stars, gas, dust Bulges –Elliptical (simple to triaxial) –Peanut shaped –Metallicity – metal poor –I(R) like Elliptical galaxies Black holes, Rings, other messes…

Irregulars No consistent characteristics (what did you expect?) Often lots of gas and dust Inconsistent star formation histories Like spirals in brightness (sort of), but fainter Some with bars

Spiral Characteristics SaSbSc MBMB -17 to to -22 Mass (M  ) D25, kpc %8%16%

Spiral/Irr Characteristics Sd/SmIm/Ir MBMB -15 to to -18 Mass (M  ) D25, kpc ~1 25%5-9%