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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
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Galaxy Characteristics • Surface Brightness Alternative to Luminosity I(R) = Flux/area = erg/s/cm2/arcsec2 I(0) – center flux I(R) = at radius R • Define R where I=25, R25 • Often R25 defines edge • “I” depends on wavelength
Ellipticals • Widest range of characteristics • Size, Brightness • LE>25 billion L – LE<few billion L • Surface Brightness goes as Re = effective radius (1/2 of light) For n>1, b=1.999n-0.327 n=4, de Vaucouleurs Law, R1/4 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 = s (km/s) Related to Elliptical galaxy luminosity Lv≈2 x 1010 L (s/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
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 – • hR = scale height (typically 1-10 kpc) • Also have variation of brightness with hz typically hz = 0.1 hR • 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! Vsys = velocity of galaxy through space V(R) = velocity at radius R (rotation curve) i = tilt from perpindicular (i=0 face on) f = 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 vmaxa vmax = maximum rotation velocity a≈ 4 a 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
