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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 b R / R I ( R) I ( Re )e 1/ n e 1 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 GE 2 KE 0 Mass distribution related to density, velocity GMm 1 2 2 mv 0 r 2 v v Can also relate surface brightness, gives a 1/R dependence (I = 1/R) v2 R M G v2 G (4r 2 ) 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! Visible X-ray Elliptical Characteristics cD MB E -22 to -25 -15 to -23 1013 108 - 1013 Mass 14 10 (M) D25, kpc 300-1000 1-200 <M/L > >100 10-100 dE dSph -13 to -19 -8 to -15 107 - 109 107 - 108 1-10 0.1-0.5 ~10 5-100 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 – I ( R) I (0)e R / hR • hR = scale height (typically 1-10 kpc) • Also have variation of brightness with hz I ( R, z) I ( R)e z / 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! Vr ( R, i ) Vsys V ( R) sin i cos f 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 vmax a 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 Spiral Characteristics MB Mass (M) D25, kpc <M /L> <Mgas/Mtotal > Sa -17 to -23 109-1012 5-100 6.2 4% Sb -17 to -23 109-1012 5-100 4.5 8% Sc -16 to -22 109-1012 5-100 2.6 16% Spiral/Irr Characteristics MB Mass (M) D25, kpc <M /L> <Mgas/Mtotal> Sd/Sm -15 to -20 108-1010 0.5-50 ~1 25% Im/Ir -13 to -18 108-1010 0.5-50 ~1 5-9%