Galaxy 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 I25, R25
• Often R25 defines edge
• I depends on wavelength

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Ellipticals

• Widest range of characteristics
• Size, Brightness
• LEgt25 billion L? LEltfew billion L ?
• Surface Brightness goes as
• Re effective radius (1/2 of light)
• For ngt1, b1.999n-0.327
• n4, de Vaucouleurs Law, R1/4 law large E

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• 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

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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
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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

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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!

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X-ray
Visible
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Elliptical Characteristics
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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?

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• 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?

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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?

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• 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

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• Goodie another formula!
• Vsys velocity of galaxy through space
• V(R) velocity at radius R (rotation curve)
• i tilt from perpindicular (i0 face on)
• f angle from motion towards/away
• This can be solve for the
• rotation curve

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• What can velocities tell us?
• Amount of velocity doesnt 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

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Spiral Structure

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

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• 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

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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

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Spiral Characteristics
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Spiral/Irr Characteristics