Surface Brightness

• The measure of the galaxy's brightness/(surface area)
  mag = -2.5log(flux) + constant
  µ = -2.5log(flux/area) + constant
  

• Measures how diffuse a galaxy's light distribution is

• Empirically, the surface brightness of a disk galaxy follows its radius as
  µ(r) = µ(0) + 1.086(r/scale_length)
  (example)

• Two galaxies with the same magnitude but difference central surface brightness can have radically different appearances:
  
  
  

  
  

  The galaxies outside Arp's "band of visibility" have faint µ(0) or small scale length