BACK TO THE BARYON PROBLEM

• We showed earlier that LSB galaxies numerically dominate the galaxies in the local universe.

  
  

• Need to decide LSB galaxies contribution to the local baryon density (i.e. the baryon mass of LSBGs)

  
  

• The baryonic contribution of a galaxy comes from its stars and gas

  
  

  M_gas = M_HI × 1.47

  M_stars = L_stars × [(M/L)_stars]

  
  

• If M/L is known for a galaxy, determining M_Total is easy
  

• For most spiral galaxies M/L is between 1 - 3
  (Measured, with not too many assumptions, from galaxy rotation curves)

• LSB galaxies, though, are weird

• We currently (as of 21 June, 2000) have two choices:
  
  
  1. Assume M/L for LSB galaxies is also between 1-3

     • Makes LSB galaxies extremely dark matter dominated

     • Keeps LSB galaxies off the baryonic Tully-Fisher relation

  2. Let M/L get as big as necessary to fix above problems

     • LSB galaxies have same dark matter content as HSB galaxies

     • Puts LSB galaxies back on the baryonic Tully-Fisher relation

     • Can result in absurd M/L values (> 10)