Who Cares?

• Missing mass

  • BB nucleosynthesis prediction: 0.021 0.014
    (=85 km/s/Mpc) (Walker et.al 1991)

  • Galactic dynamics prediction: 0.1
    (includes dark matter) (Kolb & Turner 1990).

  • HSB galaxy estimation: 0.003
    (Persic & Salucci 1992).

  • LSB and HSB estimation: 0.014 0.025
    (Impey et.al 1995)

  
  

• Galactic stucture, evolution
  • Stellar distribution, density

  • Angular velocity, momentum, luminosity relations (i.e. Fish 1964, Disney 1976).

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What is surface brightness?

• Literally, its a measure of how bright the surface of a galaxy is.

  
  

• Way back when (ancient greece) stellar brightness was broken down in to magnitudes, with the brightest stars at the lowest magnitudes.

  
  
  

• Since then its been discovered that the eye's response to light stimulus is logarithmic. That is:

  where the constant is determined (through experiment) by the filter used.

  
  
  

• In keeping with the historical logarithmic scale, then the surface brightness of an object is defined to be:

            	 
  			   or  
  		      

  (D is the diameter in arcsec, is in mag/arcsec )

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What are low surface brightness galaxies?

• Typically defined as galaxies with central surface brightness fainter than 23.0 mag/ "

• Morphology:

  LSB galaxies have a range of types as large as the Hubble sequence, though they are dominated by late-type (often dwarf) galaxies (McGaugh, et.al 1995).

  Bulge component of LSB disk galaxies are faint or totally undetectable, except in a significant subset (i.e. Malin I & II) with very bright bulges (McGaugh, et.al. 1995).

  The low sky contrast, 'clumpiness', of LSB galaxies makes classification in the Hubble scheme difficult.

  A few examples.

• Stellar composition, color:

  Most LSB galaxies are rich in H I (Schombert, et.al. 1992), though their surface density of H I is at or below the threshold criteria for star formation (Bothun, et.al., 1993, Kennicutt, 1989).

  Blue colors, lack of metal eliminate the idea that LSB galaxies are faded remnents of HSB galaxies. (i.e. McGaugh 1994)

  H II regions indicate the same IMF as HSB galaxies (McGaugh & Bothun, 1994, Schombert, et.al. 1992).

  
  
  
  

• Environment

  LSB galaxies appear to follow the same large scale (>5 Mpc) structure as HSB galaxies (Bothun, et.al. 1993).

  On small scales (<2 Mpc), though, LSB galaxies avoid high density environments (Bothun, et.al. 1993).

  LSB galaxies usually (but not always) lack companion galaxies and thus the tidal interactions necessary for star formation (Bothun et.al., 1993).

  
  

• The theories - LSB galaxies may:

  form stars at a constant rate after a late collapse(McGaugh & Bothun, 1994), or

  have a SFR which is gradually increasing with time (McGaugh & Bothun, 1994), or

  have a sporadic SFR (the best bet so far) (Block, et.al. 1995).

references