LSB Galaxies - the uproar

• 1938 Shapely announced finding two dwarf galaxies in the Sculptor cluster. One of the galaxies was a low surface brightness dwarf galaxy.

  
  

• LSB galaxies were usually considered anomalies, and about 30 years ago two papers were published supporting that belief:

  • 1964 Fish examined 29 elliptical galxies and concluded:
    ( is the potential energy of a galaxy)

    Which, combined with the assumption that mass is proportional to luminosity, gives:

    (Disney, 1976).

  • 1970, Freeman used deVanuouleur's Reference Catalog(1964) and found that all 20 of the spiral galaxies he examined have:

    Freeman's result was soon reconfirmed by others.

  
  

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• Some people, however, believed Freeman's law was due to selection effects.

• The argument for selection effects has also been around for years:

  1936 Hubble suggested that sky brightness would limit our seeing to a narrow band of magnitudes.

  1965 Arp attempted to quantify the effect by showing that galaxies plotted on an absolute magnitude vs. log(diameter) plot were restricted to a narrow band.

• 1976 Disney published an article using selection effects to obtain the results:

  for spiral and elliptical galaxies (respectively), darn close to Fish and Freeman's values ( = 24.0 B mag/ ").
  (See graph.)
  

• Numerous studies were then done which seemed to support Freeman's theory.

  Simultaneously, those studies (spec. Phillipps, et.al. 1987) and others were found to support Disney's argument.

  Each new data set gave fuel one or both sides of the argument.

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1987 til now (Malin I and friends)

• 1987 Impey, et.al. published a catalog of 137 lsb galaxies in the Virgo cluster.

  • The search was done using a new technique - photographic amplification

  • They discovered 27 new galaxies with unusually lsb and high angular size.

  • Provided much credence to the selection effects argument.

  
  

• During their survey, an extremely large low surface brightness galaxy was discovered - Malin I (Bothun et.al. 1987).

  • Malin I consists of a large, very faint disk with a prominant bulge. Both the huge disk size and the prominant bulge were unprecedented.

  • Confirms that catalogs choose objects of a luminosity and surface brightness that maximize the angular size of the discovered objects.

  • Shows the luminosity function needs modifying at low luminosities.

  • Led to speculation about existence of other massive lsb galaxies and the missing baryons question.

• Numerous successful studies were then done to find low surface brightness galaxies.

• Eventually, the existence of numerous LSB galaxies became accepted by (most) everyone, and surveys were undertaken to determine the properties of LSB galaxies and to find the true surface brightness distribution.

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Understanding selection effects

• Disney's 1976 paper was reargued twice -

  
  

  • Disney and Phillipps (1983) (DP) presented a (flawed) visability argument based on sky brightness and angular resolution (see graph).

    
    

  • Davies (1990) furthered their arguments adding in factors for the galaxies' inclination and bulge-to-disk ratios.
  
  
  

• DP's basis was sound and their argument can be restated (i.e. McGaugh, et.al. 1995), giving these results for various values of h.

  
  
  

• 1993, Phillips et.al. confirmed the predictions that the surface brightness distribution of observed galaxies narrows with distance.

  
  
  
  
  
  
  
  

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• Knowledge of the selection effects can then be used to normalize the number counts of galaxies and obtain an accurate luminosity function:

  • Choosing a normalizing surface brightness and scale length ((0)*, h*),

  • For galaxies with (0) 23.1, a luminosity-limited volume is appropriate:

  • For galaxies with (0) 23.1, a diameter-limited volume is appropriate:

• A nice normalized distribution ( McGaugh, et.al. 1995).

references