Introduction

• 1991 Walker et al used observational evidence to derive bounds on the present day density of the universe.
  Used:
  • observed abundances of the light elements
  • nuclear reaction rates
  • number of neutrino species
  • the standard cosmological model

  Obtained:

  • Present nucleon to photon ratio of
    2.8 < (n /n) x 10 < 4.0
  • Density of Baryons in the universe
    0.010< h100 < 0.015
    ( = baryons/ c, with c = 3H0/8 G)

• Persic & Salucci (1992, 1996) estimated the baryon mass density of the universe found in groups and clusters of galaxies. Found:
  
  0.0028 < < 0.0037
  

• Over 90% of the predicted baryonic mass appears to be "missing"!

• Catalog used by Persic & Salucci composed of objects bright and large enough to be detected through the noise of the night sky

• 1935 Edwin Hubble suggested that the sky brightness limits the objects we can view to a narrow band of apparent brightness (magnitude)
  Sky brightness from:
  • Sunlight
  • Moonlight
  • Terrestrial light
  • Light from stellar and extragalactic objects

• 1965 Arp attempted to quantify the effect of sky brightness:
  
  
• A good way to quantify Arp's visibility band is through the use of surface brightness

  • Surface brightness is literally the measure of the galaxy's brightness per surface area

  • Stellar brightness is traditionally defined in terms of magnitude, where
    mag = -2.5log(flux) + constant

  • Surface brightness is therefore
    µ = -2.5log(flux/area) + constant, or µ = mag - 5log(D)

  • Surface brightness is used to measure how diffuse a galaxy is.
    

  • De Vaucouleurs has shown empirically that a galaxy's surface brightness profile can be expressed as:
    

  • The brightness and size of a galaxy thus be fully described using its central surface brightness and scale length.

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

     
    Figure: From top left: M51, UGC 12522, C9-1, and P2-4
    

  • Those galaxies falling outside of Arp's band of visibility are characterized by either a low central surface brightness or a small scale length (in arcsec).
  
  

  
  
  

  Low Surface Brightness Galaxies - A Brief History

  • 1970 Freeman examined 36 galaxies (type S0-Im) and discovered all the galaxies had
    µ(0) = 21.65 +/- 0.30 mag arcsec

  • 1964 Fish examined 29 elliptical galaxies found results equivilent to
    µ(0) = 14.8 +/- 0.9 mag arcsec

  • Appears galaxies are limited in the central surface brightness they can attain
    • No known physical basis for this limitation

  • 1976 Disney suggested Fish and Freeman's results were due to selection effects.
    • Using theoretical arguments, Disney obtained
      

    • Using SL = 24.0 mag arcsec, he obtained:
      

    • Using selection effects, Disney came darn close to matching the observed central surface brightness values of galaxies.
    
    

  • Surveys were then undertaken to find the galaxies previously missed because of selection effects.
    • Some were successful
    • And some weren't

  • Mid-1980s Impey, et al surveyed the Virgo cluster of galaxies specifically looking for galaxies with µ(0) > 23.0 mag arcsec

    • Found 137 LSB galaxies, 27 of which had never before been catalogued

    • Found an extremely large low surface brightness galaxy - Malin 1

      Very large, very faint disk - µ(0)=25.7 mag arcsec,
      

      r27 = 110 kpc

      
      Total HI mass of MHI = 10M

      
      Largest HI mass of any object known!

  • Numerous successful studies have been done since to find LSB galaxies

    Large increase in the total number of galaxies in the regions surveyed

    
    Discovery of more Malin 1 type objects.

  • As technology improves and more surveys for LSB galaxies are undertaken, the number of galaxies continues to rise.

  • Theoretical arguments for selection effects have also been reworked:

    McGaugh, et al ( 1995) and McGaugh (1996) determined a probability distribution for galaxy detection

    • Detecting a galaxy depends on

      the galaxy's central surface brightness
      distance
      and apparent size

    • Showed the distribution of galaxies remains flat or may even rise as fainter surface brightnesses are reached:

  • Clearly previous galaxy surveys are lacking a significant portition of the total galaxies in the universe.

• Impey & Bothun (1997) rederived the baryonic density in the universe using a flat number density profile through µ(0) = 30.0 mag arcsec.

• The contribution to the total baryonic matter in the universe by disc galaxies rose by a factor of 17 over Persic & Salucci's value.

• Impey et al obtained 0.014< h100 <0.025

• Recall the number density obtained by Walker, et al (1991) was
  0.010< h100 < 0.015

• Impey & Bothun's values are well within the bounds set by Walker, et al.

• It is thus likely that all of the " missing " baryonic matter described by Persic & Salucci is in the form of faint and diffuse galaxies.