Simulated observations of bursting dwarf galaxies




Tyson, Neil deGrasse

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Current data on the colors and metallicities of dwarf irregular galaxies suggest that they undergo strong, short-lived bursts of star formation followed by long periods of quiescent behavior. During this quiescent phase the galaxies' surface brightness may fall well below the detection limit set by the light of the night sky. The effect of these unique objects on the faint end of the galactic luminosity function and their contribution to the mass density of the Universe are discussed in the context of a specific theoretical model for their bursting behavior. A theoretical catalogue of over 10⁵ dwarf galaxies was generated according to adopted relations between radius, mass, absolute magnitude and the dependence of burst properties on galactic radius. These relations are based on the models of Gerola et al. (1980). The frequency distribution of galaxy radii was left as a free parameter. An "observers" catalogue was drawn from the distribution, with careful attention paid to all selection effects normally encountered by a real observer. An actual data set was used to ultimately constrain the slope of the radius distribution and its normalization. The present work demonstrates that the observed apparent frequency distributions of luminosity, radius, and angular size can be matched well by the models if the true distribution of radii is a power law with slope -4.2. The calculatons also show that 1) a truly increasing galaxy luminosity function can be observed to masquerade as a distribution that has a maximum at the published values of M[subscript B] = -16.0 ± .75, 2) ten times the observed dwarf galaxy mass in a given volume of space is hidden below the detection limits of the night sky in the form of low surface brightness, small angular diameter dwarfs and 3) the true number density of dwarf galaxies may be as high as 1500/Mpc³ though their contribution to the overall mass density of the Universe remains insignificant with [Omega]dwarf [is approximately equal to] .004. The derived frequency distributions of radius, mass, surface brightness, and angular diameter is presented and a dwarf galaxy "discovery" function is introduced.




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