Virgo dwarfs - New light on faint galaxies

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Abstract

A sample of 137 low surface brightness galaxies in the Virgo Cluster has been studied, with initial selection on photographically amplified UK Schmidt plates. The limiting isophote for galaxy detection is μ_B_ ~ 27 mag arcsec^-2^. There is substantial overlap with existing catalogs of Virgo dwarf galaxies, but the new galaxies have unusually low surface brightness and large angular size. The first of the new galaxies to have a redshift turns out to be the largest, most gas-rich spiral known, located far beyond the Virgo Cluster. However, most appear to be dwarf elliptical galaxies. H I observations show the low surface brightness galaxies to be gas-poor, with M_HI_ < 3-5 x 10^6^ M_sun_. CCD images are used to derive surface photometry for 26 objects, and the radial profiles are generally well fitted by an exponential. The colors are unusually blue for ellipticals, implying low metallicity and relatively youthful ages. The star formation history that can lead to galaxies with such blue stars and such low surface mass densities is unknown. The relationships between the structural parameters of dwarf galaxies are discussed, with an emphasis on the selection effects imposed by the brightness of the night sky. The relationship between surface brightness and luminosity breaks down at M_B_ ~-16, and the range of central surface brightness for a given luminosity can be over 5 mag. Our understanding of the intrinsic distribution of dwarf structural parameters is heavily obscured by selection effects. Any survey tends to choose galaxies of a luminosity and surface brightness that give the maximum angular size on the discovery plate. There may be large populations of galaxies with extremely low surface brightness. Low-luminosity galaxies do not tend to have the bright nuclei that aid their discovery and a number of low-luminosity galaxies have flat cores. The effect of this uncertainty on the luminosity function of dwarf ellipticals (and similarly, all faint galaxies) is dramatic. In particular, a divergent exponential tail to the galaxy luminosity function cannot be ruled out.

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