The Morphological Identiication of the Rapidly Evolving Population of Faint Galaxies
نویسندگان
چکیده
The excess numbers of blue galaxies at faint magnitudes are a long-standing cosmo-logical puzzle. We present new number-magnitude counts as a function of galactic morphology from the rst deep elds of the Cycle 4 Hubble Space Telescope Medium Deep Survey project. From a sample of 301 galaxies we deene counts for elliptical, spiral and irregular/peculiar galaxies to I = 22. We nd two principal results. Firstly the elliptical and spiral galaxy counts both follow the predictions of high-normalisation no-evolution models at all magnitudes, indicating that regular Hubble types evolve only slowly to z 0:5. Secondly we nd that irregular/peculiar galaxies, including multiple-peaked, possibly merging, objects, have a very steep number-magnitude relation and greatly exceed predictions based on proportions in local surveys. These systems make up half the total counts by I = 22 and imply the rapidly-evolving component of the faint galaxy population has been identiied.
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