Astronomy and Astrophysics – Astrophysics
Scientific paper
1995-03-28
Mon.Not.Roy.Astron.Soc. 275 (1995) L19
Astronomy and Astrophysics
Astrophysics
MNRAS submitted, 6 pages, UUencoded compressed Postscript file, including figures. Postscript also available from http://www
Scientific paper
The excess numbers of blue galaxies at faint magnitudes are a long-standing cosmological puzzle. We present new number-magnitude counts as a function of galactic morphology from the first deep fields of the Cycle 4 Hubble Space Telescope {\it Medium Deep Survey} project. From a sample of 301 galaxies we define counts for elliptical, spiral and irregular/peculiar galaxies to $I=22$. We find 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 $z\sim 0.5$. Secondly we find 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 identified.
Ellis Richard
Glazebrook Karl
Griffiths Richard
Santiago Basilio
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