The following document lists the file abstract/WKEEL_53W002.abs
from catalogue VI/111.
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We have found a cluster of galaxies at redshift z=2.4, identified via Lyman alpha emission in narrow-band HST images. This cluster offers a special opportunity to determine the time of first widespread star formation in "normal" galaxies, since most of the members will not be contaminated by significant AGN emission as judged from the optical sizes and colors. We propose ISOCAM mapping of the cluster in the LW2 band, which corresponds to emitted K-band radiation. The spectral energy distributions from 0.4-8 microns will provide sensitive limits or measurements of the age of the current bursts of star formation, and detection of any substantial pre-existing stellar population. Because we are already seeing this cluster at a substantial lookback time, even a crude measure of the ages of the stellar populations gives a sensitive determination of when they first formed significant numbers of stars, which is a crucial parameter in understanding galaxy evolution. Furthermore, the HST data in this field reach limits only 1.0 mag brighter than in the Hubble Deep Field, so that these data can be used as an independent check on the representative nature of the HDF counts extending into the ISO bands. The target visibility in this region is superb (practically every orbit in the extended mission), so that this field is an important adjunct in deep survey studies as well as the immediate cluster/protogalaxy science we propose.