ISO Observations of the 53W002 Group at 6.7 microns: In Search of the   Oldest Stellar Populations at z=2.4
  We present a deep ISO observation at 6.7 microns of the 53W002 group of galaxies and AGN at z=2.4. This approximately samples the emitted K band. The faint, blue star-forming objects are not detected, as expected from their very blue color across the emitted optical and UV. However, 53W002 itself is detected at the 3-sigma level, with an emitted V-K color appropriate for a population formed starting at z=3.6-7.0 with most likely value z=4.7. This fits with shorter-wavelength data suggesting that the more massive members of this group, which may all host AGN, began star formation earlier in deeper potential wells than the compact Lyman-alpha emission objects. Two foreground galaxies are detected, as well as several stars. One additional 6.7-micron source closely coincides with an optically faint galaxy, potentially at z=2-3. The overall source counts are consistent with other ISO deep fields. 
W. C. Keel, W. Wu, P. P. van der Werf, R. A. Windorst, J. S. Dunlop,   S. A. Eales, I. Waddington, M. Holmes