ISO Observations of the 53W002 Group at 6.7 microns: In Search of the Oldest Stellar Populations at z=2.4

Abstract

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.