Infrared Space Observatory observations of the 53W002 group at 6.7 microns: In search of the oldest stellar populations at z = 2.4

We present a deep Infrared Space Observatory (ISO) observation at 6.7 μm of the 53W002 group of galaxies and active galactic nuclei (AGNs) at z = 2.4. This approximately samples the emitted K band. Faint, blue star-forming objects are not detected, as expected from their very blue color across the emitted optical and UV spectrum. However, 53W002 itself is detected at the ∼3 σ level, with an emitted V-K color that is appropriate for a population that formed starting at z = 3.6-7.0, most likely z = 4.7. This fits with shorter wavelength data, suggesting that the more massive members of this group, which may all host AGNs, began star formation earlier in deeper potential wells than the compact Lyα emission objects. Two foreground galaxies are detected, as are several stars. One additional 6.7 μm source closely coincides with an optically faint galaxy, potentially at z = 2-3. The overall source counts are consistent (within the errors of such a small sample) with the other ISO deep fields at 6.7 μm.