A deep 6.7 (mu)m survey in the SSA13 field with ISO

We present results of a deep mid-infrared survey in the SSA13 field with the Infrared Space Observatory (ISO). In order to probe the near-infrared light at high redshifts, we surveyed the field with the broad band LW2 (5-8.5 (mu)m) filter of the mid-infrared camera ISOCAM. Adopting a highly redundant imaging strategy for the 23 h observation and carefully treating gradual changes in the detector responsivity caused by a very high rate of cosmic ray impacts, we succeeded in reaching an 80% completeness limit of 16 (mu)Jy in the central 7 arcmin ^2 region. Utilizing the signal-to-noise ratio map, we detected 65 sources down to 6 (mu)Jy in the 16 arcmin^ 2 field. Integral galaxy number counts at 6.7 (mu)m are then derived, reaching 1.3 * 10^(4) deg^(-2) at the faint limit with a slope of  -1.6 between 13 (mu)Jy and 130 (mu)Jy. Integrating individual sources in this flux range, the resolved fraction of the extragalactic background light at 6.7 (mu)m is estimated to be 0.56 nW m^(-2) sr^(-1). These results, which reach a flux limit three times fainter than those in the Hubble Deep Fields, are in fairly good agreement with a model prediction by Franceschini et al. (1997). Finally, we discuss the relation of distant massive E/S0 galaxies to the faint 6.7 (mu)m galaxy population.