A deep 6.7 m survey in the SSA13 field with ISO ^*

¹ Institute of Space and Astronautical Science (ISAS), 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
² Institute of Astronomy, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
³ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁴ Astronomical Institute, Graduate School of Science, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan
⁵ Max-Planck Institut fur Extraterrestrische Physik, 85740 Garching, Germany
⁶ Gunma Astronomical Observatory, 6860-86 Nakayama, Takayama, Agatsuma, Gunma 377-0702, Japan
⁷ Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan

Corresponding author: Y. Sato, ysato@ioa.s.u-tokyo.ac.jp

Received: 29 August 2001
Accepted: 18 April 2003

Abstract

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 μm) 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 μJy in the central 7 arcmin² region. Utilizing the signal-to-noise ratio map, we detected 65 sources down to 6 μJy in the 16 arcmin² field. Integral galaxy number counts at 6.7 μm are then derived, reaching  deg^-2 at the faint limit with a slope of -1.6 between 13 μJy and 130 μJy. Integrating individual sources in this flux range, the resolved fraction of the extragalactic background light at 6.7 μm 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 μm galaxy population.