A mid-infrared deep galaxy survey in the SSA13 field

Y. Sato ^1,2,3, L.L. Cowie ⁴, Y. Taniguchi ³, D.B. Sanders ⁴, K. Kawara ⁵, H. Okuda ¹, K. Wakamatsu ⁵, T. Matsumoto ¹, R.D. Joseph ⁴, Y. Sofue ⁵, & H. Matsuhara ¹

¹ Institute of Space and Astronautical Science (ISAS)

² ISO Science Operations Centre, Astrophysics Division, ESA

³ Astronomical Institute, Tohoku University

⁴ Institute for Astronomy, University of Hawaii (UH)

⁵ Institute of Astronomy, The University of Tokyo

⁶ Department of Physics, Gifu University

We have executed deep imaging observations as the Japan/UH cosmology program using ISAS guaranteed time. This program consists of ISOCAM and ISOPHOT deep surveys and here we present the results from the former one. After the success to obtain a deep 6.7 micron ISOCAM image in the Lockman Hole (Taniguchi et al 1997), we selected SSA13 as a second target. This field is located at high ecliptic and Galactic latitudes (beta   46 degree and b   74 degree) and also has small HI column density ( ). There exist attracting high quality data such as deep optical (B and I) and NIR (K) images and spectroscopic redshifts (Cowie et al 1996).

The observations were done with the ISOCAM LW2 (6.7 micron) filter. Total observing time of 23 hours were divided into six revolutions. Separation of the revolutions into one and a half month period and use of non-integer pixel rastering offsets gave us a higher resolution image than that for the Lockman Hole. This improved reliability to identify real sources. Despite of difficulties to handle the data with larger number of cosmic ray impacts, we have reached similar results obtained in the Lockman Hole. Roughly same number (30) of sources were identified with flux of larger than 20 microJy in the central 15 sq. arcminutes. This cumulative number of detected sources is almost consistent with a prediction at 6.7 micron based on the HDF galaxies (Mobasher et al 1996). There is a room to improve the data depending on what kind of procedures were actually taken, especially those for deglitching, transition and distortion corrections are promising items.

Comparing with the optical and NIR images, we found some sources without counterparts in B, I and K bands within 6 arcsec, one pixel size well matched to the Airy disk. Surprisingly, one of the brightest sources detected in the central, thus most reliable, part of the 6.7 micron image lacks its short wavelength emission completely. A check on morphology of the associated optical objects indicates that 6.7 micron emission tends to come from the disturbed ones. Among the associated objects with spectroscopically confirmed redshifts, we found one group of galaxies at intermediate redshift ( ) and several sources with large redshifts ().