ASTRO‐F : the next generation of mid‐infrared surveys

We present basic observational strategies for ASTRO‐F [also known as the Infra‐Red Imaging Surveyor (IRIS) ] to be launched in 2004 by the Japanese Institute of Space and Astronautical Science (ISAS). We examine two survey scenarios, a deep ∼1 deg 2 survey reaching sensitivities an order of magnitude below all but the deepest surveys performed by ISO in the mid‐IR, and a shallow ∼18  deg 2 mid‐IR (7–25μm in six bands) covering an area greater than the entire area covered by all ISO mid‐IR surveys. Using two cosmological models, the number of galaxies predicted for each survey is calculated. The first model uses an enhancement of a classical (1+z) 3.1 pure luminosity evolution model by Pearson & Rowan‐Robinson. The second model incorporates a strongly evolving ultraluminous infrared galaxy component. For the deep survey, between 20 000 and 30 000 galaxies should be detected in the shortest wavebands, and ≈5000 in the longest (25‐μm) band. It is predicted that the shallow survey will detect of the order of 100 000–150 000 sources. We find that for both ASTRO‐F and other small‐aperture space telescopes, confusion due to faint sources may be severe, especially at the longest mid‐IR wavelengths. Using the exceptional range of observational options provided by ASTRO‐F (nine wavelength filters and spectroscopic ability from 2.2 to 25 μm), we show that by combining the mid‐IR observations with the near‐IR camera on ASTRO‐F , both the different galaxy populations and rough photometric redshifts can be distinguished in the colour–colour plane. In its role as a surveyor (plus near‐IR spectroscopic ability) ASTRO‐F will complement well the SIRTF space observatory mission.