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

We present basic observational strategies for ASTRO-F [also known as the In fra-Red Imaging Surveyor (IRIS)] to be launched in 2004 by the Japanese Ins titute of Space and Astronautical Science (ISAS). We examine two survey sce narios, a deep similar to1 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 similar to 18 deg(2) mid-IR (7-25 mum in six bands) coverin g an area greater than the entire area covered by all ISO mid-IR surveys. U sing two cosmological models, the number of galaxies predicted for each sur vey is calculated. The first model uses an enhancement of a classical (1+z) (3.1) pure luminosity evolution model by Pearson & Rowan-Robinson. The seco nd model incorporates a strongly evolving ultraluminous infrared galaxy com ponent. For the deep survey, between 20 000 and 30 000 galaxies should be d etected in the shortest wavebands, and approximate to 5000 in the longest ( 25-mum) band. It is predicted that the shallow survey will detect of the or der of 100 000-150 000 sources. We find that for both ASTRO-F and other sma ll-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 s pectroscopic ability from 2.2 to 25 mum), we show that by combining the mid -IR observations with the near-IR camera on ASTRO-F, both the different gal axy 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.