Infrared Imaging Surveyor (IRIS) project

In this paper we describe the concept and the design of the InfraRed Imaging Surveyor (IRIS). the first Japanese satellite solely dedicated to infrared astronomy. It will follow a successful precursor, the Infrared Telescope in Space (JRTS)1 onboard the Space Flyer Unit (SFU) in 1995. The IRIS has a 70 cm telescope cooled down to 7 K by using superfluid helium assisted by two-stage Stirling cycle coolers. The expected hold time of the super-fluid helium is one year. After consumption of the helium. near-infrared observation can be continued by using the mechanical coolers. Two focal plane instruments are planned; the InfraRed Camera (IRC) and the Far-Infrared Surveyor (FIS). The total spectral coverage is 2 to 200 microns. The major scientific objectives are to investigate birth and evolution of galaxies in the early universe by survey of young normal galaxies and starburst galaxies. The orbit is a sun-synchronous orbit, in which the cooled telescope can avoid huge emissions from the Sun and the Earth by pointing the telescope on the great circle perpendicular to the Sun. The IRIS project is expected to start in 1997 and it will be launched by a M-V rocket in 2002. key words; infrared satellite. infrared astronomy. infrared sun'ey, galaxies, cosmology, cooled telescope IRIS is a Japanese space project promoted by the Infrared Astromical Satellite Working Group ofthe Institute of Space and Astronautical Science (ISAS) and supported by scientists of Tokyo university, Nagoya university, and other universities and institutions in Japan. 1. SCIENTifIC OBJECTiVES The scientific objective of the IRIS mission is the investigation of the processes of the formation and evolution of galaxies in the early universe. The IRIS project will also contribute useful information in many important fields of astronomy and astrophysics. 1.1 Formation and Evolution of Galaxies in the Early Universe It 5 considered as the most important target of current astronomy to know how galaxies formed and how they have evolved i. Astronomers have already found distant exotic objects, such as AGN's, Seyfert galaxies, QSO's, which may have played important roles in the early universe. However. we have only limited information about normal galaxies and starburst galaxies in the early universe, although they are the majority in the vicinity of our Galaxy at present. IRIS will be able to observe the integrated starlight ofthe normal galaxies even at high redshifts and even with heavy extinction due to dust, by using its nearand mid-infrared deep survey. On the other hand. the process of the starburst (burst of star formation) must play an important role in the early universe. much more so than that in the present universe. IRIS is expected to detect a huge number of starburst galaxies in the early universe and will significantly contribute to the study of the starburst galaxies by a far-infrared survey. 1.2 Other Objectives As found by the Infrared Astronomy Satellite (IRAS), a good indicator of the evolution stage of proto-planetary disks is strong midand far-infrared continuum emission radiated by proto-planetary disks of young stellar objects, such as T-Tauri stars and that the spectral energy distribution (SED). The far-infrared survey by the IRIS will be achieved with much higher sensitivity comparable to IRAS. Therefore, proto-planetary disks in later stages of their evolution will be detected. Mid-infrared imaging capability of the IRIS is expected to be powerful in searching for brown dwarfs, which are considered a possible candidate of the missing mass of the universe. 08194-2 1251/96/$6.OO SPIE Vol. 2744 I 75 76 /SPIE Vol. 2744 10