Progress of the concept study of GREX-PLUS: Galaxy Reionization Explorer and Planetary Universe Spectrometer

Abstract

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is an ISAS/JAXA Strategic L-class mission candidate for the 2030s. The 1.0 m primary mirror, 50 K cryogenic telescope will have a wide-field camera (WFC) with a 0.5 square degree field-of-view in the 2–8 μm wavelength band. An optional high-resolution spectrometer (HRS) with a wavelength resolution of 30,000 in the 10–18 μm band is also under discussion. The WFC field-of-view is 200 times larger than that of the James Webb Space Telescope. Since the wavelength coverage of similarly wide-field imaging telescopes such as Euclid and Roman is limited to less than ∼2 μm, the GREX-PLUS coverage at wavelengths longer than 2 μm is complementary and unique, providing a versatile legacy imaging survey significantly improved from previous Spitzer imaging surveys in the same wavelength range. The wide field-of-view is realized by a Korsch three-mirror optical design for the telescope. To simplify the WFC, a filter has been assigned to each detector instead of using a filter exchange mechanism. As such, the field-of-view is divided into several filters, and multicolour images will be obtained after appropriate tiling. The spectral resolution of the HRS is 10 times higher than that of the James Webb Space Telescope, opening a new window of mid-infrared high-resolution spectroscopy from space. The telescope opto-mechanical structure design satisfying all required specifications has been developed successfully. Furthermore, the structural design of the radiative cooling system has advanced, enabling the construction of a combined structural and thermal mathematical model for the payload composed of the telescope and cooling systems. The main scientific themes are galaxy formation and evolution and planetary system formation and evolution. The GREX-PLUS WFC aims to detect the first generation of “bright” galaxies at redshift z > 15. The GREX-PLUS HRS aims to resolve the Keplerian motion of water vapor molecules and identify the location of the water “snowline” in ∼100 protoplanetary disks. Both instruments will provide unique data sets for a broad range of scientific topics, including galaxy mass assembly, the origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy for exoplanet atmospheres, planetary atmospheres in the Solar system, and so on.