Large format imaging spectrograph for the Large Submillimeter Telescope (LST)

We present a conceptual study of a large format imaging spectrograph for the Large Submillimeter Telescope (LST) and the Atacama Large Aperture Submillimeter Telescope (AtLAST). Recent observations of high-redshift galaxies indicate the onset of earliest star formation just a few 100 million years after the Big Bang (i.e., z = 12-15), and LST/AtLAST will provide a unique pathway to uncover spectroscopically-identified "first forming galaxies" in the pre-reionization era, once it will be equipped with a large format imaging spectrograph. We propose a 3-band (200, 255, and 350 GHz), medium resolution (R = 2,000) imaging spectrograph with similar to 1.5 M detectors in total based on the KATANA concept (Karatsu et al. 2019), which exploits technologies of the integrated superconducting spectrometer (ISS) and a large-format imaging array. A 1-deg(2) drilling survey (3,500 hr) will capture a large number of [O III] 88 mu m (and [C II] 158 mu m) emitters at z = 8-9, and constrain [O III] luminosity functions at z > 12.