We report the highest-redshift detection of [O i] 63 m from a luminous quasar, J20540005, at based on the Atacama Large Millimeter/sub-millimeter Array (ALMA) Band 9 observations. The [O i] 63 m line luminosity is, corresponding to the [O i] 63 m-to-far-infrared luminosity ratio of 6.7, which is consistent with the value obtained in the local Universe. Remarkably, [O i] 63 m is as bright as [C ii] 158 m, resulting in the [O i]-to-[C ii] line luminosity ratio of. Based on a careful comparison of the luminosity ratios of [O i] 63 m, [C ii] 158 m, and dust continuum emission to models of photodissociation regions, we find that J20540005 has a gas density and an incident far-ultraviolet radiation field of, showing that [O i] 63 m serves as an important coolant of the dense and warm gas in J20540005. A close examination of the [O i] and [C ii] line profiles suggests that the [O i] line may be partially self-absorbed; however, deeper observations are needed to verify this conclusion. Regardless, the gas density and incident radiation field are in broad agreement with the values obtained in nearby star-forming galaxies and objects with [O i] 63 m observations at -3 with the Herschel Space Observatory. These results demonstrate the power of ALMA high-frequency observations targeting [O i] 63 m to examine the properties of photodissociation regions in high-redshift galaxies.

We present a conceptual study of a large format imaging spectrograph for next-generation large (50-m class) single-dish telescopes, i.e., the Large Submillimeter Telescope (LST) and Atacama Large Aperture Submillimeter Telescope (AtLAST). Recent discoveries of high-redshift star-forming galaxies at z=8-9 and candidate quiescent galaxies at z~6 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 describe the preliminary of 3-band, medium resolution (R=2000) imaging spectrograph with ~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 like A-MKID.