High Frequency Channel for CHAI

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Building on our experience with developing legacy instruments for major observing facilities like Herschel and SOFIA at University of Cologne, we are currently building the Chajnantor Heterodyne Array Instrument (CHAI), to be deployed on the Fred Young Submillimeter Telescope (FYST) of the CCAT observatory on top of Cerro Chajnantor in northern Chile at 5600 m altitude. FYST is a novel 6 m aperture telescope scheduled to see first light in 2024. It will enable highly sensitive wide field observations at submillimeter wavelengths. Its instruments will address science topics ranging from the formation of stars and the evolution of the interstellar medium in the Milky Way and nearby galaxies to galaxies in the early universe and cosmology. CHAI is of particular importance, since it will be the only instrument providing high resolution spectroscopy through its multi-pixel heterodyne spectrometer covering observing frequencies from 400 to 490 GHz in its low frequency channel (LFA) at first light. Accordingly, 25% of the available observing time will be allocated to CHAI and its science cases, as guaranteed in the CCAT consortium partner agreement. We here aim to even better exploit the excellent atmospheric conditions at the CCAT observatory site by adding a high frequency channel (HFA) to CHAI, operating in the 800 GHz atmospheric window. Through simultaneous observation of the LFA and HFA, the performance of the instrument will be doubled, on top of its already unprecedented mapping speed and sensitivity for spectroscopy at these frequencies. More importantly, the second frequency window gives access to unique new science, which is necessary to fully address the research topics defined in the CCAT observatory key program GEco, the Galactic Ecology project. This survey is designed to study the matter cycle in the interstellar medium of the Milky Way and nearby galaxies from the assembly of clouds to the formation of filaments, hubs and cores within them, and eventually stars, which in turn provide stellar feedback. The full science case requires the high frequency observations as they give invaluable information for the interpretation not only of our low frequency observations, but also other large scale surveys and our understanding of the formation of structures in the interstellar medium and their evolution. The key objectives of this proposal are to secure funding to build the CHAI HFA, and thus, to enable and harvest the science that has been envisioned when securing the funding for thr FYST telescope from the DFG and partner resources.