Extremely high spectral resolution measurements of the 450 µm atmospheric window at Chajnantor with APEX^★,★★

¹ Consejo Superior de Investigaciones Cientifícas, Instituto de Física Fundamental, Serrano 123, 28006 Madrid, Spain
e-mail: jr.pardo@csic.es
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla 7630355 Santiago, Chile
⁵ Centre National de la Recherche Scientifique, Observatoire de Paris, Laboratoire d’Études du Rayonnement et la Matière en Astrophysique, 77 Avenue Denfert Rochereau, 75014 Paris, France
⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Received: 23 February 2022
Accepted: 17 May 2022

Abstract

Ground-based telescopes observing at millimeter (mm) and submillimeter (submm) wavelengths have to deal with a line-rich and highly variable atmospheric spectrum, both in space and time. Models of this spectrum play an important role in planning observations that are appropriate for the weather conditions and also calibrating those observations. Through magnetic dipolar (M1) rotational transitions and electric dipolar (E1) transitions O₂ and H₂O, respectively, dominate the atmospheric opacity in this part of the electromagnetic spectrum. Although O₂ lines, and more generally the so-called dry opacity, are relatively constant, the absorption related to H₂O can change by several orders of magnitude leading from a totally opaque atmosphere near sea level with high H₂O columns to frequency windows with good transmission from high and dry mountain sites. Other minor atmospheric gases, such as O₃ and N₂O among others, are present in the atmospheric spectrum which also includes nonresonant collision-induced absorption due to several mechanisms. The aim of our research is to improve the characterization of the mm/submm atmospheric spectrum using very stable heterodyne receivers with excellent sideband separation and extremely high (kHz) spectral resolutions at the 5000 m altitude Chajnantor site in northern Chile. This last aspect (spectral resolution) is the main improvement (by more than three orders of magnitude) in the presented data with respect to our previous work conducted ~20 yr ago from Mauna Kea in Hawai’i. These new measurements have enabled us to identify slight modifications needed in the Atmospheric Transmission at Microwaves (ATM) model to better take into account minor constituent vertical profiles, include a few missing lines, and adjust some high-energy O₃ line frequencies. After these updates, the ATM model is highly consistent with all data sets presented in this work (within ~2% at 1 GHz resolution).