Ultra-Broadband Heterodyne Receiver Development for Sub-mm/mm Astronomy

Heterodyne spectroscopy at millimeter (mm) and sub-mm wavelengths is crucial to studies of the dusty interstellar medium. The Atacama Large Millimeter/sub-mm Array (ALMA), for example, revolutionized our understanding of galaxy, star, and planet formation. The next generation of ALMA’s receiver anticipates broader RF (radio frequency) and IF (intermediate frequency) bandwidths, not only improving observation speed, but also simultaneously capturing more spectral lines. We present our current effort to develop ultra-broadband heterodyne receivers using superconductor-insulator-superconductor (SIS) mixers. Our target is to combine ALMA bands 5 and 6 (163-275 GHz) and separately cover band 7 (275-373 GHz). The end goal is implementation at the new African Millimetre Telescope and the James Clerk Maxwell Telescope. We show the electromagnetic and quantum mixing simulation performed, the predicted conversion gain and noise temperature, the mixer block and horn design, as well as our concept to simplify sideband separating architecture. Lastly, we introduce a new ALMA survey of ultra-luminous infrared galaxies (ULIRGs) as a scientific application for SIS mixer-based heterodyne receivers. ULIRGs are an extremely infrared bright (>10^12 solar luminosities) population of star-forming galaxies (SFGs) whose star formation rates (SFRs) appear to vary between local and high redshift groups. Intermediate-z (0.2<z<1) ULIRGs and their SFRs fall between these two regimes, making them ideal for probing the transition region between main sequence and starburst SFGs. We characterize the properties and kinematics of a representative intermediate-z ULIRG sample with CO J=2-1 emission to compare them to their local counterparts and investigate the significance of mergers in transitioning SFGs.