Measuring the cosmological evolution of gas and galaxies with the EXperiment for Cryogenic Large-aperture Intensity Mapping (EXCLAIM)

The EXperiment for Cryogenic Large-aperture Intensity Mapping (EXCLAIM) is a high-altitude balloon spectrometer designed to deepen our understanding of star formation in a cosmological context. It will map the sub-mm emission of redshifted carbon monoxide (CO) and singly-ionized carbon ([Cii]) lines in windows comprising 0 < z < 3.5. These lines trace the precursors of star formation but have only preliminary characterization beyond the nearby universe, with most previous efforts limited to the brightest objects or galaxies selected in other surveys. Rather than detect individual galaxies, EXCLAIM will measure the statistics of brightness fluctuations of redshifted, cumulative line emission. This approach is known as intensity mapping, and the Enduring Quests Daring Visions Roadmap establishes it as a priority in the Visionary Era, to "completely map the content of selected volumes of space that represent slices through all of cosmic time." EXCLAIM is a pathfinder to future missions that will probe early star formation and reionization. A blind, complete census of redshifted CO and [Cii] addresses New Worlds, New Horizons Decadal Survey priorities to understand baryons in galaxies and the context of their dark matter halos. EXCLAIM will address significant outstanding questions: What factors led to the dramatic decline in star formation from z~2 to the present? What is the typical abundance and excitation of the molecular gas which forms stars? How well does CO trace H_2 in our galaxy? Is intensity mapping a viable approach to push to high redshift? We will address these questions by flying six redundant spectrometers configured from 421-540 GHz (lambda/delta_lambda=R=512), permitting cross-correlation of [C ii] 158 um and CO J=6-5, 5-4, and 4-3 emission with galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS). EXCLAIM's strategy is to start with the most feasible intensity mapping demonstration: the brightest emission lines at relatively low redshift in cross-correlation with a rich spectroscopic galaxy catalog. EXCLAIM is the only CO/[C ii] intensity mapping survey specifically designed for corroborated cross-correlation, and takes advantage of overlap with BOSS that is only possible in a conventional flight from the North. Conventional flights simplify logistics and reuse, making this a versatile instrument for testing sub-mm technology in a space-like environment. Due to dramatically reduced column depth and pressure broadening at float, the atmosphere has windows that approach space-like photon backgrounds and are significantly darker than emission from 300 K telescope optics. To exploit these low photon backgrounds, EXCLAIM employs an all-cryogenic telescope (1.5 K) with a 74 cm projected aperture coupled to the microSpec on-chip spectrometer. microSpec implements an on-chip Rowland spectrometer coupled to kinetic inductance detectors. Lithographic construction achieves mass and volume much lower than free-space gratings, making the architecture appealing for future space-borne instruments. Our team has led the field in the development of large, balloon-borne cryogenic apertures, the on-chip Rowland spectrometer, and intensity mapping through cross-correlation. EXCLAIM reuses the design of the Primordial Inflation Polarization ExploreR (PIPER) gondola, housekeeping electronics, software, and 100mK cooling with a continuous adiabatic demagnetization refrigerator. Commissioning (2021) and science (2022) flights are planned. Using data from these flights, we will constrain models of galaxy evolution, and enable new approaches to studying our universe. More »