Line intensity mapping, an emerging technique for observing the large-scale structure of the Universe, provides a measure of the aggregate emission of intergalactic atomic and molecular lines without the need to resolve and detect faint sources individually. The Tomographic Ionized-carbon Mapping Experiment (TIME), developed by a collaboration between multiple institutions including ASIAA, Caltech/JPL, Cornell, RIT and the University of Arizona, is designed to study the Epoch of Reionization (EoR) — the period of time when the first stars and galaxies ionized the intergalactic medium (IGM). TIME utilizes a mm-wavelength grating spectrometer array that maps the fluctuations of the redshifted 157.7 μm emission line of singly ionized carbon ([CII]) from redshift z ≈ 5 to 9. As a tracer of star formation, the [CII] power spectrum can provide information on the sources driving cosmic reionization and complements 21 cm data, which traces neutral hydrogen in the same medium. TIME is also sensitive to the carbon monoxide (CO) rotational emission from galaxies at intermediate redshifts z ≈ 0.5 to 2, producing a rich dataset of neutral gas intensity fluctuations during the peak of cosmic star formation.
TIME employs two banks of 16 parallel-plate waveguide spectrometers (one bank per polarization) with a resolving power R ≈ 100 and a spectral range of 183 to 326 GHz. The range is divided into 60 spectral channels, of which 16 at the band edges on each spectrometer serve as atmospheric monitors. The diffraction gratings are curved to produce a compact instrument, each focusing the diffracted light onto an output arc sampled by the 60 bolometers. The bolometers are built in buttable dies of 8 (low frequency, 200–265 GHz) or 12 (high frequency, 265–300 GHz) spectral channels by 4 spatial channels and are mated to the spectrometer banks. Each detector consists of a gold micro-mesh absorber and a titanium transition edge sensor (TES). The detector arrays are engineering prototypes designed to match the high optical loading condition at the Arizona Radio Observatory 12m telescope (ARO 12m). There are a total of 1920 detectors operating from a 250 mK base temperature in a dedicated cryostat with a photon-noise-dominated noise-equivalent power (NEP) of ≈ 2×10-17 WHz-1/2.
The TIME instrument has been integrated and tested in both laboratory characterizations and on-sky observations. TIME was deployed to ARO on Kitt Peak for an engineering run in 2019 and a commissioning run in 2022, during which we demonstrated detector responsivity, optical performance and noise suppression techniques. Demonstrations of TIME spectral imaging on astrophysical sources are shown in the following figure.
TIME will be deployed to the ARO 12m again at the end of 2023 to begin scientific observations. The awarded 11-month observation time of TIME-ARO will be spread over the next three winters, when atmospheric conditions are the best for high-frequency observing. We plan to target a 1.3° by 0.45′ field on the sky and a full spectral range of 183–326 GHz, producing a thin spatial-spectral data slab by conducting line scans in azimuth. (Utilizing the same scanning strategy, the map below shows the detection of Jupiter by all TIME feedhorns — except for one which was blanked off — in a test run.) With the use of low-redshift galaxy tracers and differences in k-space projections to carefully separate the [CII] and CO signals, we anticipate a detection of the halo-halo clustering term in the [CII] power spectrum consistent with current models for star formation history.
We also plan to modify the TIME detectors for future measurements performed at the Leighton Chajnantor Telescope (LCT), the relocated Caltech Submillimeter Observatory (CSO) 10m telescope currently in preparations at the Chajnantor Plateau in northern Chile for its first light in 2025. Exploiting the lower atmospheric and telescope optical loading, an upgraded TIME-LCT with a re-optimized focal plane can offer more than 10 times the survey power, yielding much tighter constraints on the process of reionization and the cosmic optical depth in a 3-year observation.