TIME: A Millimeter-Wavelength Grating Spectrometer Array for [CII] / CO Intensity Mapping

In this thesis I review the design, fabrication, and initial engineering deployment of the TIME (Tomographic Ionized-carbon Mapping Experiment) instrument. TIME seeks to make a first detection of the clustering amplitude of the power spectrum of redshifted [CII] emission from the Epoch of Reionization (z = 5-9). [CII], the 157.7 µm fine-structure line of singly ionized carbon, traces star formation on large scales, providing a new method for constraining the contribution of star formation to the Reionization process. [CII] intensity mapping complements traditional galaxy surveys by using spatially-broad beams to integrate signal from the many faint sources thought to be responsible for the bulk of the integrated emission from galaxies. TIME covers the 200-300 GHz atmospheric window, which also enables the study of lower-redshift CO emission (z = 0.5-2), a tracer of molecular gas in the period following the peak of cosmic star formation. The full TIME instrument consists of 32 single-polarization grating spectrometers with a resolution R ~ 100. Each spectrometer consists of an input feedhorn coupled to parallel plate waveguide with a curved diffraction grating, which focuses the diffracted light onto an output arc populated by 60 transition-edge sensor (TES) bolometers at 250 mK. The 1920 total detectors couple to the output of the parallel plate waveguide with a direct-absorbing micro-mesh and are organized into buttable arrays covering 4 spatial by either 12 (HF) or 8 (LF) spectral pixels. A partial TIME instrument was field tested in early 2019 on the ARO APA 12m dish at Kitt Peak. We intend to return to Kitt Peak in late 2020 to begin initial science observations.</p

TIME millimeter wave grating spectrometer

The Tomographic Ionized-carbon Mapping Experiment (TIME) utilizes grating spectrometers to achieve instantaneous wideband coverage with background-limited sensitivity. A unique approach is employed in which curved gratings are used in parallel plate waveguides to focus and diffract broadband light from feed horns toward detector arrays. TIME will measure singly ionized carbon fluctuations from 5 < z < 9 with an imaging spectrometer. 32 independent spectrometers are assembled into two stacks of 16, one per polarization. Each grating has 210 facets and provides a resolving power R of ~ 200 over the 186–324 GHz frequency range. The dispersed light is detected using 2-D arrays of transition edge sensor bolometers. The instrument is housed in a closed-cycle 4K–1K–300mK cryostat. The spectrometers and detectors are cooled using a dual-stage 250/300 mK refrigerator

A Foreground Masking Strategy for [CII] Intensity Mapping Experiments Using Galaxies Selected by Stellar Mass and Redshift

Intensity mapping provides a unique means to probe the epoch of reionization (EoR), when the neutral intergalactic medium was ionized by the energetic photons emitted from the first galaxies. The [CII] 158$\mu$m fine-structure line is typically one of the brightest emission lines of star-forming galaxies and thus a promising tracer of the global EoR star-formation activity. However, [CII] intensity maps at $6 \lesssim z \lesssim 8$ are contaminated by interloping CO rotational line emission ($3 \leq J_{\rm upp} \leq 6$) from lower-redshift galaxies. Here we present a strategy to remove the foreground contamination in upcoming [CII] intensity mapping experiments, guided by a model of CO emission from foreground galaxies. The model is based on empirical measurements of the mean and scatter of the total infrared luminosities of galaxies at $z 10^{8}\,\rm M_{\rm \odot}$ selected in $K$-band from the COSMOS/UltraVISTA survey, which can be converted to CO line strengths. For a mock field of the Tomographic Ionized-carbon Mapping Experiment (TIME), we find that masking out the "voxels" (spectral-spatial elements) containing foreground galaxies identified using an optimized CO flux threshold results in a $z$-dependent criterion $m^{\rm AB}_{\rm K} \lesssim 22$ (or $M_{*} \gtrsim 10^{9} \,\rm M_{\rm \odot}$) at $z < 1$ and makes a [CII]/CO$_{\rm tot}$ power ratio of $\gtrsim 10$ at $k=0.1$ $h$/Mpc achievable, at the cost of a moderate $\lesssim 8\%$ loss of total survey volume.Comment: 14 figures, 4 tables, re-submitted to ApJ after addressing reviewer's comments. Comments welcom

Detector modules and spectrometers for the TIME-Pilot [CII] intensity mapping experiment

This proceeding presents the current TIME-Pilot instrument design and status with a focus on the close-packed modular detector arrays and spectrometers. Results of laboratory tests with prototype detectors and spectrometers are discussed. TIME-Pilot is a new mm-wavelength grating spectrometer array under development that will study the Epoch of Reionization (the period of time when the first stars and galaxies ionized the intergalactic medium) by mapping the fluctuations of the redshifted 157:7 μm emission line of singly ionized carbon ([CII]) from redshift z ~ 5:2 to 8:5. As a tracer of star formation, the [CII] power spectrum can provide information on the sources driving reionization and complements 21 cm data (which traces neutral hydrogen in the intergalactic medium). Intensity mapping provides a measure of the mean [CII] intensity without the need to resolve and detect faint sources individually. We plan to target a 1 degree by 0.35 arcminute field on the sky and a spectral range of 199-305 GHz, producing a spatial-spectral slab which is 140 Mpc by 0.9 Mpc on-end and 1230 Mpc in the redshift direction. With careful removal of intermediate-redshift CO sources, we anticipate a detection of the halo-halo clustering term in the [CII] power spectrum consistent with current models for star formation history in 240 hours on the JCMT. TIME-Pilot will use two stacks of 16 parallel-plate waveguide spectrometers (one stack 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 freqeuency) or 12 (high frequency) spectral channels by 8 spatial channels and are mated to the spectrometer stacks. Each detector consists of a gold micro-mesh absorber and a titanium transition edge sensor (TES). The detectors (1920 total) are designed to operate from a 250 mK base temperature in an existing cryostat with a photon-noise-dominated NEP of ~2 * 10^(-17) WHz^(-1-2). A set of flexible superconducting cables connect the detectors to a time-domain multiplexing SQUID readout system

Probing Cosmic Reionization and Molecular Gas Growth with TIME

Line intensity mapping (LIM) provides a unique and powerful means to probe cosmic structures by measuring the aggregate line emission from all galaxies across redshift. The method is complementary to conventional galaxy redshift surveys that are object-based and demand exquisite point-source sensitivity. The Tomographic Ionized-carbon Mapping Experiment (TIME) will measure the star formation rate (SFR) during cosmic reionization by observing the redshifted [CII] 158$\mu$m line ($6 \lesssim z \lesssim 9$) in the LIM regime. TIME will simultaneously study the abundance of molecular gas during the era of peak star formation by observing the rotational CO lines emitted by galaxies at $0.5 \lesssim z \lesssim 2$. We present the modeling framework that predicts the constraining power of TIME on a number of observables, including the line luminosity function, and the auto- and cross-correlation power spectra, including synergies with external galaxy tracers. Based on an optimized survey strategy and fiducial model parameters informed by existing observations, we forecast constraints on physical quantities relevant to reionization and galaxy evolution, such as the escape fraction of ionizing photons during reionization, the faint-end slope of the galaxy luminosity function at high redshift, and the cosmic molecular gas density at cosmic noon. We discuss how these constraints can advance our understanding of cosmological galaxy evolution at the two distinct cosmic epochs for TIME, starting in 2021, and how they could be improved in future phases of the experiment.Comment: 30 pages, 18 figures, accepted for publication in Ap