Pre-Flight Simulation of DESHIMA2.0
Observation of a High-Redshift Galaxy with the Current DESHIMA2.0 Instrument on the ASTE Telescope
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Abstract
To better understand the formation of stars in dusty star-forming galaxies (DSFGs), and the evolution of this type of submillimeter galaxies (SMGs), it is of high significance to perform submillimeter/far-infrared surveys. The Deep Spectroscopic High-redshift Mapper 2.0 (DESHIMA2.0) on the ASTE telescope in Chile has been designed to observe the redshifted emission lines of these DSFGs to measure both their (spectroscopic) redshifts and molecular compositions. DESHIMA2.0 is designed to observe across the 220-440 GHz frequency band using its 347 channel integrated superconducting spectrometer (ISS) chip.
The current DESHIMA2.0 instrument has been tested in the lab. Its 332 filter channels with center frequencies in the range of 204-391 GHz have a spectral resolution of f/δf ≈ 340. Their coupling efficiencies can be approximated by Lorentzian functions.
The gravitationally lensed ultraluminous high-redshift DSFG J1329+2243 with redshift z = 2.04 shows emission lines of molecular gases like CO and H2O in the frequency band of DESHIMA2.0. Using the Time-dependent End-to-end Model for Post-process Optimization (TiEMPO), an 8-hour observation of the J1329+2243 galaxy has been simulated for both the lab-measured chip and the designed chip. Two measures of sensitivity, the noise equivalent flux density (NEFD) and the minimum detectable line flux (MDLF), are theoretically derived and subsequently used to be compared with the results of the simulations. The results yielded from these simulations are ultimately used to report on the overall performance of the lab-measured chip.
To run a TiEMPO simulation using the lab-measured chip, adaptations had to be made to the model for it to accept customizable chip data. Within TiEMPO, variable spectral resolutions and coupling efficiencies had to be introduced as well as a crucial addition to enable the creation of a new filterbank. Given the results of the simulations, it can be stated that this implementation of the lab-measured chip has been successful.
After applying an ON-OFF (dual) sky chopping technique to the simulated observation to cancel fluctuations of the atmospheric transmission, the atmosphere-corrected antenna temperature of the J1329+ 2243 galaxy could be found. The standard deviation of the noise showed to be scaling inversely proportional to the square root of the integration time. For five separate emission lines of the galaxy within the range of 200-310 GHz, a signal-to-noise ratio (SNR) analysis was performed and compared to the estimated theoretical proportionality to the square root of the integration time. For lines that show overlap in the spectrum, the detection was proven to be more difficult. Optimizations in the strategy used to define these signal-to-noise ratios would improve this finding.
The performance of the current DESHIMA2.0 instrument is sufficient to detect (SNR≥5) the bright CO(7-6) line of an ultraluminous high-redshift galaxy like J1329+2243 after 5.5 minutes of observation time. After 50 minutes of observation time, it is also capable of identifying the CO(6-5), the H2O(211-202), the [CI](2-1), and the CO(8-7) emission line.