Browsing by Author "Hu, Wenkai"

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    Concerto at apex on-sky performance in continuum
    (EDP Sciences, 2024) Hu, Wenkai; Beelen, Alexandre; Lagache, Guilaine
    CarbON CII line in post-rEionisation and ReionisaTiOn epoch (CONCERTO) instrument is a low-resolution mapping spectrometer based on lumped element kinetic inductance detector (LEKIDs) technology, operating at 130-310 GHz. It was installed on the 12-metre APEX telescope in Chile in April 2021 and was in operation until May 2023. CONCERTO’s main goals were the observation of [CII]-emission line fluctuations at high redshift and of the Sunyaev–Zel’dovich (SZ) signal from galaxy clusters. Aims. We present the data processing algorithms and the performance of CONCERTO in continuum by analysing the data from the commissioning and scientific observations.Methods. We developed a standard data processing pipeline to proceed from the raw data to continuum maps. Using a large dataset of calibrators (Uranus, Mars, and quasars) acquired in 2021 and 2022 at the APEX telescope across a wide range of atmospheric conditions, we measured the CONCERTO continuum performance and tested its stability against observing conditions. Further, using observations on the COSMOS field and observations targeting a distant sub-millimetre galaxy in the UDS field, we assessed the robustness of the CONCERTO performance on faint sources and compared our measurements with expectations.
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    FAST drift scan survey for HI Intensity mapping: simulation of Bayesian-stacking-based HI mass function estimation
    (Institute of Physics, 2025) Hu, Wenkai; Wang, Jiaxin; Li, Yichao
    This study investigates the estimation of the neutral hydrogen (Hi) mass function (HiMF) using a Bayesian stacking approach with simulated data for the Five-hundred-meter Aperture Spherical radio Telescope (FAST) Hi intensity mapping (HiIM) drift-scan surveys. Using data from the IllustrisTNG simulation, we construct Hi sky cubes at redshift 𝑧 ∼ 0.1 and the corresponding optical galaxy catalogs, simulating FAST observations under various survey strategies, including pilot, deep-field, and ultradeep-field surveys. The HiMF is measured for distinct galaxy populations– classified by optical properties into red, blue, and bluer galaxies– and injected with systematic effects such as observational noise and flux confusion caused by the FAST beam. The results show that Bayesian stacking significantly enhances HiMF measurements. For red and blue galaxies, the HiMF can be well constrained with pilot surveys, while deeper surveys are required for the bluer galaxy population. Our analysis also reveals that sample variance dominates over observational noise, emphasizing the importance of wide-field surveys to improve constraints. Furthermore, flux confusion shifts the HiMF toward higher masses, which we address using a transfer function for correction. Finally, we explore the effects of intrinsic sample incompleteness and propose a framework to quantify its impact. This work lays the groundwork for future HiMF studies with FAST HiIM, addressing key challenges and enabling robust analyses of Hi content across galaxy populations.
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    Searching for Axion Dark Matter Gegenschein of the Vela Supernova remnant with FAST
    (Institute of Physics, 2025) Hu, Wenkai; Yang, Wenxiu; Sun, Yitian
    Axions are one of the leading dark matter candidates. If we are embedded in a Milky Way dark matter halo comprised of axions, their stimulated decay would enable us to observe a counterimage (“axion gegenschein”) with a frequency equal to half the axion mass in the opposite direction of a bright radio source. This spectral line emission will be broadened to Δν/ν ∼ σd/c ∼ 10−3 due to the velocity dispersion of dark matter, σd. In this pilot study, we perform the first search for the expected axion gegenschein image of Vela supernova remnant with 26.4 hr of effective ON–OFF data from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) Lband (1.0–1.5 GHz) 19 beam receiver. Our null detection limits the axion–photon coupling strength to be gaγγ ≲ 2 × 10−10 GeV−1 in the mass ranges of 8.7 μeV � ma � 9.44 μeV and 10.85 μeV � ma � 12.01 μeV. These results provide a stronger constraint on gaγγ in this axion mass range than the current limits obtained by the direct search of an axion decay signal from a dwarf galaxy that uses FAST observations, but are a factor of ∼3 times weaker than the current CERN Axion Solar Telescope limit. Based on our observation strategy, data processing methods, and results, the expected sensitivity will reach ∼10−11 GeV−1 with ∼2000 hr of observation in the future.