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This study investigates the estimation of the neutral hydrogen (H i) mass function (HiMF) using a Bayesian stacking approach with simulated data for the Five-hundred-meter Aperture Spherical radio Telescope (FAST) H i intensity mapping (HiIM) drift-scan surveys. Using data from the IllustrisTNG simulation, we construct H i 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 H i content across galaxy populations.

We present the H i galaxy observation results of FATHOMER, a pilot drift scan survey by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The survey comprises 28 hr of observations over seven nights in 2021, covering a 60 deg2 sky area in the frequency range 1.05–1.45 GHz. The H i galaxies are identified using both a matched-filtering algorithm and the SoFiA source-finding pipeline, which yield consistent detections. We derive the velocity width (W50), flux density, and Hi mass for detected galaxies. A total of 702 galaxies are identified with H i mass above 106.2 M⊙, signal-to-noise ratio greater than 5, and redshift z < 0.09. Among these, 331 are previously known from the ALFALFA survey. Of the newly detected sources, 9 have spectroscopic confirmation from the Sloan Digital Sky Survey (SDSS), 285 are matched to SDSS or DESI photometric data, and 77 lack optical counterparts–possible candidates for dark or faint galaxies. Comparison with ALFALFA shows that FAST enables the detection of galaxies at higher redshifts and with lower Hi fluxes, despite the radio frequency interference (RFI) and partial data masking. A preliminary Hi mass function analysis reveals a higher characteristic mass and steeper low-mass slope than ALFALFA, indicating FAST’s enhanced sensitivity to massive and distant H i systems. These results demonstrate FAST’s strong potential for future deep H i surveys and highlight the importance of improved RFI mitigation and completeness correction.

This work presents the initial results of the drift-scan observation for the neutral hydrogen (Hi) intensity mapping survey with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The data analyzed in this work were collected in night observations from 2019 through 2021. The primary findings are based on 28 hr of drift-scan observation carried out over 7 nights in 2021, which covers 60 deg2 sky area. Our main findings are, first, our calibration strategy can successfully correct both the temporal and bandpass gain variation over the 4 hr drift-scan observation. Second, the continuum maps of the surveyed region are made with frequency resolution of 28 kHz and pixel area of 2.95arcmin2 . The pixel noise levels of the continuum maps are slightly higher than the forecast assuming T sys = 20 K, which are 36.0 mK (for 10.0 s integration time) at the 1050–1150 MHz band, and 25.9 mK (for 16.7 s integration time) at the 1323–1450 MHz band, respectively. Third, the flux-weighted differential number count is consistent with the NRAO-VLA Sky Survey (NVSS) catalog down to the confusion limit ∼7 mJy beam−1. Finally, the continuum flux measurements of the sources are consistent with those found in the literature. The difference in the flux measurement of 81 isolated NVSS sources is about 6.3%. Our research offers a systematic analysis for the FAST Hi intensity mapping drift-scan survey and serves as a helpful resource for further cosmology and associated galaxies sciences with the FAST drift-scan survey.

Neutral hydrogen (H i) intensity mapping (IM) presents great promise for future cosmological large-scale structure surveys. However, a major challenge for H i IM cosmological studies is to accurately subtract the foreground contamination. An accurate beam model is crucial for improving the quality of foreground subtraction. In this work, we develop a stacking-based beam reconstruction method utilizing the radio continuum point sources within the drift-scan field. Based on the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we employ two sets of drift-scan survey data and merge the measurements to construct the beam patterns of the 19 FAST L-band feeds. To model the beams, we utilize the Zernike polynomial, which effectively captures asymmetric features of the main beam and the different side lobes. Due to the symmetric location of the beams, the main features of the beams are closely related to the distance from the center of the feed array, e.g., as the distance increases, side lobes become more pronounced. This modeling pipeline leverages the stable drift-scan data to extract beam patterns while accounting for and excluding the reflector’s changing effects. It provides a more accurate measurement beam and a more precise model beam for FAST H i IM cosmology surveys.

Filaments stand as pivotal structures within the cosmic web. However, direct detection of the cold gas content of the filaments remains challenging owing to its inherently low brightness temperature. With the TNG hydrodynamical simulations, we demonstrate the effectiveness of isolating faint filament H i signal from the FAST H i intensity mapping (IM) survey through pairwise stacking of galaxies, which yields an average H i filament signal amplitude of ∼0.29 μK at z ≃ 0.1. However, our simulations reveal a nonnegligible contribution from H i-rich galaxies within or near the filaments. Particularly, the faint galaxies dominantly contribute to the extra filament H i signal. Our simulation also shows that the measurement uncertainty is produced by both thermal noise and background variation caused by brightness leakage from surrounding random galaxies. Given a fixed total observation time, a wide-field H i IM survey, which includes a large number of galaxy pairs, can simultaneously reduce thermal noise to below the filament signal level and minimize background variation to a negligible level. Through the end-to-end simulation, this work demonstrates the critical role of the galaxy pairwise stacking method in future filament H i detection, outlining a road map for filament H i detection in the next-generation H i IM surveys.

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) L-band (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.

Peaked-spectrum (PS) sources exhibit turnover characteristics in their broad radio spectra. However, the mechanism underlying this phenomenon remains elusive. The two most common hypotheses are synchrotron self-absorption (SSA) and free–free absorption (FFA). By incorporating multiple absorption scenarios, we propose a multimechanism hybrid model, which aligns well with current observational data and provides a good physical explanation. Using data from the GLEAM survey, we identified a sample of 4315 sources with peak frequencies between approximately 72 and 3000 MHz, most of which are MHz-peaked-spectrum sources. Our analysis shows that FFA rather than SSA is the dominant mechanism in producing the spectral turnover for most of the sources in this sample. The index of the optically thick spectrum αthick has a lower boundary due to FFA, and the steeper αthick indicates a complex multiabsorption mechanism. In particular, the external FFA produces substantial αthick, which exhibits a weak correlation with the peak frequency. Future ultralong-wavelength observations would also provide data on the spectrum of these sources at even lower frequencies. Determining the absorption mechanism that shaped the spectrum of these sources would be a crucial part of understanding their nature.

We present an extragalactic H i 21 cm absorption lines catalog from a blind search at z ≤ 0.35, using drift-scan data collected in 1325.6 hr by the ongoing Commensal Radio Astronomy Fast Survey and FAST All Sky H i Survey, which spans a sky area of 6072.0 deg2 and covers 84,533 radio sources with a flux density greater than 12 mJy. Fourteen previously identified H i absorbers and 20 newly discovered H i absorbers were detected, comprising 15 associated systems, 10 intervening systems, and nine systems with undetermined classifications. Through spectral stacking, the mean peak optical path, mean velocity-integrated optical path, mean FWHM, and mean H i column density are measured to be 0.47 and 0.30; 27.19 and 4.36 km s−1; 42.61 and 9.33 km s−1; 0.49 and 0.08 Ts × 1020 cm−2 K−1, for the associated and intervening samples, respectively. Statistical analysis also reveals that associated systems tend to be hosted by red (g − r > 0.7) galaxies at lower redshifts, whereas galaxies hosting intervening H i absorption are typically found at higher redshifts and are of a bluer (g − r ≤ 0.7) type. A noticeable difference is observed in the positions of foregrounds, backgrounds of intervening systems, and high-redshift and low-redshift associated systems on the Wide-field Infrared Survey Explorer color–color diagram. All identified foreground sources in our sample have W1 – W2 magnitudes below 0.8, suggesting no active galactic nuclei (AGNs). In contrast, backgrounds of intervening systems tend to have W1 – W2 magnitudes above 0.8, indicating AGN presence. For associated absorption, most low-redshift (z ≤ 0.5) systems show W1 – W2 values below 0.8, while higher-redshift associated absorption (z > 0.5) displays a broader range of W1 − W2 values.

Ozone (O 3 ) is a significant global air pollutant. Recent epidemiological studies have established a correlation between O 3 exposure and an increased risk of neurological disorders. However, the underlying mechanisms by which O 3 induces cognitive deficits remain unclear. This study demonstrated that exposure to environmentally relevant O 3 levels resulted in significant cognitive impairment in mice. These deficits arose from hippocampal synaptic injury, characterized by reduced dendritic spine density, disrupted synaptic ultrastructure, and impaired long-term potentiation. Mechanistically, O 3 activated the liver complement pathway, leading to increased levels of complement component 3 (C3) and its subsequent release into the bloodstream. Furthermore, O 3 compromised the integrity of the blood–brain barrier, allowing peripheral C3 to infiltrate the hippocampus. Notably, C3 served as a key signal that triggered local pro-inflammatory microglial activation and enhanced their phagocytosis of excitatory synapses, ultimately resulting in synaptic loss and cognitive decline. Importantly, both the microglial inhibitor minocycline and liver-specific C3 knockdown suppressed pro-inflammatory microglial activation and restored synaptic plasticity and cognitive function. These findings systematically reveal a novel liver–brain axis in O 3 neurotoxicity, whereby peripheral C3 drives central microglial phagocytosis of excitatory synapses, offering new mechanistic insights and potential therapeutic targets for O 3 -related neurological diseases.

We present the calibration procedures and validation of source measurement with the data of the Commensal Radio Astronomy FAST Survey for H i intensity mapping by the Five-hundred-meter Aperture Spherical Radio Telescope. Using a 70 hr drift-scan observation with the L-band (1.05–1.45 GHz) 19 beam receiver, we obtain the data covering a 270 deg2 sky area. We employ both the pulsar backend and the spectrum backend to calibrate the spectral time-ordered data (TOD) before projecting them onto HEALPix maps. We produce calibrated TOD with a frequency resolution of 30 kHz and time resolution of 1 s and the map data cube with a frequency resolution of 30 kHz and spatial resolution of 2.95 arcmin2. We examine the pointing errors, noise overflow, radio-frequency interference (RFI) contamination, and their effect on the data quality. The resulting noise level is ∼5.7 mJy for the calibrated TOD and 1.6 mJy for the map, consistent with the theoretical predictions within 5% at RFI-free channels. We also validate the data by principal component analysis and find that the residual map looks thermal noise dominated after removing 30 modes. We identify 447 isolated bright continuum sources in our data matching the NRAO VLA Sky Survey catalog, with relative flux error of 8.3% for TOD and 6.6% for the map level. We also measure the H i emission of 90 galaxies with redshift z < 0.07 and compare them with H i-MaNGA spectra, yielding an overall relative H i integral flux error of 16.7%. These results provide an important first step in assessing the feasibility of conducting cosmological H i detection with CRAFTS.