Yichao Li

We investigate the relative importance of active galactic nucleus (AGN) feedback and environmental processes using a large sample of H i galaxies from the Arecibo Legacy Fast ALFA Survey (ALFALFA) and FAST All Sky H i Survey (FASHI) surveys. By applying the optical spectroscopy from Sloan Digital Sky Survey DR7/DR8 and the Dark Energy Spectroscopic Instrument survey, we analyse the gas content and physical properties of AGN-hosting galaxies in group environments. Our results show that AGN-hosting galaxies exhibit significantly suppressed star formation rates and H i gas fraction, approximately one order of magnitude lower than star-forming counterparts, regardless of their group-centric position. AGN-hosting satellites exhibit a significant and persistent deficit in both gas fraction and star formation rate relative to normal satellites without an AGN, even at the halo virial radius ( R / R ₁₈₀ ≈ 1). This suggests that cold gas depletion is primarily driven by internal AGN feedback before these galaxies experience intense environmental interactions. The relatively flat radial profiles of gas fraction and specific star formation rate further indicate that the evolution of AGN-hosting satellites is governed by internal physical processes rather than environmental interactions. Moreover, the apparent increase in H i gas at R / R ₁₈₀ < 0.3 is identified as an artifact of beam confusion. We conclude that for the AGN-hosting population, internal feedback is likely the prior quenching mechanism, while environmental effects act as a secondary, subsequent process.

We present the first blind search for OH 18 cm absorption with the Five-hundred-meter Aperture Spherical Telescope (FAST), conducted alongside the H i 21 cm absorption search. Our previous FAST blind H i absorption search identified 34 systems. In this work, we extend the search using 2024 and part of the 2025 CRAFTS and FAST All Sky H i Survey data (394.4 hr and 1622.1 deg ² ) together with FATHOMER observations, yielding four known and three new H i absorbers, for a total of 41 H i absorption systems. We search for OH absorption in 19 H i absorption systems whose OH redshifted frequencies fall within the FAST band. The known OH absorber toward PKS 1413+135 was redetected, making our survey the first blind survey to detect OH absorption. No new OH absorbers were identified. We examine the relationship between N _OH and N _HI , applying survival analysis to account for upper limits. The analysis does not provide statistically significant evidence for either an N _OH – N _HI correlation or redshift evolution of N _OH / N _HI . Finally, spectral stacking sets 3 σ OH column density upper limits of 4.93, 1.64, and 1.72 T _ex / c _f,OH × 10 ¹² cm ⁻² K ⁻¹ for associated, intervening, and combined samples, corresponding to [OH]/[H i ] ratios of −8 , −8 , and −8 , assuming T _ex = 10 K for OH and T _s = 100 K for H i . These results place the strongest constraints to date on the OH content in radio-selected H i absorbers and establish a blind-survey benchmark for future studies of molecular gas in H i -selected systems. They also demonstrate that known H i 21 cm absorbers provide an effective parent sample for systematic OH absorption searches, paving the way for future larger surveys.

We present the beam pattern measurement of the Tianlai Cylinder Pathfinder Array. As it is a pure drift-scan instrument, we exploit the North-South motion of the Sun to demonstrate that the primary beam is factorizable. Leveraging this property, we decompose the primary beam into independent East-West (E-W) and North-South (N-S) components. Using the Sun as a calibration source, we obtain the E-W beam profiles at various elevations, applying normalization to eliminate the effects of solar activity. Subsequently, we simulate the observed signals using a sky map model to derive the best-fit N-S beam. The results of this work are consistent with previous expectations.

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 deg ² 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 ( W ₅₀ ), flux density, and Hi mass for detected galaxies. A total of 702 galaxies are identified with H i mass above 10 ^6.2 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.

The repeating behavior of fast radio bursts (FRBs) is regarded as a key clue to understanding their physical origin, yet reliably distinguishing repeaters from apparent nonrepeaters with current observations remains challenging. Here we propose a physically interpretable and practically quantifiable classification framework based on spectral morphology. Using dimensionality reduction, clustering, and feature-importance analysis, we identify the spectral running r and spectral index γ as the most critical parameters for distinguishing repeaters from apparent nonrepeaters in the CHIME/FRB sample. In the γ– r space, repeaters preferentially occupy regions with steeper, narrower-band spectra, whereas nonrepeaters cluster in flatter, broader-band regions, resulting in a clear density separation. We further construct a probability map in the γ – r space based on Gaussian mixture model posterior analysis, revealing a clear gradient of repetition likelihood from ∼67% in the high-repetition region to ∼7% in the low-repetition region. This model also identifies several apparent nonrepeaters with high inferred repetition probability, highlighting them as priority targets for future monitoring. This framework provides a simple and generalizable tool for assessing repeatability in the CHIME/FRB sample and highlights the diagnostic power of spectral morphology in unveiling FRB origins.

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.

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 ∼ σ _d /c ∼ 10⁻³ due to the velocity dispersion of dark matter, σ _d . 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 g _aγγ ≲ 2 × 10⁻¹⁰ GeV⁻¹ in the mass ranges of 8.7 μeV ≤ m _a ≤ 9.44 μeV and 10.85 μeV ≤ m _a ≤ 12.01 μeV. These results provide a stronger constraint on g _aγγ 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.