Pulsars, as extreme celestial objects with exceptional physical properties, serve as vital tools for probing fundamental physics and astrophysical phenomena. The Nanshan 25 m Radio Telescope, a core facility of China's Very Long Baseline Interferometry Network, possesses unique geographical and hardware advantages for pulsar observations, particularly in long-term timing monitoring and transient detection. Based on its long-term (2002-2009) timing data targeting 74 bright pulsars (delta > -40 degrees), the database and data retrieval environment compliant with the standards of a Virtual Observatory (VO) have been constructed in this work, which implements the functions of cone search and multi-object constraint search. The retrieval results can be transmitted to standard VO tools via the Simple Application Messaging Protocol for further data processing. For the archived pulsar data, the typical sampling rate during observations is 1 ms, and the duration of each observation is determined by the flux density of the pulsar in the corresponding observation band, generally ranging from 2 to 16 minutes. All data have undergone preprocessing using standard software packages including PSRchive and TEMPO, with the data format unified as PSRFITS and the file extension designated as .FTp. The database supports two access modes, namely web-based online retrieval and VO tool-based retrieval, facilitating flexible data access to meet diverse research requirements. As a valuable supplement to the global pulsar data resources, the dataset released in this work can be applied to carry out research on pulsar-related topics.

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 <1.66 × 10 ^−8 , <1.42 × 10 ^−8 , and <0.90 × 10 ^−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.

The faint 21 cm signal emitted by neutral hydrogen in cosmic filaments is expected to be detectable. However, due to its weakness, stacking techniques are required. We assessed two stacking methods—pair stacking and filament stacking—using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) and IllustrisTNG simulations. Pair stacking leverages the fact that cosmic filaments connect massive structures (i.e., knots) in the cosmic web, while filament stacking directly aggregates filaments identified from galaxy distributions. Our analysis indicates that, although pair stacking is convenient, it faces contamination from massive structures; after removing this contamination, the filament signal is significantly reduced. In contrast, H I detection via filament stacking appears more promising. The column density in filament stacking reaches ∼10 ^16 –10 ^17 cm ^−2 , even when all halos are masked, whereas pair stacking does not reach this level, even without masking, and is further suppressed by several orders of magnitude once masking is applied. The effectiveness of filament stacking can be further improved with higher galaxy number density and better spatial resolution in radio intensity mapping observations. With the advent of upcoming optical and radio data, the detection of H I in cosmic filaments remains promising.

The FAST All Sky HI Survey (FASHI) conducted with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) has mapped ∼ 19500 deg^2 of the sky north of DEC = -14^∘, detecting 156411 extragalactic HI sources at z< 0.09 with a median sensitivity of 0.57 mJy beam^-1 at a velocity resolution of 6.4 km s^-1. The survey achieves unprecedented depth and area coverage, significantly improving upon previous single-dish surveys. Through a detailed completeness analysis that accounts for the survey's non-uniform sensitivity and line-width dependence, we construct a robust HI mass function (HIMF) using a completeness-corrected sample of over 109000 sources. The HIMF is robustly constrained down to M_HI∼ 10^6.2 M_⊙. When systematic uncertainties are included, the HIMF is well described by a single-Schechter function with a characteristic mass log (M_* / h_70^-2M_⊙) = 9.89± 0.02, low-mass end slope α= -1.31± 0.02, and amplitude ϕ_* = (6.38± 0.49)× 10^-3 h_70^3 Mpc^-3 dex^-1. The derived cosmic HI density is Ω_HI = (4.71± 0.03_stat± 0.40_sys)× 10^-4 h_70^-1. FASHI provides the most extensive and sensitive HI catalog to date, establishing an important benchmark for studies of gas accretion, galaxy evolution, and large-scale structure in the local universe.

The Hundred-deg2 H I Deep (HD2) survey carried out with the Five-hundred-meter Aperture Spherical Telescope is planned to map a contiguous region within the DESI DR1 footprint, achieving an effective integration time of 20 minutes for each pointing and a uniform detection sensitivity of 0.28 mJy beam(-1) at 4.8 km s(-1) resolution. We present early results from the pilot HD2 survey: a 10 deg(2) field overlapping with the Hyper Suprime-Cam Subaru Strategic Program and the DESI early data release SV3, observed with an integration time of 7.3 minutes beam(-1) and an rms of 0.45 mJy beam(-1) at 4.8 km s(-1) resolution. We identify 339 H I sources at z < 0.09, corresponding to similar to 34 detections per deg2, nearly 6 times higher than the detection rate of the wide-field surveys. Optical counterparts are primarily identified using DESI redshifts, yielding a matching rate and correctness exceeding 90% for galaxies with r < 19.5 mag, a substantial improvement over the Sloan Digital Sky Survey. Under the constraint of r < 17.8 mag and 0.01 < z < 0.05, nearly 50% of galaxies in the DESI Bright Galaxy Survey samples have H I detections in this pilot survey. The optical properties of these H I-detected galaxies span nearly the entire parameter range of the DESI sample. The gas fraction scaling relations versus stellar mass, stellar mass surface density, near-UV (NUV) - r, and specific star formation rate are consistent with previous surveys, e.g., ALFALFA, DINGO, and xGASS. These results justify the feasibility of the full HD2 survey, which will build a high-completeness H I census over a contiguous area to probe the cold gas scaling relations of galaxies over different scales.

We present the first blind search for OH 18-cm absorption with the Five-hundred-meter Aperture Spherical Telescope (FAST), conducted alongside the HI 21-cm absorption search. Our previous FAST blind HI absorption search identified 34 systems. In this work, we extend the search using 2024 and part of the 2025 CRAFTS and FASHI data (394.4 hr and 1622.1 deg^2) together with FATHOMER observations, yielding three known and four new HI absorbers, for a total of 41 HI absorption systems. We search for OH absorption in 19 HI absorption systems whose OH redshifted frequencies fall within the FAST band. The known OH absorber towards PKS 1413+135 was re-detected, 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 3σ OH column density upper limits of 4.93, 1.64, and 1.72 T_ex/c_f,OH×10^12cm^-2K^-1 for associated, intervening, and combined samples, corresponding to [OH]/[HI] ratios of <1.66×10^-8, <1.42×10^-8, and <0.90×10^-8, assuming T_ex=10K for OH and T_s=100K for HI. These results place the strongest constraints to date on the OH content in radio-selected HI absorbers and establish a blind-survey benchmark for future studies of molecular gas in HI-selected systems. They also demonstrate that known HI 21-cm absorbers provide an effective parent sample for systematic OH absorption searches, paving the way for future larger surveys.

Radio telescopes are susceptible to radio frequency interference (RFI) during observations, which introduces noise and artificial signals into astronomical data. Failure to properly process RFI-contaminated data can severely compromise data reliability and even lead to erroneous scientific conclusions. Consequently, RFI identification and mitigation have become critical scientific challenges in radio astronomy. This study proposes an RFI recognition and data cleaning method based on a deep learning image classification algorithm (RFI-MobileNetV2, RFI-MN). By performing feature extraction and preliminary classification on visual astronomical data, the method aims to effectively mitigate RFI, improve the data signal-to-noise ratio, and provide more reliable data support for astronomical research. Using the lightweight convolutional neural network MobileNetV2 as the core architecture, the model achieves efficient extraction and classification of diverse RFI features, thereby providing an effective solution for subsequent RFI suppression tasks. For the experiments, a dataset containing multiple RFI types was constructed as a training sample. During model optimization, performance was significantly enhanced through the integration of an attention mechanism, modification of the activation function in inverted residuals, and multiscale feature fusion. Evaluation metrics including precision, recall, and F1 score were employed to verify the scheme’s effectiveness through baseline training, inference, multimodel comparison, and ablation experiments. The results demonstrate that the optimized RFI-MN achieves performance exceeding 94%, substantially outperforming other comparative models and providing an effective solution for RFI-affected astronomical data preprocessing.

Stray radiation is a considerable challenge for radio telescopes, requiring careful assessment of its effects. This is crucial when the strong background flux from side lobes significantly affects the total flux, especially for extended sources. In this study, we introduced the beam pattern of the L-band receiver on the Five-hundred-meter Aperture Spherical Telescope (FAST), covering various frequencies based on recent observations. We discovered that the main beam efficiency of all beams exceeds 90% throughout the L band frequencies, with efficiency decreasing slowly as frequency increases. Subsequently, we developed a module to mitigate stray radiation effects, incorporating it into FAST's standard HI data reduction process, referred to as HiFAST. Our analysis shows that side lobe flux's influence, particularly for extended sources with significant surface density gradients, necessitates detailed evaluation. Corrections for the extended M33 galaxy can reach up to 20%. Moreover, the pattern data presented here is vital for studying HI intensity maps at high redshift. The module, along with HiFAST and beam pattern data across 15 frequency bins, can be accessed at https://hifast.readthedocs.io. The datasets of beam pattern presented in this paper are openly available at https://doi.org/10.57760/sciencedb.j00113.00266.

We present a deep learning framework to reconstruct the three-dimensional cosmic dark matter density field from observed halo distributions. By training a feed-forward neural network (FNN) on the Millennium Simulation, we establish a nonlinear mapping from the halo group catalogs to the underlying dark matter density. At spatial resolutions of (10, 5, 2) h(-1) Mpc, our method significantly outperforms the standard halo cloud-in-cell approach, especially in low-density regions where halo tracers are sparse. Our scheme could recover both the density probability distribution function and the power spectrum statistics as well. The FNN achieves reconstruction errors of less than half an order of magnitude across the entire density dynamic range, demonstrating robust generalization across cosmic environments. These results establish a foundation for sparse galaxy tracers of the real dark matter density field in the forthcoming large-scale galaxy surveys (DESI, LSST, and Euclid).

The dark matter halo mass function is one of the most fundamental predictions of structure formation theory and cosmological simulations. We present the full halo mass function in the Λ cold dark matter (ΛCDM) model, ranging from a planetary mass (10^-6 M_⊙; the thermal cutoff in the initial power spectrum for a fiducial CDM particle mass of 100 GeV) to the mass of a rich galaxy cluster (10^15.5 M_⊙), and from redshift, z=30 to the present. To span this very large dynamic range, we combine our earlier Voids-within-Voids-within-Voids (VVV) set of simulations (Wang et al) with large volume, lower resolution cosmological simulations. We develop a subsampling method to extract subvolumes from the original simulations, allowing us to reconstruct the global halo mass function from the biased underdense VVV regions. We show that the results agree reasonably well among the sets of simulations on different scales and environments. We provide a fitting formula for the dark matter halo mass function based on the work of Reed et al. calibrated with our simulations, such that it can be applied at all scales, all environments and all times, with deviations of ∼2-3% at z < 2 and ∼ 7% at higher redshift z ≳ 5. This formula is also accurate at least for a restricted set of models we tested with modest deviations from ΛCDM in the values of some of the cosmological parameters. A python code is publicly available at https://github.com/haonan-zheng/hmfc.

Tidal-torque theory predicts that galaxy angular momenta are imprinted by the primordial tidal field acting on proto-structures and that they can retain information about the early Universe through cosmic evolution. Here we test this prediction by comparing observed galaxy angular momentum vectors with those predicted from the primordial density field reconstructed by ELUCID for the nearby Universe. Among the galaxy populations considered, the gas component of central massive elliptical galaxies provides the clearest signal, exhibiting a strong direction correlation at a significance of about 7σ. These results provide robust observational evidence for tidal-torque theory and open a window for cosmological measurements of neutrino mass and other cosmological parameters. Combining galaxy kinematics with a reconstruction of the primordial density field has enabled the detection of a substantial imprint of primordial tidal torques in present-day galaxy spins. This strengthens evidence that the angular momentum of galaxies originates from large-scale tidal forces on their proto-halos at very early times.

Aims. This paper aims to investigate the galaxy-halo connection using a large sample of individual galaxies with H I-integrated spectra. We determined their dark matter content by applying a dynamical method based on H I line widths measured with the curve-of-growth technique, together with inclination corrections inferred from optical images. Methods. We built a sample of 2453 gas-rich, predominantly late-type galaxies spanning a stellar mass range of 10(8.7) M-circle dot to 10(11.4) M-circle dot by matching them one-to-one with their counterparts from the ALFALFA survey and the TNG100 simulation, ensuring a direct match of stellar mass and H I radius. We generated mock images and mock H I-integrated spectra for TNG100 galaxies, and applied the same dynamical method to both ALFALFA and TNG100 mock galaxies to infer their dark matter masses.. Results. Across all stellar mass bins, ALFALFA galaxies exhibit lower median dark matter masses than the mock TNG100 simulation results. In each bin, this offset is driven by a tail of galaxies with comparatively low dark matter content, which becomes more prominent toward higher stellar masses. In the highest mass bin (M-* > 10(11) M-circle dot), late-type ALFALFA galaxies show a median dark matter mass that is 23% lower than that of their counterparts in the TNG100 dark-matter-only simulation, with 32% of ALFALFA galaxies having M-DM(< R-HI) < 10(11.5) M-circle dot, compared to 17% in the mock TNG100 sample. These results suggest that a larger fraction of massive late-type galaxies reside in relatively less massive dark matter haloes than predicted by the TNG100 simulation.

The impact of cosmic web environments on galaxy properties plays a critical role in understanding galaxy formation. Using the state-of-the-art cosmological simulation IllustrisTNG, we investigate how satellite galaxy abundance differs between filaments and the field, with filaments identified using the DisPerSE algorithm. When filaments are identified using galaxies as tracers, we find that, across all magnitude bins, central galaxies in filaments tend to host more satellite galaxies than their counterparts in the field, in qualitative agreement with observational results from the Sloan Digital Sky Survey. The average ratios between satellite luminosity functions in filaments and the field are 3.49, 2.61, and 1.90 in the central galaxy r -band magnitude bins of M _r _,cen ∼ −22, −21, and −20, respectively. We show that much of this excess can be attributed to the higher host halo masses of galaxies in filaments. After resampling central galaxies in both environments to match the halo mass distributions within each magnitude bin, the satellite abundance enhancement in filaments is reduced by up to 79%. Additionally, the choice of tracers used to identify filaments introduces a significant bias: when filaments are identified using the dark matter density field, the environmental difference in satellite abundance is reduced by more than 70%; after further resampling in both magnitude and halo mass, the difference is further suppressed by another ∼60%–95%. Our results highlight the importance of halo mass differences and tracer choice biases when interpreting and understanding the impact of environment on satellite galaxy properties.

Real-time processing of ultrawideband pulsar data requires handling massive parallel data streams that exceed single-machine capacity. We designed and implemented Ultra-wideband Radio Astronomy Novel Utility for Shared Ring Buffer Pipeline (URANUS) to support the multifunctional, real-time processing of ultrawideband, multisub-band pulsar baseband data. We employed the Linked-list Double-buffer Receive Packet (LDRP) algorithm to address the challenges of real-time reception and alignment of 4/3 oversampled dual-polarization data. The data are formatted using the Very Long Baseline Interferometry Data Interchange Format (VDIF). LDRP facilitates real-time reception of dual-polarization VDIF packets and aligns them by parsing the VDIF header for time-stamp information. In the event of packet loss, LDRP performs frame padding (zero-filling) to prevent downstream errors. We constructed a multifunctional data processing pipeline using a shared ring buffer, decomposing complex operations into multiple processes with efficient data transfer through the shared buffer. Currently, URANUS supports fold, search, and baseband pulsar observation modes. We observed multiple pulsars using the C-band receiver (4468-7796 MHz) at the NanShan 26 m Radio Telescope. Each sub-band was correctly folded to produce pulse profiles, and broadband profiles synthesized from multiple sub-bands were accurately aligned. These results demonstrate URANUS is ready for deployment on the QiTai Radio Telescope ultrawideband low-frequency receiving system, where it will enable high-precision pulsar timing and real-time transient searches.

We report the discovery of a pair of H I clouds near M51 (NGC 5194) using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). These clouds have no optical counterparts and are potential candidates for Reionization-Limited H I Clouds (RELHICs). We search for compact H I sources in deep FEASTS observations using SoFiA and remove objects with optical counterparts through cross-matching with the DESI Legacy Imaging Surveys. The remaining candidates are modelled as hydrostatic H I structures embedded in Navarro-Frenk-White dark matter haloes and compared with RELHIC predictions and TNG50 simulations. We identify two H I clouds, Cloud S and Cloud N, at projected distances of 70–90 kpc from M51. Each cloud has an H I mass of approximately 10^6.5 solar masses, a velocity dispersion of about 20 km/s, and no detectable optical counterpart down to a g-band surface brightness limit of approximately 27.5 mag arcsec^-2. Their stellar luminosities are constrained to be below 10^5 solar luminosities. Their H I properties are consistent with RELHIC predictions, corresponding to host halo masses of 3.7 +- 0.4 * 10^9 solar masses. Cloud S and Cloud N are promising but not definitive RELHIC candidates. A tidal origin remains possible in the interacting M51 system, especially because the clouds are unresolved by FAST and Cloud N may show a velocity gradient. Future high-resolution interferometric observations will be crucial for distinguishing between starless dark matter haloes and tidal debris.

Aims. This paper aims to investigate the galaxy-halo connection using a large sample of individual galaxies with H I-integrated spectra. We determined their dark matter content by applying a dynamical method based on H I line widths measured with the curve-of-growth technique, together with inclination corrections inferred from optical images. Methods. We built a sample of 2453 gas-rich, predominantly late-type galaxies spanning a stellar mass range of 108.7 M⊙ to 1011.4 M⊙ by matching them one-to-one with their counterparts from the ALFALFA survey and the TNG100 simulation, ensuring a direct match of stellar mass and H I radius. We generated mock images and mock H I-integrated spectra for TNG100 galaxies, and applied the same dynamical method to both ALFALFA and TNG100 mock galaxies to infer their dark matter masses. Results. Across all stellar mass bins, ALFALFA galaxies exhibit lower median dark matter masses than the mock TNG100 simulation results. In each bin, this offset is driven by a tail of galaxies with comparatively low dark matter content, which becomes more prominent toward higher stellar masses. In the highest mass bin (M* > 1011 M⊙), late-type ALFALFA galaxies show a median dark matter mass that is 23% lower than that of their counterparts in the TNG100 dark-matter-only simulation, with 32% of ALFALFA galaxies having MDM(< RHI) < 1011.5 M⊙, compared to 17% in the mock TNG100 sample. These results suggest that a larger fraction of massive late-type galaxies reside in relatively less massive dark matter haloes than predicted by the TNG100 simulation.

Filaments are crucial components of the cosmic web, representing the extensive and aligned distributions of galaxies and gas. Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we report the detection of a filament in the Ursa Major supergroup using atomic-hydrogen (HI) observations. This filament consists of sixteen various types of galaxies and five starless gas clumps, spanning a length of approximately 0.9 Mpc. Notably, it is extremely thin, with a thickness comparable to the diameter of a galaxy. We observed a galaxy-filament spin alignment and a velocity gradient within the filament. These findings strongly suggest a cold accretion flow along the filament, potentially contributing to the formation and growth of the galaxies. The thin filament, as a small group, is likely to be merged into the Ursa Major supergroup in the context of hierarchical structure formation.

The Hundred-deg^2 HI Deep (HD^2) survey carried out with the Five-hundred-meter Aperture Spherical Telescope (FAST) is planned to map a contiguous region within the DESI DR1 footprint, achieving an effective integration time of 20 minutes for each pointing and a uniform detection sensitivity of 0.28 mJy beam^-1 at 4.8 km s^-1 resolution. We present early results from the pilot HD^2 survey: a 10 deg^2 field overlapping with HSC-SSP and the DESI EDR SV3, observed with an integration time of 7.3 minutes per beam and the rms of 0.45 mJy beam^-1 at 4.8 km s^-1 resolution. We identify 339 HI sources at z<0.09, corresponding to ∼34 detections per deg^2, nearly six times higher than the detection rate of the wide-field surveys. Optical counterparts are primarily identified using DESI redshifts, yielding a matching rate and correctness exceeding 90

With the continuous increase in the observational bandwidth of radio telescopes, digital backend are facing new challenges in real-time control, heterogeneous resource management, and system stability. To address the requirements of ultrawideband radio observations for low latency, high concurrency, and visualization capabilities, this paper presents the design and implementation of an ultrawideband digital backend visualization control system tailored for heterogeneous distributed environments. The system adopts a layered and decoupled architecture to centrally manage FPGA front-end devices, GPU computing nodes, and general-purpose computing resources, while containerization technologies are employed to enable standardized deployment of data processing tasks. At the communication layer, a gRPC framework based on HTTP/2 and Protocol Buffers is introduced to reduce the serialization and transmission overhead of monitoring data and control signals. Through comparative experiments on end-to-end latency and high-concurrency throughput, the performance characteristics of gRPC and traditional REST interfaces are quantitatively evaluated under varying payload sizes and concurrency levels. The proposed system has been deployed in the C-band (4468-7796 MHz) receiver system of the Nanshan 26 m Radio Telescope and validated through practical observations in pulsar timing, pulsar searching, and baseband observation modes. Experimental results demonstrate that the system operates stably under real ultra-wideband observing conditions, supports multi-sub-band data processing and real-time visualization monitoring, and provides an effective engineering reference for the implementation of ultrawideband radio telescope digital backend systems.