Research Overview
My research centers on the baryon cycle across cosmic time—from the initial conditions of high-mass star formation within Milky Way molecular clouds to the large-scale neutral hydrogen (H i) content of galaxies in the nearby Universe. A unifying theme in my work is the use of observational data to understand how gas is accreted, converted, and regulated in galaxies. I have a particular focus on exploiting the unprecedented sensitivity of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) to conduct wide-field H i surveys and to establish a statistical benchmark for galaxy evolution studies.
1. The FASHI Survey: Building the Largest H i Galaxy Database
I am a core leader of the FAST All-Sky H i Survey (FASHI), the most sensitive wide-field H i survey ever conducted (Zhang et al. 2024b). The survey has mapped approximately 19,500 square degrees of the sky and detected over 156,000 extragalactic H i sources at redshifts z < 0.09, with a median sensitivity of 0.57 mJy beam−1 (Zhang et al. 2026a).
FASHI represents a significant advance in our ability to study the atomic gas content of the local Universe. Compared to previous surveys such as ALFALFA, it achieves a five-fold larger sample size and four times greater sensitivity, enabling the first robust measurement of the H i mass function down to 106 M⊙ (Ma et al. 2025; Zhang et al. 2026a). This catalog provides a critical benchmark for studies of gas accretion, galaxy evolution, and large-scale structure. The data reduction is performed using the HiFAST pipeline (Jing et al. 2024), with dedicated RFI mitigation techniques developed for FAST's L-band observations (Zhang et al. 2022).
2. Cosmic H i Density and the Decline of Star Formation
A central question in galaxy evolution is why cosmic star formation has declined so dramatically over the past several billion years while the total H i reservoir remains relatively stable. Using the unique synergy between FASHI and the Dark Energy Spectroscopic Instrument (DESI), I measured the cosmic H i density (ΩH i) over the past 4.5 Gyr with unprecedented precision (Zhang et al. 2026b). By stacking H i spectra from 2.5 million DESI galaxies across 12,000 square degrees, I found that ΩH i declined by only a factor of 1.35 over this period, while the cosmic star formation rate density declined by a factor of 2.46.
Key Insight: This quantitative comparison demonstrates that the late-time decline in star formation is not driven by depletion of the H i reservoir. Instead, the results point to decreasing efficiency in converting H i into molecular hydrogen as the primary cause. This work provides a stringent observational benchmark for models of gas accretion, phase conversion, and star-formation regulation.
3. H i Content in the Cosmic Web and Galaxy Environments
The FASHI catalog also enables statistical studies of how H i traces large-scale structure. FASHI sources broadly follow the filamentary cosmic web seen in optical surveys, yet notable differences exist: optical voids are not devoid of H i galaxies, confirming that H i-selected systems preferentially reside in underdense environments (Zhang et al. 2026a).
I have conducted detailed studies of H i gas in galaxy groups and clusters:
- Our FAST observations of the NGC 5055 group revealed a warped H i disk extending to 61.7 kpc, suggesting ongoing gas stripping (Liu et al. 2024).
- In the Ursa Major cluster, we found a shallower low-mass end slope of the H i mass function, consistent with gas stripping in dense environments (Yu et al. 2025).
These results provide key observational constraints on environmental quenching mechanisms.
4. H i–Stellar Mass Scaling Relations and Their Evolution
Using the cross-matched FASHI-DESI sample of over 64,000 galaxies, I established precise measurements of the H i gas fraction as a function of stellar mass across a broad mass range, from 106 to 1011 M⊙ (Zhang et al. 2026a; Zhang et al. 2026b). The relation exhibits remarkable invariance with redshift, evolving by less than 0.2 dex over the past 4.5 Gyr at fixed stellar mass. This population-wide trend implies that the weak evolution of H i is not the result of compensating changes among different galaxy populations, but rather a fundamental feature of the baryon cycle.
In interacting systems:
- In the NGC 3395/3396 pair, we detected extended gas structures and ring-like tidal features (Yu et al. 2024).
- In the M101 group, we found a significantly extended and asymmetric H i disk (Xu et al. 2021).
These observations highlight the role of interactions in shaping the gas content of galaxies.
5. Discovery of Exotic and Elusive H i Sources
The high sensitivity of FAST enables the detection of rare astrophysical objects. Using FASHI data, I have:
- 🔭 Discovered isolated dark dwarf galaxy candidates (Xu et al. 2023a; Liu et al. 2025);
- 📡 Detected over 120 H i absorption line galaxies, with more than 80 being first detections (Zhang et al. 2025);
- 💥 Identified over 40 OH megamaser sources, with more than 20 being first detections (Zhang et al. 2024a).
These discoveries provide unique laboratories for studying dark matter, cold gas dynamics, and galaxy mergers.
6. Early Massive Star Formation in the Milky Way
Prior to my work on extragalactic H i, I studied the initial conditions of high-mass star formation within the Milky Way. This work provided the foundation for my later focus on the baryon cycle, connecting the earliest phases of star formation to the larger-scale evolution of galaxies. My contributions include:
- Fragmentation of dense cold cores: Using PdBI and VLA interferometric data, I revealed how massive clumps fragment into cores and condensations across different physical scales (Zhang et al. 2014; Zhang et al. 2016a).
- Deuterium chemistry: Through NH2D and other deuterated molecules, I constrained the physical conditions and evolutionary timescales of pre-stellar and protostellar cores (Zhang et al. 2016a).
- Triggered star formation: I investigated how ionizing radiation and stellar winds from massive stars compress molecular gas, triggering new generations of star formation in infrared dust bubbles (Zhang & Wang 2012; Zhang et al. 2013; Zhang et al. 2017).
- Large-scale surveys: I led a subproject of the TOP-SCOPE survey, studying the gas and dust properties of 64 Planck Galactic Cold Clumps (Zhang et al. 2017).
Additional studies on Giant Molecular Clouds (Zhang et al. 2012) and Infrared Dark Clouds (Xu et al. 2016) further contributed to our understanding of the earliest phases of the star formation process.