Research programme

Research

I use neutral atomic hydrogen, molecular gas, and multiwavelength observations to understand how galaxies exchange matter with their surroundings.

How does cold gas travel through the environments that shape galaxy evolution?

My work approaches this question across an unusually broad range of physical scales. I map faint H I around nearby galaxies and groups, resolve cold clouds in the Milky Way halo, and connect those reservoirs to star formation and black-hole activity with CO, optical spectroscopy, and ultraviolet absorption.

Galaxy environments & the CGM

Diffuse neutral gas beyond galaxies

DECaLS g, r, and z composite image of HCG 100 and its surrounding galaxies — **Optical RGB**DECaLS *g/r/z* view of the 42-arcmin field. Add H I

False-colour image of diffuse FAST H I emission over the optical field around HCG 100 — Optical **H I false colour**Diffuse FAST emission reveals the extended neutral-gas envelope. Fig. 1

Published HCG 100 map with FAST H I contours, VLA contours, observing footprints, source labels, and velocity-gradient arrows — H I **Published map**FAST and VLA contours, survey footprints, sources, and coherent velocity gradients.

Swipe, scroll, or use the arrows to move through the comparison.

Neutral hydrogen becomes exceptionally difficult to detect once it spreads beyond galactic disks. I use ultra-deep FAST mapping to reach this low-column-density regime and reveal gas that conventional surveys miss.

In HCG 100, our observations uncover a coherent H I envelope extending over roughly one megaparsec. The structure connects the compact group to neighboring galaxies and contains about 1.4 × 10¹⁰ M_☉ of diffuse neutral gas. Its origin—long-lived tidal debris, accretion from the cosmic web, or a combination—offers a direct test of baryon cycling in dense environments.

Yu et al. 2026, ApJL arXiv

Explore FAST coverage & sensitivity

~1 Mpc: diffuse H I envelope
~0.6 Mpc: coherent velocity gradient
40–50%: of H I outside galaxies

Interacting galaxies

Cold gas through the merger sequence

A merger-driven gas-cycle diagram connecting ionized circumgalactic gas, atomic hydrogen, molecular hydrogen, star formation, and AGN activity — Cold-gas cycling through galaxy interactions: H I, H₂, star formation, AGN activity, and the ionized CGM.

Galaxy encounters rearrange angular momentum and compress gas, but the response of the atomic and molecular reservoirs changes from first approach to coalescence. I introduced kinematic asymmetry as an empirical merger-stage indicator and compared paired galaxies with carefully matched isolated controls.

Combining MaNGA integral-field spectroscopy with FAST H I and IRAM 30-m CO observations, I find that close passages can enhance star formation efficiency even when the total atomic reservoir changes only modestly. The current programme extends this picture from galaxy pairs to compact groups, where repeated interactions act over longer timescales.

H I study · ApJ 2022 CO study · ApJS 2024

Black-hole ecosystems

H I absorption, AGN feeding & feedback

SDSS images and FAST H I absorption spectra with Gaussian decompositions for J1332+2634, J1512+3050, and J1628+2529 — Host galaxies and decomposed FAST H I absorption profiles from the pilot survey (Yu et al. 2023, Fig. 4).

Absorption against compact radio continuum sources can detect cold atomic gas independently of distance. My FAST programme extends this technique to 159 low-power radio sources; across the pilot and completed survey, we identify 15 absorbers and an overall detection rate of about 10%.

The resolved line profiles distinguish regular rotating structures from disturbed, redshifted or blueshifted gas. Most low-power systems are narrow and close to systemic velocity, while comparison with higher-power samples shows an increasing fraction of blueshifted absorbers—evidence that stronger radio activity more effectively drives atomic outflows.

Pilot survey · ApJ 2023 Full survey · Su et al. 2026

Corresponding author and FAST PI for the full survey.

The Milky Way laboratory

High-velocity clouds in a multiphase halo

Our location inside the Milky Way offers a close-up view of processes that are unresolved in external galaxies. I use sensitive FAST and GBT H I mapping to find and characterize faint high-velocity clouds, then compare them with ultraviolet metal-line absorption and hydrodynamical predictions.

Two connected directions anchor this work: tracing how the Magellanic Stream mixes with the Galactic circumgalactic medium, and identifying cool clouds embedded in the Milky Way's nuclear outflow. Together they reveal how cold material survives, fragments, and exchanges mass with a hot wind.

EmissionDeep H I mapping and cloud finding
AbsorptionUV ions along background sightlines
ModellingPPV structure and cloud evolution

Selected work

Publications

Complete list on ADS

2026
Megaparsec-Scale Neutral Hydrogen Flows in the Neighborhood of Hickson Compact Group 100
Yu et al. · The Astrophysical Journal Letters
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2026
A FAST Survey of H I Absorption in Low-power Radio Sources
Su, Yu^† et al. · The Astrophysical Journal · ^†Corresponding author
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2024
CO Observations of Early–mid Stage Major Mergers in the MaNGA Survey
Yu et al. · The Astrophysical Journal Supplement Series
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2023
H I Absorption in Low-power Radio AGNs Detected by FAST
Yu et al. · The Astrophysical Journal
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2022
On the H I Content of MaNGA Major Merger Pairs
Yu et al. · The Astrophysical Journal
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