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Peer-reviewedCC BY 4.0The Astrophysical Journal Supplement Series

FASHI DR2: A Catalog of 132 Low-redshift H I 21 cm Absorption Systems

  • Chuan-Peng Zhang1,2
  • Ming Zhu1,2
  • Peng Jiang1,2
  • Hong Guo1
  • Yizhou Gu3
  • Cheng Cheng4
  • Jin-Long Xu1,2
  • Nai-Ping Yu1,2
  • Xiao-Lan Liu1,2
  • Bo Zhang1,2
4 affiliations
  1. 1Chinese Academy of Sciences
  2. 2Guizhou University
  3. 3Shanghai Jiao Tong University
  4. 4Chinese Academy of Sciences South America Center for Astronomy
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Abstract

We present an untargeted survey of 21 cm H i absorption systems based on the second data release of the FAST All Sky H I survey (FASHI DR2), covering approximately 19,500 deg ² at z ≲ 0.09. A total of 132 H i absorbers are identified, including approximately 60 new discoveries, forming one of the largest homogeneous samples of low-redshift H i absorbers assembled to date. The sample extends to continuum flux densities as low as 2.6 mJy, substantially below the limits of previous flux-limited surveys. The absorber population is dominated by narrow systems ( W ₅₀ < 100 km s ⁻¹ ), while broad absorbers ( W ₅₀ > 200 km s ⁻¹ ) account for 13.6% of the sample. Most absorbers are optically thin, with a median optical depth of τ HI ≈ 0.14. The velocity-offset distribution is broadly symmetric about the systemic velocities of the host galaxies. The associated absorbers are preferentially found in massive, actively star-forming galaxies. We find tentative evidence for a weak anticorrelation between H i column density and stellar mass, although the relation exhibits substantial scatter. These results provide the first statistical characterization of the low-redshift H i absorber population based on the FASHI DR2 sample and establish a valuable benchmark for future H i absorption surveys with next-generation radio facilities.

Versions

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Cite

BibTeXRIS
BibTeX
@article{zhang2026fashi,
  title = {{FASHI DR2: A Catalog of 132 Low-redshift H I 21 cm Absorption Systems}},
  author = {Chuan-Peng Zhang and Ming Zhu and Peng Jiang and Hong Guo and Yizhou Gu and Cheng Cheng and Jin-Long Xu and Nai-Ping Yu and Xiao-Lan Liu and Bo Zhang},
  journal = {The Astrophysical Journal Supplement Series},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4365/aea991},
  url = {https://iopscience.iop.org/article/10.3847/1538-4365/aea991},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - FASHI DR2: A Catalog of 132 Low-redshift H I 21 cm Absorption Systems
AU  - Chuan-Peng Zhang
AU  - Ming Zhu
AU  - Peng Jiang
AU  - Hong Guo
AU  - Yizhou Gu
AU  - Cheng Cheng
AU  - Jin-Long Xu
AU  - Nai-Ping Yu
AU  - Xiao-Lan Liu
AU  - Bo Zhang
PY  - 2026
DA  - 2026/10/27
JO  - The Astrophysical Journal Supplement Series
DO  - 10.3847/1538-4365/aea991
UR  - https://iopscience.iop.org/article/10.3847/1538-4365/aea991
AB  - We present an untargeted survey of 21 cm H i absorption systems based on the second data release of the FAST All Sky H I survey (FASHI DR2), covering approximately 19,500 deg ² at z ≲ 0.09. A total of 132 H i absorbers are identified, including approximately 60 new discoveries, forming one of the largest homogeneous samples of low-redshift H i absorbers assembled to date. The sample extends to continuum flux densities as low as 2.6 mJy, substantially below the limits of previous flux-limited surveys. The absorber population is dominated by narrow systems ( W ₅₀ < 100 km s ⁻¹ ), while broad absorbers ( W ₅₀ > 200 km s ⁻¹ ) account for 13.6% of the sample. Most absorbers are optically thin, with a median optical depth of τ HI ≈ 0.14. The velocity-offset distribution is broadly symmetric about the systemic velocities of the host galaxies. The associated absorbers are preferentially found in massive, actively star-forming galaxies. We find tentative evidence for a weak anticorrelation between H i column density and stellar mass, although the relation exhibits substantial scatter. These results provide the first statistical characterization of the low-redshift H i absorber population based on the FASHI DR2 sample and establish a valuable benchmark for future H i absorption surveys with next-generation radio facilities.
SN  - 0067-0049
SN  - 1538-4365
ER  -