FASHI DR2: A Catalog of 132 Low-Redshift HI 21 cm Absorption Systems
Abstract
We present an untargeted survey of 21 cm HI absorption systems based on the second data release of the FAST All Sky HI survey (FASHI DR2), covering approximately 19,500 deg2 at z≲0.09. A total of 132 HI absorbers are identified, including approximately 60 new discoveries, forming one of the largest homogeneous samples of low-redshift HI 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 (W50<100 km s−1), while broad absorbers (W50>200 km s−1) account for 13.6% of the sample. Most absorbers are optically thin, with a median optical depth of τHI≈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 anti-correlation between HI column density and stellar mass, although the relation exhibits substantial scatter. These results provide the first statistical characterization of the low-redshift HI absorber population based on the FASHI DR2 sample and establish a valuable benchmark for future HI absorption surveys with next-generation radio facilities.
Versions
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BibTeX
@misc{zhang2026fashi,
title = {{FASHI DR2: A Catalog of 132 Low-Redshift HI 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},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.23385},
eprint = {2608.23385},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.23385},
}
RIS
TY - UNPB
TI - FASHI DR2: A Catalog of 132 Low-Redshift HI 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 - 2026/08/26
PB - arXiv
DO - 10.48550/arxiv.2608.23385
UR - https://arxiv.org/abs/2608.23385
AB - We present an untargeted survey of 21 cm HI absorption systems based on the second data release of the FAST All Sky HI survey (FASHI DR2), covering approximately 19,500 deg^2 at z≲0.09. A total of 132 HI absorbers are identified, including approximately 60 new discoveries, forming one of the largest homogeneous samples of low-redshift HI 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 anti-correlation between HI column density and stellar mass, although the relation exhibits substantial scatter. These results provide the first statistical characterization of the low-redshift HI absorber population based on the FASHI DR2 sample and establish a valuable benchmark for future HI absorption surveys with next-generation radio facilities.
ER -