The FAST HI 21-cm Absorption Blind Survey. III. OH Absorption Search in the 21-cm Absorber Sample and Continued HI Absorption Search
Abstract
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 deg2) together with FATHOMER observations, yielding four known and three 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 NOH and NHI, applying survival analysis to account for upper limits. The analysis does not provide statistically significant evidence for either an NOH-NHI correlation or redshift evolution of NOH/NHI. Finally, spectral stacking sets 3σ OH column density upper limits of 4.93, 1.64, and 1.72 Tex/cf,OH×1012cm−2K−1 for associated, intervening, and combined samples, corresponding to [\rm{OH}]/[\rm{HI}] ratios of <1.66×10−8, <1.42×10−8, and <0.90×10−8, assuming Tex=10K for OH and Ts=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.
Versions
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BibTeX
@misc{hu2026fast,
title = {{The FAST HI 21-cm Absorption Blind Survey. III. OH Absorption Search in the 21-cm Absorber Sample and Continued HI Absorption Search}},
author = {Wenkai Hu and Yougang Wang and Jeremy Darling and Zheng Zheng and James R. Allison and Elaine M. Sadler and Wenxiu Yang and Yichao Li and Yidong Xu and Jie Wang and Fengquan Wu and Di Li and Ming Zhu and Xuelei Chen},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2607.15595},
eprint = {2607.15595},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2607.15595},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The FAST HI 21-cm Absorption Blind Survey. III. OH Absorption Search in the 21-cm Absorber Sample and Continued HI Absorption Search
AU - Wenkai Hu
AU - Yougang Wang
AU - Jeremy Darling
AU - Zheng Zheng
AU - James R. Allison
AU - Elaine M. Sadler
AU - Wenxiu Yang
AU - Yichao Li
AU - Yidong Xu
AU - Jie Wang
AU - Fengquan Wu
AU - Di Li
AU - Ming Zhu
AU - Xuelei Chen
PY - 2026
DA - 2026/10/09
PB - arXiv
DO - 10.48550/arxiv.2607.15595
UR - https://arxiv.org/abs/2607.15595
AB - 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 four known and three 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 [\rm{OH}]/[\rm{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.
ER -