Qingze Chen, Jie Wang, Yingjie Jing, Chen Xu, Tiantian Liang, Zerui Liu, Zhipeng Hou, Yiwei Xu, Cheng Cheng, Wei Du
Context. We report the discovery of a pair of H I clouds in the vicinity of M51 (NGC 5194) using the FAST telescope. These clouds, which share comparable properties and do not display any optical counterpart, are potential candidates for reionisation-limited H I cloud (RELHIC) modelling. Aims. We characterised this newly discovered pair and assessed whether their physical characteristics align with predictions from the RELHIC model. Methods. We performed a systematic search for compact H I sources in the deep H I observations from the FAST Extended-Atlas-of-Selected-Targets Survey (FEASTS) using S O F I A. The resulting catalogue was then cross-matched with the DESI Legacy Imaging Surveys to remove all objects with optical counterparts. Candidate RELHICs were subsequently modelled as hydrostatic H I structures residing in NFW halos and assessed using TNG50 simulations. Results. We report the detection of two H I clouds (Cloud S and Cloud N) located in the outer regions of the M51 system, at projected separations of 70–90 kpc from its centre. Each cloud has an H I mass of ∼10 6.5 M ⊙ and a velocity dispersion of ∼20 km s −1 . Both clouds lack any observable optical counterpart down to a g -band surface brightness limit of ∼27.5 mag arcsec −2 . Their stellar luminosities have been constrained to below 10 5 L ⊙ . The measured H I characteristics are compatible with RELHIC model predictions and based on our fiducial model, they correspond to characteristic host dark matter halo masses of 3.7 ± 0.4 × 10 9 M ⊙ . Conclusions. Cloud S and Cloud N are promising, but not definitive, RELHIC candidates. A tidal origin remains a serious alternative with respect to the interacting environment of M51, particularly since both clouds are unresolved by FAST and the fact that Cloud N might, in fact, display an internal velocity gradient. Their H I masses and line widths are compatible with RELHIC predictions, but future high-resolution interferometric observations will be essential to distinguish among these possibilities.