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

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.