The FAST H <scp>i</scp> 21 cm Absorption Blind Survey. II. Statistical Exploration for Associated and Intervening Systems

Abstract We present an extragalactic H i 21 cm absorption lines catalog from a blind search at z ≤ 0.35, using drift-scan data collected in 1325.6 hr by the ongoing Commensal Radio Astronomy Fast Survey and FAST All Sky H i Survey, which spans a sky area of 6072.0 deg 2 and covers 84,533 radio sources with a flux density greater than 12 mJy. Fourteen previously identified H i absorbers and 20 newly discovered H i absorbers were detected, comprising 15 associated systems, 10 intervening systems, and nine systems with undetermined classifications. Through spectral stacking, the mean peak optical path, mean velocity-integrated optical path, mean FWHM, and mean H i column density are measured to be 0.47 and 0.30; 27.19 and 4.36 km s −1 ; 42.61 and 9.33 km s −1 ; 0.49 and 0.08 T s × 10 20 cm −2 K −1 , for the associated and intervening samples, respectively. Statistical analysis also reveals that associated systems tend to be hosted by red ( g − r &gt; 0.7) galaxies at lower redshifts, whereas galaxies hosting intervening H i absorption are typically found at higher redshifts and are of a bluer ( g − r ≤ 0.7) type. A noticeable difference is observed in the positions of foregrounds, backgrounds of intervening systems, and high-redshift and low-redshift associated systems on the Wide-field Infrared Survey Explorer color–color diagram. All identified foreground sources in our sample have W1 – W2 magnitudes below 0.8, suggesting no active galactic nuclei (AGNs). In contrast, backgrounds of intervening systems tend to have W1 – W2 magnitudes above 0.8, indicating AGN presence. For associated absorption, most low-redshift ( z ≤ 0.5) systems show W1 – W2 values below 0.8, while higher-redshift associated absorption ( z &gt; 0.5) displays a broader range of W1 − W2 values.