The FAST HI 21-cm absorption blind survey. II -- statistic exploration for associated and intervening systems

This is the authors' abstract. We don't add key points for this paper.

We present an extragalactic HI 21-cm absorption lines catalog from a blind search at z ≤ 0.35, using drift-scan data collected in 1616.9 hours by the ongoing Commensal Radio Astronomy FasT Survey (CRAFTS) and FAST All Sky HI Survey (FASHI), which spans a sky area of 7456.8 deg² and covers 84,533 radio sources with a flux density greater than 12 mJy. 14 previously identified HI absorbers and 20 newly discovered HI absorbers were detected, comprising 14 associated systems, 11 intervening systems, and 9 systems with undetermined classifications. We fit HI profiles with multi-component Gaussian functions and calculate the redshift, width, flux density, optical depth, and HI column densities for each source. Through spectral stacking, the mean peak optical path, mean velocity-integrated optical path τ, mean FWHM and mean HI column density NHI are measured to be 0.46 and 0.34; 25.85 km/s and 4.62 km/s; 39.80 km/s and 8.95 km/s; 0.470 and 0.085 Tₛ × 10²⁰cm⁻²K⁻¹, for the associated and intervening samples, respectively. Statistical analysis also reveals that associated systems tend to be hosted by red (g$-$r$>$0.7) galaxies at lower redshifts, whereas galaxies hosting intervening HI absorption are typically found at higher redshifts and are of a bluer (g$-$r≤0.7) type. Additionally, it has been demonstrated that associated HI 21-cm absorptions connected to compact radio sources display higher NHI values compared to those linked with extended radio sources.