The FAST HI 21-cm absorption blind survey. II. -- Statistic Exploration for Associated and Intervening systems

TL;DR

This paper presents a catalog of HI 21-cm absorption lines from a large sky survey, analyzing the properties and host galaxy types of associated and intervening systems to understand their distribution and characteristics.

Contribution

It provides the first extensive statistical analysis of HI 21-cm absorption systems from a blind survey, including new detections and insights into their host galaxy properties and redshift distributions.

Findings

Associated systems are hosted by red galaxies at lower redshifts.

Intervening systems are typically found at higher redshifts and in bluer galaxies.

Distinct WISE color signatures differentiate foreground and background sources.

Abstract

We present an extragalactic HI 21-cm absorption lines catalog from a blind search at z $\leqslant$ 0.35, using drift-scan data collected in 1325.6 hours by the ongoing Commensal Radio Astronomy FasT Survey (CRAFTS) and FAST All Sky HI Survey (FASHI), which spans a sky area of 6072.0 deg$^{2}$ and covers 84533 radio sources with a flux density greater than 12 mJy. 14 previously identified HI absorbers and 20 newly discovered HI absorbers were detected, comprising 15 associated systems, 10 intervening systems, and 9 systems with undetermined classifications. Through spectral stacking, the mean peak optical path, mean velocity-integrated optical path, mean FWHM and mean HI 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$_{s} \times$ 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>$0.7) galaxies at lower redshifts, whereas galaxies hosting intervening HI absorption are typically found at higher redshifts and are of a bluer ($g-r\leqslant$0.7) type. A noticeable difference is observed in the positions of foregrounds, backgrounds of intervening systems, and high-redshift and low-redshift associated systems on the WISE color-color diagram. All identified foreground sources in our sample have W1-W2 magnitudes below 0.8, suggesting no Active Galactic Nuclei (AGN). In contrast, backgrounds of intervening systems tend to have W1-W2 magnitudes above 0.8, indicating AGN presence. For associated absorption, most low-redshift ($z\leqslant$0.5) systems show W1-W2 values below 0.8, while higher-redshift associated absorption ($z>$0.5) displays a broader range of W1-W2 values.