2023A&A...675A..40H

We present the early science results from a blind search of the extragalactic H I 21 cm absorption lines at z ≤ 0.09 with the drift-scan observation of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We carried out the search using the data collected over 643.8 h by the ongoing Commensal Radio Astronomy FasT Survey (CRAFTS), which spans a sky area of 3155 deg² (∼81% of CRAFTS sky coverage up to January 2022) and covers 44827 radio sources with a flux density greater than 12 mJy. Due to the radio frequency interference (RFI), only the relatively clean data in the frequency range of 1.3-1.45 GHz are used in the present work. Under the assumption of T_s/c_f = 100 K, the total completeness-corrected comoving absorption path length spanned by our data and sensitive to Damped Lyman α Absorbers (DLAs; N_H I_ ⩾ 2 × 10²⁰ cm^–2) is ΔX^inv = 8.33 × 10³ (Δz^inv = 7.81 × 10³) for intervening absorption. For associated absorption, the corresponding value is ΔX^asc = 1.28 × 10¹ (Δz^asc = 1.19 × 10¹). At each time point of the drift scan, a matched-filtering approach is used to search H I absorbers. Combining the information of observation mode and the distribution of the beams that detect the same candidates, spurious absorbers are successfully excluded. Three known H I absorbers (UGC 00613, 3C 293, and 4C +27.14) and two new H I absorbers (towards the direction of NVSS J231240-052547 and NVSS J053118+315412) are detected blindly. We fitted the H I profiles with multi-component Gaussian functions and calculated the redshift (0.063, 0.066), width, flux density, optical depth, and H I column densities for each absorption. Our results demonstrate the power of FAST in blindly searching H I absorbers. For absorption towards NVSS J231240-052547, the optical counterparts are faint and currently lack existing spectra. The most likely interpretation is that a radio-loud active galactic nucleus (AGN) is faint in the optical as the background source, with a faint optical absorber in between. NVSS J053118+315412 exhibits an associated absorption with a complex profile, which may suggest unsettled gas structures or gas accretion onto the supermassive black hole (SMBH). The expanding collection of blind radio detections in the ongoing CRAFTS survey offers a valuable opportunity to study AGNs, associated interstellar medium (ISM) interaction, and intervening absorbers optically without overwhelming quasi-stellar object (QSO) background light.