Detections of 21 cm absorption with a blind FAST survey at z ≤ 0.09

Abstract

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 Ts/cf = 100 K, the total completeness-corrected comoving absorption path length spanned by our data and sensitive to Damped Lyman α Absorbers (DLAs; NH I ⩾ 2 × 1020 cm−2) is ΔXinv = 8.33 × 103 (Δzinv = 7.81 × 103) for intervening absorption. For associated absorption, the corresponding value is ΔXasc = 1.28 × 101 (Δzasc = 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.