FASHI: An untargeted survey of the 21 cm HI absorption galaxies with FAST

Abstract: The FAST All Sky H I survey (FASHI) will cover the entire observable sky ($\sim$22000 square degrees) with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). With the currently released data, we perform an untargeted survey of 21 cm HI absorption galaxies at redshift $z\lesssim0.09$ over an area of about 10000 square degrees. We have detected 51 HI absorbers, including 21 previously known and 30 new ones. The probability of occurrence for the HI absorbers in all HI galaxies is 1/1078. The radio flux densities of the FASHI absorbers are mainly concentrated in the range of $S_{\rm 1.4GHz}=10\sim100$ mJy, but also as low as $2.6\pm0.4$ mJy. We find that the host galaxies of the associated HI absorbers have relatively high star formation rates, and there is a negative correlation between the HI column density and the stellar mass in the host galaxy. Consequently, FAST has significantly improved the capabilities and performance for HI absorption observations and has provided a true untargeted survey of 21 cm HI absorption galaxies for such studies.