We present the results of a pilot survey for neutral hydrogen ( HI ) 21 cm absorption in the Arecibo Legacy Fast Arecibo L-Band Feed Array ( ALFALFA ) Survey .
This project is a wide-area “ blind ” search for HI absorption in the local universe , spanning -650 ~ { } { km~ { } s } ^ { -1 } < cz < 17 , 500 ~ { } { km~ { } s } ^ { -1 } and covering 517.0 deg ^ { 2 } ( 7 % of the full ALFALFA survey ) .
The survey is sensitive to HI absorption lines stronger than 7.7 mJy ( 8983 radio sources ) and is 90 % complete for lines stronger than 11.0 mJy ( 7296 sources ) .
The total redshift interval sensitive to all damped Ly \alpha ( DLA ) systems ( N _ { HI } \geq 2 \times 10 ^ { 20 } cm ^ { -2 } ) is \Delta z = 7.0 ( 129 objects , assuming T _ { s } = 100 K and covering fraction unity ) ; for super-DLAs ( N _ { HI } \geq 2 \times 10 ^ { 21 } cm ^ { -2 } ) it is \Delta z = 128.2 ( 2353 objects ) .
We re-detect the intrinsic HI absorption line in UGC 6081 but detect no intervening absorption line systems .
We compute a 95 % confidence upper limit on the column density frequency distribution function f ( N _ { HI } ,X ) spanning four orders of magnitude in column density , 10 ^ { 19 } ( T _ { s } / 100 ~ { } K ) ( 1 / f ) cm ^ { -2 } < N _ { HI } < 10 ^ { 23 } ( T _ { s } / 100 ~ { } K ) ( 1 / f ) cm ^ { -2 } , that is consistent with previous redshifted optical damped Ly \alpha surveys and the aggregate HI 21 cm emission in the local universe .
The detection rate is in agreement with extant observations .
This pilot survey suggests that an absorption line search of the complete ALFALFA survey — or any higher redshift , larger bandwidth , or more sensitive survey , such as those planned for Square Kilometer Array pathfinders or a low frequency lunar array — will either make numerous detections or will set a strong statistical lower limit on the typical spin temperature of neutral hydrogen gas .