The ALFALFA HI Absorption Pilot Survey: A Wide-Area Blind Damped Lyman
  Alpha System Survey of the Local Universe
  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 < cz < 17,500 km/s and covering 517.0 square
degrees (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 Lyman alpha (DLA) systems (N_HI >= 2x10^20 cm^-2) is
Delta z = 7.0 (129 objects, assuming T_s = 100 K and covering fraction unity);
for super-DLAs (N_HI >= 2x10^21 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.
