The ALFALFA H I Absorption Pilot Survey: A Wide-area Blind Damped Lyα System Survey of the Local Universe

• Published in: The Astrophysical journal 2011-11, Vol.742 (1), p.60-jQuery1323901702630='48'
• Main Authors: Darling, Jeremy, Macdonald, Erin P, Haynes, Martha P, Giovanelli, Riccardo
• Format: Article
• Language: English
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/742/1/60

We present the results of a pilot survey for neutral hydrogen (H I) 21 cm absorption in the Arecibo Legacy Fast Arecibo L-Band Feed Array (ALFALFA) Survey. This project is a wide-area 'blind' search for H I absorption in the local universe, spanning -650 km s super(-1) < cz < 17, 500 km s super(-1) and covering 517.0 deg super(2) (7% of the full ALFALFA survey). The survey is sensitive to H I 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 sub(HI) greater than or equal to 2 x 10 super(20) cm super(-2)) is Delta z = 7.0 (129 objects, assuming Ts = 100 K and covering fraction unity); for super-DLAs (N sub(HI) greater than or equal to 2 x 10 super(20) cm super(-2)) it is Delta z = 128.2 (2353 objects). We re-detect the intrinsic H I 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 sub(HI), X) spanning four orders of magnitude in column density, 10 super(19) (Ts /100 K) (1/f) cm super(-2), that is consistent with previous redshifted optical DLA surveys and the aggregate H I 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.