The Arecibo Legacy Fast ALFA Survey. I. Science Goals, Survey Design, and Strategy

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map7000 square degrees of the high galactic latitude sky visible from Arecibo,providing a HI line spectral database covering the redshift range between -1600km/s and 18,000 km/s with 5 km/s resolution. Exploiting Arecibo's largecollecting area and small beam size, ALFALFA is specifically designed to probethe faint end of the HI mass function in the local universe and will provide acensus of HI in the surveyed sky area to faint flux limits, making itespecially useful in synergy with wide area surveys conducted at otherwavelengths. ALFALFA will also provide the basis for studies of the dynamics ofgalaxies within the Local and nearby superclusters, will allow measurement ofthe HI diameter function, and enable a first wide-area blind search for localHI tidal features, HI absorbers at z < 0.06 and OH megamasers in the redshiftrange 0.16 < z < 0.25. Although completion of the survey will require some fiveyears, public access to the ALFALFA data and data products will be provided ina timely manner, thus allowing its application for studies beyond thosetargeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimumintrusion, drift scan observing technique which samples the same region of skyat two separate epochs to aid in the discrimination of cosmic signals fromnoise and terrestrial interference. Survey simulations, which take into accountlarge scale structure in the mass distribution and incorporate experience withthe ALFA system gained from tests conducted during its commissioning phase,suggest that ALFALFA will detect on the order of 20,000 extragalactic HI linesources out to z=0.06, including several hundred with HI masses of less than10^{7.5} msun.