The Arecibo Legacy Fast ALFA Survey. IV. Strategies for Signal Identification and Survey Catalog Reliability

We present a signal extraction utility written for the purposes of theArecibo Legacy Fast ALFA survey (ALFALFA). This survey, when completed, willhave covered 7000 square degrees of the high galactic latitude sky and shoulddetect over 20,000 extragalactic objects. It is the most sensitive blind HIsurvey to date. The large size of the survey justifies in itself the need foran automated way of identifying signals in the data set. The matched-filteringsignal extractor proposed is based on convolutions in the Fourier domain oftemplates of varying widths with each spectrum. The chosen templates are builtfrom a simple combination of Hermite functions to mimic the shape of typicalgalactic HI profiles of varying widths. The main advantages of thismatched-filtering approach are a sensitivity to the total flux of the signals(and not only to peak flux), robustness against instabilities and shortcomputing times. The details of the algorithm are given here, as well asresults of simulations that assess the reliability and completeness of theprocess.