The Arecibo Legacy Fast ALFA (ALFALFA) Extragalactic HI Survey

TL;DR

The ALFALFA survey aims to catalog thousands of extragalactic HI sources over a large sky area, providing new insights into galaxy populations and their properties in the local universe.

Contribution

It presents the design, progress, and preliminary results of the ALFALFA survey, which is detecting more HI sources than previously predicted and offers detailed data on galaxy characteristics.

Findings

Detects about twice as many HI sources as predicted

Identifies a significant number of low-mass and dwarf galaxies

Most high HI mass galaxies are massive disks with diverse properties

Abstract

The Arecibo Legacy Fast ALFA (ALFALFA) survey is a program aimed at obtaining a census of HI-bearing objects over a cosmologically significant volume of the local universe. When complete in ~3-4 years, it will cover 7000 square degrees of high latitude sky using the 305m telescope and the seven-beam Arecibo L-band feed array (ALFA). As of May 1, 2008, almost 60% of the required observations are complete and a catalog exists in preliminary form for 25% of the final sky area. ALFALFA is detecting about twice as many HI sources as predicted based on previously published HI mass functions and should deliver a final catalog of >25000 extragalactic HI sources. ALFALFA will detect hundreds of galaxies with HI masses less than 10**7.5 solar masses and similarly large numbers greater than 10**10.3 Msun. Its centroiding accuracy allows for the immediate identification of highly probably optical counterparts to each HI detection. Fewer than 3% of all extragalactic HI sources, and < 1% of ones with HI masses > 10**9.5 Msun cannot be identified with a stellar counterpart. The hundreds of HI sources with observed line widths of 20-30 km/s include a population of optically faint dwarf galaxies. The objects with highest HI masses exhibit a range of morphologies, optical colors and surface brightnesses, but most appear to be massive disk systems. The latter represent the population likely to dominate future studies of HI at high redshift.