The Arecibo Legacy Fast ALFA project, ("ALFA" is the name of a new seven-beam radio camera: the Arecibo L-band Feed Array) at the 305-m radio telescope in Puerto Rico is an ongoing cutting-edge survey which will map approximately 17% of the entire sky. Upon its completion in 2010, approximately 20,000 extragalactic neutral-hydrogen line sources will be found. This survey is the deepest, highest resolution blind extragalactic neutral hydrogen survey ever conducted over a comparable area of the sky. The Principal Investigator and his team lead the survey, which is organized as an open collaboration and based on principles of enabling maximal scientific return and engaging the broadest community of researchers. The Virgo Cluster is a top target in the survey area, which makes the project potentially even more interesting as it is likely that important new structures may be found. Graduate students are involved in all phases of the project, helping to plan and execute the observing program, developing software for data processing and analysis, and producing public data products, as well as undertaking their own science projects. The group will provide a rich and homogeneous dataset, which enables many applications by the broadest possible community ans is useful in synergy with wide area surveys conducted at other wavelengths.
Galaxies started assembling when the Universe was a few million years old. They evolved interacting with the surrounding intergalactic medium and other nearby galaxies, converting gas into stars and progressively altering their chemical composition, mass, morphology and star-forming rate. Their properties have been extensively studied by measuring the properties of their starlight. Since the discovery in 1951 of the emission by interstellar atomic Hydrogen (HI) of a spectral line of wavelength near 21cm - the "HI line" -, large radio telescopes have been used to investigate galaxy evolution as seen through the radio window of the electromagnetic spectrum. Thousands of galaxies, as revealed by their starlight, have also been observed in the HI line. However, only in the last decade have unbiased studies of the HI properties of galaxies been carried out in a "blind" manner, i.e. by surveying large regions of the sky and analyzing the detected HI sources independently on whether an "optical" galaxy, i.e. one revealed by its starlight, is also present or not. ALFALFA is such a survey, covering about 1/6 of the whole sky out to a distance of 750 million light years from us. It is the richest of such surveys and the only one capable of providing a snapshot of a cosmologically fair sample of the Universe. ALFALFA was carried out over a period of 6.5 years with the largest radio telescope on Earth, which is located in Arecibo, PR. It required 4741.5 hours of telescope time to complete, spread over 808 observing sessions, mostly run remotely from Cornell University. The expected statistical characteristics of the survey were simulated in advance, for sets of different, possible "HI mass functions", telescope/hardware sensitivity parameters and observing modes (the HI mass function describes the average cosmic density of HI sources, separately binned by intervals of HI mass). This allowed the selection of survey parameters that would optimize reach of the science goals of the project. The survey was thus designed to be one of minimal "intrusion", revisiting each pointed direction twice. By "minimal intrusion" we mean maintaining the telescope configuration fixed while acquiring data, letting the sky drift by as dictated by Earth's rotation, and keeping the frequency bandwidth fixed in the reference frame of the observatory, rather than electronically compensating for the continuous variation due to the Earth's motion. The second visit at each sky location, carried out with time separation from the first of several months, provided a useful check for removal of spurious signals produced by radio frequency interference or system malfunction. The analysis of the data is underway. To date (June 2013), 57 peer-reviewed scientific papers based on ALFALFA data have been produced, and 2/3 of the data have been fully processed and sources cataloged. PhD theses of 10 students - at Cornell alone - are based on ALFALFA data, and many younger students have chosen careers in the physical sciences, after participating in a most successful program - "Undergraduate ALFALFA" - which involved many undergaduate instiutions around the country. Preliminary statistical results have been obtained, based on early data releases. Some of the interesting results that have emerged: * The ALFALFA HI mass function disagrees with previous observational reports and the simulations based on theory. ALFALFA finds that the more massive HI sources were previously severely undercounted. * It also finds that the low mass sources observed are not as abundant as expected from theory, confirming previous reports. The result holds true even when the total baryon mass (i.e. the mass of stars plus that of interstellar material, both made of atomic - rather than dark - matter) is considered. Thus, dark matter halos of dwarf galaxies are progressively less able to retain all their baryons, as their mass decreases. The average cosmic baryonic-to-dark mass fraction is thought to be about 1/6, while for the extreme, dwarf galaxies detected by ALFALFA that ratio appears to be less than 1/100. * The baryonic mass of giant galaxies like the Milky Way is usually dominated by that of stars, the interstellar medium contributing less than 10%. Yet ALFALFA has detected a category of massive systems in which the baryonic mass is mainly in the form of interstellar gas, rather than stars. These "anomalous" systems appear to be due to the fact that their disks are rotating very fast, which inhibits the conversion of gas into stars. * ALFALFA detects a number of HI sources which do not appear to have matching optical counterparts. Some of those systems may have resulted from tidal interactions between galaxies in tight groups, and challenge us to produce possible scenarios for such interactions; others are probably very low mass systems which have been unable to convert much of their gas into stars; others again may be remnants of the interaction between galaxies and hot gas populating the denser regions of galaxy clusters.
