The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map 7000 square degrees of the high galactic latitude sky visible from Arecibo, providing a HI line spectral database covering the redshift range between -1600 km/s and 18,000 km/s with 5 km/s resolution. Exploiting Arecibo's large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the HI mass function in the local universe and will provide a census of HI in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local and nearby superclusters, will allow measurement of the HI diameter function, and enable a first wide-area blind search for local HI tidal features, HI absorbers at z < 0.06 and OH megamasers in the redshift range 0.16 < z < 0.25. Although completion of the survey will require some five years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique which samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic HI line sources out to z=0.06, including several hundred with HI masses of less than 10^{7.5} msun.Comment: 37 pages, including 11 figures; to appear in Astron. J.; see http://egg.astro.cornell.edu/alfalfa/pubs.php Authors' list contains 40 names: see paper for full detai

We derive the atomic hydrogen content for a total of 1900 spirals in the fields of eighteen nearby clusters. By comparing the HI deficiency distributions of the galaxies inside of and outside of one Abell radius of each cluster, we find that two thirds of the clusters in our sample show a dearth of neutral gas in their interiors. Possible connections between the gaseous deficiency and the characteristics of both the underlying galaxies and their environment are investigated in order to gain insight into the mechanisms responsible for HI depletion. While we do not find a statistically significant variation of the fraction of HI-deficient spirals in a cluster with its global properties, a number of correlations emerge that argue in favor of the interplay between spiral disks and their environment. In the clusters in which neutral gas deficiency is pronounced, we see clear indications that the degree of HI depletion is related to the morphology of the galaxies and not to their optical size: early-type and, probably, dwarf spirals are more easily emptied of gas than the intermediate Sbc--Sc types. The wealth of 21-cm data collected for the Virgo region has made it possible to study the 2D pattern of HI deficiency in that cluster. We also find evidence that gas-poor spirals in HI-deficient clusters move on orbits more radial than those of the gas-rich objects. The implications of all these results on models of how galaxies interact with their environment are reviewed. Hydrodynamic effects appear as the most plausible cause of HI removal. (abridged)Comment: Accepted for publication in the Astrophysical Journa

Spiral galaxies that are deficient in neutral hydrogen are observed on the outskirts of the Virgo cluster. If their orbits have crossed the inner parts of the cluster, their interstellar gas may have been lost through ram pressure stripping by the hot X-ray emitting gas of the cluster. We estimate the maximum radius out to which galaxies can bounce out of a virialized system using analytical arguments and cosmological N-body simulations. In particular, we derive an expression for the turnaround radius in a flat cosmology with a cosmological constant that is simpler than previously derived expressions. We find that the maximum radius reached by infalling galaxies as they bounce out of their cluster is roughly between 1 and 2.5 virial radii. Comparing to the virial radius of the Virgo cluster, which we estimate from X-ray observations, these H I-deficient galaxies appear to lie significantly further away from the cluster center. Therefore, if their distances to the cluster core are correct, the H I-deficient spiral galaxies found outside of the Virgo cluster cannot have lost their gas by ram pressure from the hot intracluster gas.

We have applied a relativistic Tolman-Bondi model of the Virgo cluster to a sample of 183 galaxies with measured distances within a radius of 8 degrees from M 87. We find that the sample is significantly contaminated by background galaxies which lead to too large a cluster mean distance if not excluded. The Tolman-Bondi model predictions, together with the HI deficiency of spiral galaxies, allows one to identify these background galaxies. One such galaxy is clearly identified among the 6 calibrating galaxies with Cepheid distances. As the Tolman-Bondi model predicts the expected distance ratio to the Virgo distance, this galaxy can still be used to estimate the Virgo distance, and the average value over the 6 galaxies is Mpc. Well-known background groups of galaxies are clearly recovered, together with filaments of galaxies which link these groups to the main cluster, and are falling into it. No foreground galaxy is clearly detected in our sample. Applying the B-band Tully-Fisher method to a sample of 51 true members of the Virgo cluster according to our classification gives a cluster distance of Mpc, larger than the mean Cepheid distance. Finally, the same model is used to estimate the Virgo cluster mass, which is within 8 degrees from the cluster center (2.2 Mpc radius), and amounts to 1.7 virial mass.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

(Abridged) The distances and HI contents of 161 spiral galaxies in the region of Virgo cluster are used to gain insight into the complicated structure of this galaxy system. The 3D galaxy distribution has been inferred from quality distance estimates obtained by averaging distance moduli based upon the TF relationship taken from eight published datasets previously homogenized. Previous findings that the spiral distribution is substantially more elongated along the line-of-sight than in the plane of the sky are confirmed by the current data. In addition, an important east-west disparity in this effect has been detected. The overall width-to-depth ratio of the Virgo cluster region is about 1:4, with the most distant objects concentrated in the western half. The filamentary structure of the spiral population and its orientation are also reflected by the HI-deficient objects alone. The HI deficiency pattern shows a central enhancement arising from galaxies that belong to the Virgo cluster proper. However, significant gas deficiencies are also detected outside the main body of the cluster in a probable group of galaxies at line-of-sight distances 25-30 Mpc, as well as in various foreground galaxies. In the Virgo region, the HI content of the galaxies then is not a straightforward indicator of cluster membership.Comment: 35 pages, 7 figures. Accepted for publication by the Astronomical Journa