The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map $7000 deg 2 of the high Galactic latitude sky visible from Arecibo, providing a H i line spectral database covering the redshift range between À1600 and 18,000 km s À1 with $5 km s À1 resolution. Exploiting Arecibo's large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H i mass function in the local universe and will provide a census of H i 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 Supercluster and nearby superclusters, allow measurement of the H i diameter function, and enable a first wide-area blind search for local H i tidal features, H i 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 5 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 that 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 H i line sources out to z $ 0:06, including several hundred with H i masses M H i < 10 7:5 M .

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We present the first installment of HI sources extracted from the Arecibo Legacy Fast ALFA (ALFALFA) extragalactic survey, initiated in 2005. Sources have been extracted from 3-D spectral data cubes exploiting a matched filtering technique and then examined interactively to yield global HI parameters. A total of 730 HI detections are catalogued within the solid angle 11 h 44 m < R.A.(J2000) < 14 h 00 m and +12 • < Dec.(J2000) < +16 • , and redshift range −1600 km s −1 < cz < 18000 km s −1 . In comparison, the HI Parkes All-Sky Survey (HIPASS) detected 40 HI signals in the same region. Optical counterparts are assigned via examination of digital optical imaging databases. ALFALFA HI detections are reported for three distinct classes of signals: (a) detections, typically with S/N > 6.5; (b) high velocity clouds in the Milky Way or its periphery; and (c) signals of lower S/N (to ∼ 4.5) which coincide spatially with an optical object of known similar redshift. Although this region of the sky has been heavily surveyed by previous targeted observations based on optical flux-or size-limited samples, 69% of the extracted sources are newly reported HI detections. The resultant positional accuracy of HI sources is dependent on S/N: it averages 24 ′′ (20 ′′ median) for all sources with S/N > 6.5 and is of order ∼17 ′′ (14 ′′ median) for signals with S/N > 12. The median redshift of the sample is ∼7000 km s −1 and its distribution reflects the known local large scale structure including the Virgo cluster and the void behind it, the A1367-Coma supercluster at cz ∼7000 km s −1 and a third more distant overdensity at cz ∼13000 km s −1 . Distance uncertainties in and around the Virgo cluster perturb the derived HI mass distribution. Specifically, an apparent deficiency of the lowest HI mass objects can be attributed, at least in part, to the incorrect assignment of some foreground objects to the cluster distance. Several extended HI features are found in the vicinity of the Virgo cluster. A small percentage (6%) of HI detections have no identifiable optical counterpart, more than half of which are high velocity clouds in the Milky Way vicinity; the remaining 17 objects do not appear connected to or associated with any known galaxy. Based on these initial results, ALFALFA is expected to fulfill, and even exceed, its predicted performance objectives in terms of the number and quality of HI detections.

Observations of 170 local (z 0.08) galaxy clusters in the northern hemisphere have been obtained with the Wendelstein Telescope Wide Field Imager (WWFI). We correct for systematic effects such as PSF broadening, foreground stars contamination, relative bias offsets and charge persistence. Scattered light induced background inhomogeneities are reduced down to ∆SB > 31 g' mag arcsec −2 by large dithering and subtraction of night-sky flats. Residual background inhomogeneities brighter than SB σ < 27.6 g' mag arcsec −2 caused by galactic cirrus are detected in front of 23% of the clusters. However, the large field of view allows to discriminate between accretion signatures and galactic cirrus. We detect accretion signatures in form of tidal streams in 22%, shells in 9.4%, multiple nuclei in 47% and two Brightest Cluster Galaxies (BCGs) in 7% of the clusters / BCGs.We measure semi-major axis surface brightness profiles of the BCGs and their surrounding Intracluster Light (ICL) down to a limiting surface brightness of SB = 30 g' mag arcsec −2 . The spatial resolution in the inner regions is increased by combining the WWFI light profiles with those that we measured from archival Hubble Space Telescope images or deconvolved WWFI images. We find that 71% of the BCG+ICL systems have SB profiles that are well described by a single Sérsic (SS) function whereas 29% require a double Sérsic (DS) function to obtain a good fit. SS BCGs, having more symmetric isophotal shapes and fewer detected accretion signatures than DS BCGs, appear to have slightly more relaxed morphology than their DS counterparts. Members of the latter type encompass S2 = 52 ± 21% of their total light in the outer Sérsic component. There is a wide scatter in transition radii r × between the two Sérsic components and surface brightnesses at the transition radii SB(r × ). The integrated brightnesses of the BCG+ICL systems correlate only weakly with S2, r × and SB(r × ). That indicates that the outer Sérsic component is unlikely to trace the dynamically hot ICL since BCG+ICL systems grow at present epoch predominantly in their outskirts.We find that BCGs have scaling relations that differ markedly from those of normal ellipticals, likely due to their indistinguishable embedding in the ICL. The most extended BCG+ICL systems have luminosities and radii comparable to whole clusters. We use different plausible estimates for the ICL component (based on an integrated brightness threshold, SB thresholds and profile decompositions) and find that they do not affect our conclusions about the properties of the ICL. On average, the ICL seems to be better aligned than the BCG with the host cluster in terms of position angle and centering. That makes it a potential Dark Matter tracer. We find positive correlations between BCG+ICL brightness and cluster mass, cluster radius, cluster richness and integrated satellite brightness, confirming that BCG/ICL growth is indeed coupled with cluster growth.

In preparation for the full Arecibo Legacy Fast ALFA (ALFALFA) extragalactic H i survey, precursor observations were carried out in 2004 August-September with the seven-beam Arecibo L-band Feed Array (ALFA) receiver system and the Wideband Arecibo Pulsar Processor spectral processors. While these observations were geared mainly at testing and debugging survey strategy, hardware, and software, approximately 48 hr of telescope time yielded science-quality data. The efficiency of system usage (allowing for minor malfunctions and the impact of radiofrequency interference) during that time was 75%. From those observations, an initial list of 730 tentative detections of varying degrees of reliability was extracted. Ninety-eight high signal-to-noise ratio candidates were deemed to be bona fide H i line detections. To test our ability to discriminate cosmic signals from radio-frequency interference and noise, 165 candidates ranging in reliability likelihood were reobserved with the single-beam L-band wide system at Arecibo in 2005 January-February. Of those, 41% were confirmed as real. We present the results of both the ALFA and the single-beam observations for the sample of 166 confirmed H i sources, as well as our assessment of their optical counterparts. Of the 166 sources, 62 coincided with previously known H i sources, while optical redshifts were available for an additional 18 galaxies; thus, 52% of the redshifts reported here were previously unknown. Of the 166 H i detections, 115 are identified with previously cataloged galaxies of either known or unknown redshift, leaving 51 objects identified for the first time. Because of the higher sensitivity of the Arecibo system, fewer than 10% of the 166 H i sources would have been detected by a HIPASS-like survey of the same region. Three of the objects have H i masses less than 10 7 M . The full ALFALFA survey, which commenced in 2005 February, should detect more than 100 times as many objects of similarly low H i mass over the next 5 years.