The Arecibo L-Band Feed Array Zone of Avoidance (ALFA ZOA) Deep Survey is the deepest and most sensitive blind Hi survey undertaken in the ZOA. ALFA ZOA Deep will cover about 300 square degrees of sky behind the Galactic plane in both the inner (30 d
eg < l < 75 deg; b < |2 deg|) and outer (175 deg < l < 207 deg; -2 deg < b < +1 deg) Galaxy, using the Arecibo Radio Telescope. First results from the survey have found 61 galaxies within a 15 square degree area centered on l = 192 deg and b = -2 deg. The survey reached its expected sensitivity of rms = 1 mJy at 9 km/s channel resolution, and is shown to be complete above integrated flux, F_HI = 0.5 Jy km/s. The positional accuracy of the survey is 28 arcsec and detections are found out to a recessional velocity of nearly 19,000 km/s. The survey confirms the extent of the Orion and Abell 539 clusters behind the plane of the Milky Way and discovers expansive voids, at 10,000 km/s and 18,000 km/s. 26 detections (43%) have a counterpart in the literature, but only two of these have known redshift. Counterparts are 20% less common beyond v_hel = 10,000 km/s and 33% less common at extinctions higher than AB = 3.5 mag. ALFA ZOA Deep survey is able to probe large scale structure beyond redshifts that even the most modern wide-angle surveys have been able to detect in the Zone of Avoidance at any wavelength.
Two 5 square degree regions around the NGC 7332/9 galaxy pair and the isolated galaxy NGC 1156 have been mapped in the 21-cm line of neutral hydrogen (HI) with the Arecibo L-band Feed Array out to a redshift of ~0.065$ (~20,000$ km/s) as part of the
Arecibo Galaxy Environment Survey. One of the aims of this survey is to investigate the environment of galaxies by identifying dwarf companions and interaction remnants; both of these areas provide the potential for such discoveries. The neutral hydrogen observations were complemented by optical and radio follow-up observations with a number of telescopes. A total of 87 galaxies were found, of which 39 (45 per cent) were previously cataloged and 15 (17 per cent) have prior redshifts. Two dwarf galaxies have been discovered in the NGC 7332 group and a single dwarf galaxy in the vicinity NGC 1156 . A parallel optical search of the area revealed one further possible dwarf galaxy near NGC 7332.
Many observations indicate that dark matter dominates the extra-galactic Universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously we have suggested that VIRGOHI 21, a 21-cm source we found in t
he Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line-width (~200 km/s) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral-hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ~10^8 years ago with a perturber of ~10^11 solar masses. This we take as additional evidence for the massive nature of VIRGOHI 21 as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the HI and find none down to an I band surface brightness limit of 31.1 +/- 0.2 mag/sq. arcsec.
