A study of a 21 centimeter-selected sample of galaxies. I - The survey

Known galaxies are rare in the portion of the sky obscured by our Galaxy. This so-called zone of avoidance blocks our view of about one- fifth of the extragalactic sky. A full understanding of the large-scale structure of the universe will eventually require a complete sky mapping, so methods of surveying the zone of avoidance must be found. This paper concerns a pilot survey for galaxies at 21 cm conducted with the NRAO 300 foot (91 m) telescope, which was done to test the feasibility of finding unseen galaxies through the detection of the radiation emitted by neutral hydrogen. This "blind search" method is appropriate in optically obscured regions because the long-wave-length radiation from H I is not blocked by interstellar dust in the Milky Way. As a control experiment, the same style of H I search was conducted in unobscured parts of the sky. The resulting collection of 21 cm-selected galaxies, 37 extragalactic objects, of which 19 were previously known, forms a unique sample. No one has ever investigated the characteristics of objects found in this way before. I have undertaken the study of the 21 cm-selected sample to see if these galaxies differ from optically selected samples. H I mapping of all of the uncataloged galaxies, and a few of the cataloged sources, was done at the VLA to determine accurately the galaxies H I line widths and masses. Although the purpose of this work was not to map large-scale structure, a region of galaxian underdensity, or cosmic void, was evident in the data. It seems that voids are not filled by low surface brightness, dwarf galaxies, but rather are truly lacking in bright and faint galaxies. This paper, the first of two, presents the results of the single-dish and VLA studies and discusses the appearance of large-scale structures in the data. The second paper will detail the analysis of this H I-selected sample, in particular, the comparison with optical- and infrared-selected samples.