An Extragalactic Slice of the Hydrogen Sky

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Scientific paper

We have just concluded the most definitive systematic search ever attempted for sources of extragalactic atomic hydrogen (HI) emission at 21 cm using the Arecibo 300m telescope. We have completely surveyed 60 square degrees at b>30(deg) out to 8000km s(-1) . In the process we have discovered 39 ``new" extragalactic objects and ``rediscovered" 35 previously cataloged ones. A number of the new objects may have been overlooked previously because of their unusual morphologies and/or very low surface brightnesses (and some have not yet been identified optically pending November observations at Kitt Peak). We use these data to characterize the completeness of current catalogs and optical surveys in general. We have obtained (or are obtaining) CCD images at B, R, I, and H-alpha of all objects encountered in the HI survey using the Kitt Peak 0.9m telescope. Comparisons of the HI content, velocity width, optical emission, and spatial position of the members of the two samples reveal a number of interesting distinctions between them. The ``new" objects do not reach the highest HI and dynamical masses of the cataloged galaxies; otherwise the samples are quite similar except that the new objects have a lower relative optical luminosity. The spatial distributions of both samples generally trace large-scale structure, although the new objects tend to be more isolated. We also find an excess of low-luminosity objects at low redshifts in the new sample as compared to the cataloged sample. Some initial analysis of these distinctions will be presented with the goal of better understanding the completeness of the current extragalactic census.

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