HI Searches in the Zone of Avoidance: Past and Present (and Future)

P.A. Henning, PASA, 14 (1), 21.

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Clustering and Space Density of the HI-Selected Sample

We have determined that an HI-selected sample is not made up of high optical surface brightness galaxies, which are the types usually used to trace large-scale structures. Because blind 21-cm searches are rarely done, the relative distributions of HI-selected galaxies versus optical samples are not well known. For our sample, we can ask if the HI galaxies out of the ZOA lie along structures delineated by optically-selected samples. In particular, we considered the magnitude-limited CfA redshift survey (Huchra et al. 1983) and a volume-limited survey of dwarf and other low surface brightness galaxies. Analysis by Thuan et al. (1991) indicates that the high- and low-surface brightness galaxies lie along the same three-dimensional structures. We have eleven HI-selected galaxies in regions covered by these surveys. Each of the eleven lies along structures traced by the high- and low-surface brightness samples. This result is consistent with the underdensity of HI galaxies in the Local Void, where it lies behind the Milky Way (Henning 1995).

Finally, we consider the space densities of galaxies of various HI masses. In particular, are we missing a large population of low optical surface brightness, HI-rich objects when relying on optical catalogs? To answer this, we have constructed an HI mass function, analogous to optical luminosity functions. Figure 1 (Figure 6 of Henning 1995) shows the number density of galaxies as a function of Mtex2html_wrap_inline140, weighted inversely by the volume in which the galaxies could have been detected. Uncertainty is introduced by the presence of large-scale structures in the survey volume, but this effect should be relatively minor due to the all-sky distribution of the search points.

 
figure34

Figure 1: Plotted points represent the observed HI mass function of Henning 1995. The sensitivity of the search is shown by the solid line, and the curves labelled TS2 and TS5 represent the predictions of the model by Tyson & Scalo (1988) discussed in the text.

This function is flat over high Mtex2html_wrap_inline140 then shows an upturn at Mtex2html_wrap_inline144; however, we do not see the large population of very low mass (10tex2html_wrap_inline146 to 10tex2html_wrap_inline134 tex2html_wrap_inline138) low surface brightness dwarfs predicted by the model of dwarf galaxy formation developed by Tyson and Scalo (1988). The curves TS2 and TS5 show their predicted number density of dwarf galaxies with HI column densities of tex2html_wrap_inline152 and tex2html_wrap_inline154, following Briggs (1990). Our search sensitivity would have allowed detection of this population. While the number density increases with decreasing mass, low mass objects do not hold the majority of the neutral hydrogen mass. High HI mass objects, with log tex2html_wrap_inline156 9.5, still contain the bulk (60%) of the HI mass. This is consistent with the study of the HI content of optically-selected galaxies by Rao and Briggs (1993). This HI mass function is also consistent with the analysis (Schneider 1996) of the blind survey conducted with the Arecibo telescope by Spitzak (1996). However, compared with his more recent analysis (Schneider 1997) in which he applies a larger correction for sensitivity, our HI mass function falls significantly below his. We note that much of our searching probed the Local Void; further, these small blind surveys conducted to date all may suffer uncertainties due to large scale structure.


Next Section: Surveys in the North
Title/Abstract Page: HI Searches in the
Previous Section: Types of Galaxies found
Contents Page: Volume 14, Number 1

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