Introduction

There have been speculations that low surface brightness (LSB) galaxies and intergalactic clouds might constitute a substantial portion of the population of nearby extragalactic objects and that they might contain comparable mass to that in normal galaxies. The LSB population would escape detection optically and would not be included in the galaxy luminosity functions that are used to compute the visible baryonic content of the local Universe (Disney 1976, McGaugh 1996). On the other hand, estimates of the HI mass function (HIMF) based on published observations (Briggs 1990) have seemed to indicate that there is probably not any substantial population that has been missed. Weinberg et al (1991) and Szomoru et al (1994) have come to the same conclusion. Until recently, there were no galaxy samples that could be used to address this question empirically, since the galaxies were all first identified optically.

In this paper we present results from the Arecibo HI Strip Survey, an unbiased 21cm survey with adequate sensitivity to detect HI of very low surface density. It is of sufficient length (approximately 15 hours of RA) and depth (7400 km ) that it should be immune to fluctuations due to the large scale structure. The total sky coverage was square degrees. In the main beam, which has a FWHM of 3.2 arcmin, the survey was capable of detecting HI masses of at 7 Mpc and at the full depth of the survey. The details of the Arecibo Strip Survey are described by Sorar (1994) and Briggs (1996).

The survey yielded a total of 61 detections, of which about half could be associated with cataloged galaxies listed in the NASA Extragalactic Database (NED). About five detections with galactic latitude , where extinction is not a problem, have no obvious counterparts on the Digitized Sky Survey (DSS). The HI selected galaxies generally follow the same structures as optical selected galaxies. This is consistent with (1) results from Szomoru et al (1996) who fail to detect large numbers of HI selected galaxies in selected void fields and (2) the finding that LSB galaxies and gas-rich dwarfs lie on structures delineated by normal, high surface brightness galaxies (Mo et al 1994).

© Copyright Astronomical Society of Australia 1997