Minchin et al. have recently placed limits on the cosmological significance of gas-rich low surface brightness (LSB) galaxies as a proportion of the total population of gas-rich galaxies by carrying out a very deep survey (HIDEEP) for neutral hydrogen (H i) with the Parkes multibeam system. Such a survey avoids the surface brightness selection effects that limit the usefulness of optical surveys for finding LSB galaxies. To complement the HIDEEP survey, we have digitally stacked eight 1-h R -band Tech Pan films from the UK Schmidt Telescope covering 36 deg 2 of the survey area to reach a very deep isophotal limit of 26.5 R mag arcsec −2 . At this level, we find that all of the 129 H i sources within this area have optical counterparts and that 107 of them can be identified with individual galaxies. We have used the properties of the galaxies identified as the optical counterparts of the H i sources to estimate the significance of LSB galaxies (defined to be those at least 1.5 mag dimmer in effective surface brightness than the peak in the observed distribution seen in optical surveys). Two different methods of correcting for ease of detection do not yield significantly different results: LSB galaxies make up 62 ± 37 per cent of gas-rich galaxies by number according to our first method (weighting by H i mass function), which includes a correction for large-scale structure, or 51 ± 20 per cent when calculated by our second method (1/ V max correction). We also find that LSB galaxies provide 30 ± 10 per cent of the contribution of gas-rich galaxies to the neutral hydrogen density of the Universe, 7 ± 3 per cent of their contribution to the luminosity density of the Universe, 9 ± 4 of their contribution to the baryonic mass density of the Universe, 20 ± 10 per cent of their contribution to the dynamical mass density of the Universe, and 40 ± 20 per cent of their cross-sectional area. We do not find any ‘crouching giant’ LSB galaxies such as Malin 1, nor do we find a population of extremely low surface brightness galaxies not previously found by optical surveys. Such objects must be either rare, gas-poor or outside the survey detection limits
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