First results from the HI Jodrell All Sky Survey: inclination-dependent selection effects in a 21-cm blind survey

Details are presented of the HI Jodrell All Sky Survey (HIJASS). HIJASS is a blind neutral hydrogen (HI) survey of the northern sky (δ > 22°), being conducted using the multibeam receiver on the Lovell Telescope (full width at half-maximum beamwidth 12 arcmin) at Jodrell Bank. HIJASS covers the velocity range -3500 to 10 000 km s^-1, with a velocity resolution of 18.1 km s^-1 and spatial positional accuracy of ~2.5 arcmin. Thus far about 1115 deg² of sky have been surveyed. The average rms noise during the early part of the survey was around 16 mJy beam^-1. Following the first phase of the Lovell Telescope upgrade (in 2001), the rms noise is now around 13 mJy beam^-1. We describe the methods of detecting galaxies within the HIJASS data and of measuring their HI parameters. The properties of the resulting HI-selected sample of galaxies are described. Of the 222 sources so far confirmed, 170 (77 per cent) are clearly associated with a previously catalogued galaxy. A further 23 sources (10 per cent) lie close (within 6 arcmin) to a previously catalogued galaxy for which no previous redshift exists. A further 29 sources (13 per cent) do not appear to be associated with any previously catalogued galaxy. The distributions of peak flux, integrated flux, HI mass and cz are discussed. We show, using the HIJASS data, that HI self-absorption is a significant, but often overlooked, effect in galaxies with large inclination angles to the line of sight. Properly accounting for it could increase the derived HI mass density of the local Universe by at least 25 per cent. The effect that this will have on the shape of the HI mass function will depend on how self-absorption affects galaxies of different morphological types and HI masses. We also show that galaxies with small inclinations to the line of sight may also be excluded from HI-selected samples, since many such galaxies will have observed velocity widths that are too narrow for them to be distinguished from narrow-band radio-frequency interference. This effect will become progressively more serious for galaxies with smaller intrinsic velocity widths. If, as we might expect, galaxies with smaller intrinsic velocity widths have smaller HI masses, then compensating for this effect could significantly steepen the faint-end slope of the derived HI mass function.