First results from the H I Jodrell All Sky Survey: Inclination-dependent selection effects in a 21-cm blind survey

Details are presented of the H I Jodrell All Sky Survey (HIJASS). HIJASS is a blind neutral hydrogen (H I) 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 H I parameters. The properties of the resulting H I-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, H I mass and cz are discussed. We show, using the HIJASS data, that H I 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 H I mass density of the local Universe by at least 25 per cent. The effect that this will have on the shape of the H I mass function will depend on how self-absorption affects galaxies of different morphological types and H I masses. We also show that galaxies with small inclinations to the line of sight may also be excluded from H I-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 H I masses, then compensating for this effect could significantly steepen the faint-end slope of the derived Hi mass function.