Multiwavelength study of the nuclei of a volume-limited sample of galaxies- I. X-ray observations

We discuss ROSAT HRI X-ray observations of 33 very nearby galaxies, sensiti ve to X-ray sources down to a luminosity of approximately 10(38) erg s(-1). The galaxies are selected from a complete, volume-limited sample of 46 gal axies with d < 7 Mpc for which we have extensive multiwavelength data. For an almost complete subsample with M-B < -14 (29/31 objects) we have HRI ima ges. Contour maps and source lists are presented within the central region of each galaxy, together with nuclear upper limits where no nuclear source was detected. Nuclear X-ray sources are found to be very common, occurring in similar to 35 per cent of the sample. Nuclear X-ray luminosity is statis tically connected to host galaxy luminosity there is not a tight correlatio n, but the probability of a nuclear source being detected increases strongl y with galaxy luminosity, and the distribution of nuclear luminosities seem s to show an upper envelope that is roughly proportional to galaxy luminosi ty. While these sources do seem to be a genuinely nuclear phenomenon rather than nuclear examples of the general X-ray source population, it is far fr om obvious that they are miniature Seyfert nuclei. The more luminous nuclei are very often spatially extended, and H-II region nuclei are detected jus t as often as LINERs. Finally, we also note the presence of fairly common s uperluminous X-ray sources in the off-nuclear population - out of 29 galaxi es we find nine sources with a luminosity greater than 10(39) erg s(-1). Th ese show no particular preference for more luminous galaxies. One is alread y known to be a multiple SNR system, but most have no obvious optical count erpart and their nature remains a mystery.