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, sensitive to X-ray sources down to a luminosity of approximately 1038ergs-1. The galaxies are selected from a complete, volume-limited sample of 46 galaxies with LX ∝ L1.5host d <7 MPc for which we have extensive multiwavelength data. For an almost complete subsample with MB <-14 MB (29/31 objects) we have HRI images. 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 ~35per cent of the sample. Nuclear X-ray luminosity is statistically connected to host galaxy luminosity - there is not a tight correlation, but the probability of a nuclear source being detected increases strongly with galaxy luminosity, and the distribution of nuclear luminosities seems to show an upper envelope that is roughly proportional to galaxy luminosity. 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 from obvious that they are miniature Seyfert nuclei. The more luminous nuclei are very often spatially extended, and Hii region nuclei are detected just as often as LINERs. Finally, we also note the presence of fairly common superluminous X-ray sources in the off-nuclear population - out of 29 galaxies we find nine sources with a luminosity greater than 1039ergs-1. These show no particular preference for more luminous galaxies. One is already known to be a multiple SNR system, but most have no obvious optical counterpart and their nature remains a mystery.